It can prevent you from taking off from the same runway you did the day before. It will sap power from your engine. It can eliminate any chance of a climb rate on departure. It can drastically increase your takeoff and landing rolls. What aviation phenomenon has this much power over your flying? Density altitude. And if you fly without paying it due attention, you may find yourself staring down the end of a runway without hope of stopping or taking off. Even if you do make it in the air, high-density altitudes can cause you to quickly meet up with terrain that has a gradient superior to your ascent.

Yet there’s no way a pilot can grasp what density altitude is all about without knowing a little about the word “density.” Its official definition is “the quantity of something per unit measure, especially per unit length, area or volume.” In the case of density altitude, it should read “the quantity of gas molecules per unit of volume.” Each gas (air) molecule has a certain mass or weight. The more air molecules per unit of volume, the more tightly populated is that unit of volume. Thus, we say that parcel of air is more dense.

According to the laws of physics, if the temperature and/or pressure of a gas are altered, density (remember: number of molecules per unit of volume) also will change. For example, if you heat gas, the air molecules begin to move faster and strike each other. As they perform this dance, they spread out. Think about it like breaking the balls on a pool table. One fast-moving molecule comes in and hits some other molecules, speeding them up and spreading them out. In the long run, fewer molecules will occupy a given unit of volume, such as a square foot.

The opposite is true with decreases in temperature—molecules slow down and become more closely packed. Take water, for instance. There are more water molecules in a cubic foot of liquid (cooler) water than in a cubic foot in gaseous (hotter) form. If you don’t believe me, weigh them and see for yourself (more molecules equals more weight!).

Pressure also affects density. Increasing pressure smooshes molecules together, packing in more per unit of volume. In other words, density increases. The reverse occurs if pressure is decreased. When pressure is released, molecules can stretch out and have some breathing room. As they spread out, there are fewer per square foot, or whatever measurement you use, as density decreases. While variations in weather conditions have an effect on the ambient pressure, the biggest influence is altitude. Remember that for every 1,000 feet in altitude change, there’s one inch of mercury change in pressure. So the difference in molecular compression due to ambient pressure is much less in Denver, Colo., at around 5,000 feet versus Tampa, Fla., which sits near sea level.

Density altitude is commonly referred to as the actual altitude at which the plane “feels” it’s flying. For instance, an airplane taking off in Billings, Mont., (elevation 3,500 feet) with an altimeter setting of 29.82 and a temperature of 40 degrees C is being flown at a density altitude of 7,100 feet—the aircraft actually “feels” as if it’s flying at 7,100 feet. So the engine, wings and propeller act as though they’re much higher than what is read off of the altimeter. Hopefully, every pilot is aware that his or her plane flies a lot different at higher-density altitudes and some care is in order. That’s the layman’s version. Just like everything else in aviation, though, density altitude has an official defini-tion: “pressure altitude corrected for non-standard temperature.” Do you recall how temperature and pressure both influence density? Aren’t those two key words found in the definition of density altitude? Absolutely. So, basically, by calculating density altitude, we’re figuring out how atmospheric pressure and ambient temperature affect the airplane.

In fact, increases in density altitude, that is, fewer molecules, decreases the available horsepower created by the aircraft’s engine and steals performance from the wings and propellers. It also causes the aircraft’s true airspeed to increase. But how can the number of air molecules, which are so small they can’t even be seen by the naked eye, keep airplanes from becoming airborne and rob them of vital performance?

In order for engines to create power, oxygen is required so that fuel can be burned. If you have more oxygen (molecules) available, you can burn more gas and, in turn, create more power. If there’s less oxygen available, which is the case at higher-density altitudes, less power can be produced. Furthermore, the engine likes a particular ratio of fuel to oxygen. This is why at density altitudes near sea level, which are ripe with oxygen molecules, full rich mixture is used. Then, climbing up to higher altitudes where air molecules, including oxygen, become more and more scarce, pilots must reduce the fuel supplied to the engine by leaning the mixture. This keeps the engine happy by maintaining its desired fuel-to-air ratio.

The wings and propellers function best in thick air, which is chock-full of molecules. This is due to the part of the lift equation dealing with Newton’s Third Law of Motion: For every action, there is an equal and opposite reaction. As air flows over a wing, it’s dumped downward off of the aft end. Each air molecule that makes this trip brings about an equal and opposite reaction. More molecules, more reaction. If there is, say, 5,000 pounds of air molecules being pushed down by the wing, there will be an equal and opposite reaction of 5,000 pounds augmenting aircraft lift. At higher-density altitudes, however, there are fewer molecules available. Therefore, there’s less equal and opposite reaction, or less lift.

Unfortunately, propellers can only go so fast, usually somewhere in the neighborhood of 2,500 rpm, which in thick air pushes lots of air molecules, yielding more thrust than 2,500 rpm in thin air with its fewer particles. But wings have to develop a certain amount of lift to support the weight of the aircraft or else, obviously, the plane won’t fly! So how could you make up for the decreased number of air molecules passing over the wing at higher-density altitudes? What happens in the real world is that aircraft must travel faster. The faster the wing goes, the more molecules will be encountered at a given moment. More molecules, more lift.

Dig deep into your memory. What’s the definition of true airspeed (TAS)? It’s “the speed of the airplane through the relatively undisturbed air mass.” We just learned that in order to keep the plane flying at high-density altitudes, the aircraft must travel through the air mass at a high rate of speed. Hence, what’s actually increasing is the true airspeed. Going back to our primary training, we know if true airspeed increases, so does groundspeed. If your groundspeed is higher during landing, you know the ground roll will be longer because there’s more speed to dissipate. And if more speed is required to pass enough air molecules over the wing to make it fly, the longer the takeoff roll will be as well.

How come the fact that the plane is traveling faster doesn’t show up on the airspeed indicator? It’s because the airspeed indicator displays indicated airspeed, which is derived from the impact pressure—the number of molecules jammed into the pitot tube at a given moment. Since there are fewer molecules available at higher-density altitudes, the pitot tube must pass through the atmosphere faster to jam as many air molecules down its throat as it would passing through thicker, more densely populated air.

This is why a 65-knot final approach speed is used whether you’re at sea level (in thick air) or at high altitude (thin air), or on a hot day, etc. At sea level, the plane travels around 65 knots TAS to encounter enough air molecules to stuff 65 knots’ worth of impact pressure into the pitot tube. While at higher altitudes, lower pressures or higher temperatures, the plane has to travel, say, 80 knots TAS to pack the pitot tube full of enough molecules to yield 65 knots worth of impact pressure.

Thus far, we’ve seen how increases in temperature and decreases in pressure both lead to less dense air, thus higher-density altitudes. We’ve also seen how high-density altitudes can decrease aircraft performance. There’s another factor that many people neglect to take into account when determining density altitude—humidity. Water vapor molecules can and do displace nitrogen, oxygen and other gases. Considering that water molecules weigh less than those of nitrogen or oxygen, if water displaces these other elements in a particular parcel of air, it ends up weighing less and is thus less dense (O2 has an atomic weight of 32, N2 has an atomic weight of 28, and H2O is the lightest at an atomic weight of 18).

The lesser mass of the water molecules translates into less potential energy when they’re pushed down off the back of a wing or propeller. The equal and opposite reaction from the water molecules is less than if there were oxygen or nitrogen molecules making the trip instead. Also, water doesn’t burn, so whenever water displaces oxygen, there’s less of the latter available to the combustion cylinders of the engine.

Keep in mind, too, that hotter air can hold more water than cool air. At a given relative humidity, air at 15 degrees C contains less water vapor than the air at a temperature of 30 degrees C. Of course, if there’s more water in the air, it results in a higher-density altitude (less atmospheric density). For example, a field with a pressure altitude of 5,000 feet, 37 degrees C and zero percent humidity bears a density altitude of around 8,600 feet. Increase the humidity to 100%, and the density altitude jumps to 9,500 feet. Evidently, you shouldn’t listen to the endless references that humidity doesn’t have an affect on density altitude!

Considering how important density altitude is for the ability of the wing, the propeller(s) and the engine to do their jobs, pilots should always go the distance and check it prior to flying. Certainly, pilots need to use caution when dealing with the three Hs: hot, high and humid conditions. While the performance charts of most aircraft have a density-altitude correction built in to the process of calculation, it’s not a bad idea to figure out density altitude itself if for nothing more than shock value. When was the last time you maneuvered an airplane, or even more important, taken off or landed above 8,000 feet? Put that into consideration when the density altitude you uncover is up there. At high-density altitudes, the plane acts differently; it performs more sluggishly, if it performs at all. So don’t let density altitude sneak up on you by being dense about it and its dangers.

The post Don’t Be Dense About Density Altitude appeared first on Plane & Pilot Magazine.

While there are many factors and influences that can cause pilots trouble in flight, weather is one of the most pervasive and prominent. Pilots who don’t give Mother Nature the proper respect often find themselves humbled, scared and, in the worst of cases, injured or dead. According to AOPA’s Nall Report, approximately 4% of general aviation accidents are weather related, yet these accidents account for more than 25% of all fatalities.

The lethality rate of weather accidents, in other words the chances a fatality will occur in such an event, is around 63%, one of the highest among all accident types. Of these weather-related accidents, half involved attempts to continue to fly under Visual Flight Rules (VFR) into Instrument Meteorological Conditions (IMC). Among these continued VFR into IMC accidents, more than 72% were fatal versus 17% among other types of general aviation events.

A prominent study by human factor and weather accident experts Juliana Goh and Douglas Wiegmann stated VFR into IMC accidents are “a major safety hazard within general aviation,” a fact clearly supported by accident statistics.

Another reason why there appears to be so much interest in these types of accidents is they’re highly preventable, because as is often the case, the pilot intentionally presses on with a flight that clearly should be terminated due to deteriorating weather conditions.

And, of course, in order to have a good plan for prevention, we need to better understand the phenomenon. Thus, Embry-Riddle Aeronautical University-Worldwide (ERAU-WW) recently conducted a study to attempt to better inform pilots about VFR into IMC and offer suggestions for prevention.

Causes And Factors
Because of the extraordinary incidence and high lethality of VFR into IMC, numerous other studies have been conducted by academia, FAA, AOPA and the NTSB on the subject in an effort to better understand causes and factors related to these accidents, all of which guided the current ERAU-WW inquiry.

Of particular interest has been the relationship between flight time and accident occurrence, as well as between pilot certification and accident occurrence. Evidence supports the fact that pilots with low flight time (less than 500 hours) are involved in nearly half of all general aviation accidents. Private pilots make up around 38% of pilots, yet are involved in 49% of accidents. Commercial pilots make up 21% of the pilot population, yet are involved in 28% of accidents. Other types of pilots actually are involved in lower percentages of accidents in relation to their percentage of the population. Interestingly, student pilots make up 15% of pilots, but are involved in only 6% of accidents.

Among studies specifically focusing on weather and VFR into IMC, it has been identified that pilots often misunderstand weather or don’t receive adequate education on how to interpret weather reports. According to a study by Coyne, “Despite its importance for flight safety, manypilots believe weather is the most difficult and least understood subject in their pilot training.” While VFR into IMC is often assumed an unintentional digression by pilots, human factors expert Wiegmann noted that the cause of such occurrences is “often found to be a willful disregard for the regulations and cues that dictated an alternative and safer course of action.”

AOPA’s findings agreed with this statement as “most often, these fatal accidents resulted from pilots deciding to continue VFR flight into instrument meteorological conditions.” Goh and Wiegmann dug deeper into VFR into IMC accidents, finding that the median number of flight hours of pilots involved in such events was lower than among other accident types. There was also a higher incidence of VFR into IMC among pilots at, or below, the private pilot certification level in comparison to higher levels of certification. Two additional studies looked at some pilot demographics and environmental factors, but few statistics were offered as support.

In an effort to better understand what may influence pilots to make and continue flights in which they’re threatened by poor weather, faculty at ERAU-WW utilized guidance from the aforementioned studies, as well as new research concepts to conduct a new inquiry on continued VFR into IMC accidents.

The current investigation used a type of regression analysis, which is a method to understand how strongly variables are related to, or predict, an outcome. In this case, terrain, time of day, receipt of a weather briefing, filing a flight plan, pilot age, pilot flight time, pilot certification and communication with air traffic control were examined to see how they were related to or influenced the outcome of either a VFR into IMC accident, or an accident-unrelated VFR into IMC. In order to complete this study, 40 VFR into IMC accident reports and 40 non-VFR into IMC accident reports were pulled from the NTSB database and were mined for the previously listed factors. The resultant model indicated that the identified factors were capable of correctly classifying an accident in more than 76% of cases (which was found to be statistically significant). In short, the predictors were collectively good indicators as to whether an accident was VFR into IMC.

The study found two particular factors that provided a statistically significant influence on VFR into IMC accidents: terrain and the receipt of a weather briefing. It’s not surprising that high or mountainous terrain would be more deadly for a pilot that inadvertently flies into poor weather, but there’s likely more to this “story.” Weather conditions are often poor or rapidly changing in mountainous areas, making it more likely that pilots be exposed to such conditions in these areas.

Moreover, pilots may be accustomed to poor weather in high terrain and be more likely to “push the limits,” which seemed to be the case in many accidents in the state of Alaska. In the lower 48, the
opposite was true, with many accident pilots unfamiliar with mountainous terrain and weather getting caught up in the grip of the unique weather phenomena around hills and mountains.

As is often the case, the pilot intentionally presses on with a flight that clearly should be terminated due to deteriorating weather conditions.

Oddly, the significant majority of VFR into IMC accident pilots received a weather briefing, meaning they were concerned about the weather, or at the very least, knew of poor forecasted conditions. A significant number of weather briefings included “VFR not recommended” statements that clearly went unheeded. Therefore, the problem isn’t so much that pilots aren’t checking the weather, but instead are misinterpreting or ignoring clues given by Mother Nature.

Other interesting findings from the ERAU study show that as pilot certification level increased, the likelihood of VFR into IMC went down. This indicates that with experience and more advanced education, pilots are more likely to avoid such occurrences.

However, there was also a positive correlation between flight time and VFR into IMC accidents, meaning that pilots involved in these accidents had higher flight time. Coupled with the previous finding, it appears that higher-time, lower- certification-level pilots are more at risk.

Of course, higher-time pilots are more likely to be exposed to poor weather. But it also speaks to some of those hazardous attitudes we should all avoid, specifically overconfidence in one’s ability to cope with weather, that may exceed expectations and abilities. Some other relationships uncovered by the current study were that older pilots were less likely to be involved in these types of accidents.

Weather-briefer training could be modified or augmented to better provide pilots with more emphasis on warnings and hazards that may positively influence pilot decision-making.

Also, pilots flying in mountainous terrain were less likely to be on a flight plan, perhaps counterintuitive to what might be considered good practice. Other factors, such as time of day and air traffic control (ATC) communications, had weak associations with VFR into IMC, indicating such events were less likely to occur during the day and when in communication with ATC. These findings are intuitive, as poor weather is easier to see and avoid during the day, and ATC can provide assistance to pilots in trouble especially in high terrain.

Lessons Learned
So what are the “takeaways” from this study? One is that perhaps we need to examine pilot weather education. Clearly, those with lower certification levels could benefit from situational-based training (SBT) that concentrates on weather decision-making and risk assessment. With the high incidence of VFR into IMC in elevated terrain, more focus should be paid to SBT in mountainous terrain, complete with their unique weather attributes. This could easily be reproduced in simulation.

Moreover, weather-briefer training could be modified or augmented to better provide pilots with more emphasis on warnings and hazards that may positively influence pilot decision-making. Other findings, such as the fact that pilots with low certification levels and high flight times having a higher incidence of VFR into IMC, beckons improved recurrent and flight review training to include SBT and hazardous attitude evaluation specifically related to flights in or around rapidly changing or deteriorating weather conditions.

Also, the findings indicate some actions to mitigate risk in marginal weather conditions, such as avoiding night flights in such conditions, filing of flight plans and using all available resources to help keep them safe, namely interaction with ATC. Thankfully, studies such as these highlight improvements that pilots, as well as the industry as a whole, can make to improve the knowledge base and amend operational habits to make flying safer.

David Ison, Ph.D., holds a masters of aeronautical science from Embry-Riddle Aeronautical University-Worldwide. He’s an assistant professor of aeronautics and program chair of the masters of aeronautical science program. A copy of his research for this study can be requested by contacting Ison at david.ison@erau.edu.

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Pilots can use these approximations, which deal mostly with operations and performance, as shortcuts to information that normally requires much more drudgery to uncover. It’s important to note that while aviation rules of thumb often provide answers that are almost identical to those produced longhand, they are, in fact, estimates. If you find that aircraft performance or the like is particularly critical, it’s best to use charts and computation in lieu of a rule of thumb. With that said, there are many excellent rules of thumb out there. Let’s take a look at the most utilitarian.

10. A True Rule Of Thumb.
What good is a rule of thumb if you can’t really use your thumb? Well, believe it or not, your stubby finger is good for something other than hitchhiking. For the average individual, the length between the tip of one’s thumb to its midpoint (the knuckle where it bends) equates to about 10 nm on a sectional chart. This can be helpful when eyeballing distances, such as for a quick deviation, although it’s not recommended to use this method to measure an entire route or to stay clear of unfriendly airspace.

9. Avoid Being Crossed.
Many a headache has been caused by the stress over how much crosswind component exists for a particular flight. Even more throbbing ensues upon pulling out the age-old crosswind chart. There’s an easier way! If the wind differs from the runway heading by 15 degrees, the crosswind component is one-quarter (25%) of the wind velocity. If the difference between the wind and runway is 30 degrees, the crosswind is half of the reported wind speed. If the wind makes a 45-degree angle with the runway, the crosswind component is three-quarters (75%) of the overall wind speed. And when the windsock is pointing 60 degrees or more from the runway centerline, just assume the crosswind is the same as the total wind (it’s pretty close, and you’d only be overestimating the crosswind component, which is probably a good thing anyway).

8. Starting Down.
One thing pilots of all experience levels struggle to grasp is when to start down from cruise. I remember riding in the jumpseat of a regional jet while the pilot flying was having a bad day determining when to descend. We ended up overhead the airport at several thousand feet, i.e., a bit high. Knowing when to start down so the descent remains at a reasonable rate is a critical piece of information, regardless of the type of aircraft flown. In most circumstances, it’s smart to plan on a three-degree descent, which equates to a gradient of 318 feet per nautical mile (the problem is that 318 isn’t a mathematically friendly number).

The descent rule of thumb is used to determine when you need to descend in terms of the number of miles prior to the point at which you desire to arrive at your new altitude. This is accomplished by dividing the altitude needed to be lost by 300 (clearly a much more pleasant number to work with). So let’s say you’re cruising at 7,000 feet and you want to get down to a pattern altitude of 1,000 feet. The altitude you want to lose is 6,000 feet, which when divided by 300 results in 20. Therefore, you need to start your descent 20 nm out (of course, you’ll want to leave some extra room so that you’re at pattern altitude prior to the proper entry, as applicable). The beauty of this rule of thumb is that you can use it to determine visual descent points (VDPs) as well. Just divide the height above threshold by 300, and you’ll get a VDP in miles from the runway.

7. How Fast To Descend?
While it’s nice to figure out when to descend, that’s only part of the picture. It’s also necessary to know what rate of descent (ROD) to use. Consequently, rules of thumb No. 8 and No. 7 go hand in hand. To determine ROD for a three-degree path, simply multiply your groundspeed by 5. If you’re going 120 knots, your ROD to fly the desired path would be 600 feet per minute (120 x 5 = 600). This makes sense. In No. 8, it was determined that a descent should be initiated at 20 nm to lose 6,000 feet. If the groundspeed is 120 knots, that means the aircraft is zooming along at 2 nm per minute. So to go 20 nm, it will take 10 minutes. Ten minutes at 600 feet per minute means you’ll lose 6,000 feet. Voilà!

6. 10/20 Rule Of Speed.
Rule of thumb No. 6 deals with speed and ground roll for both takeoff and landing. If you increase your groundspeed by 10%, your ground roll will increase by at least 20%. The actual amount the ground roll will change varies among aircraft (thus, the words “at least” have been emphasized). According to the Cessna 172P takeoff and landing charts, “for operation with tailwinds up to 10 knots, increase distances by 10% for each two knots [of wind].” A 10% change in groundspeed, which would be about five knots, brings forth an increase in ground roll of 25% (unmistakably more than the rule’s 20%). In general, though, if you fly too fast, you’ll land long.

5. 10/20 Rule Of Weight.
Rule of thumb No. 5 states that a 10% change in weight will cause at least a 20% change in takeoff and landing distances. More weight requires more runway. This rule, too, has some variation in ground-roll numbers among aircraft. A review of Cessna 172, Piper Warrior II and Beech Duchess data shows that a 10% addition of weight yields a 22% to 25% increase in distances. Obviously, if performance is critical, you’ll need to do some calculating. Even so, both 10/20 rules steer you to consider the influences of weight and speed on aircraft performance.

4. Easy Density Altitude.
Ever see a Koch chart? It’s used to determine density altitude and it can be more than a little perplexing. Instead of mulling over yet another “spaghetti chart,” use rule No. 4. For every degree of Celsius variation from standard temperature, density altitude (DA) changes by 120 feet. Increases in temperature cause DA to go up; decreases make DA go down. There’s even a formula: DA equals pressure altitude plus 120 times the difference between actual air temperature and standard. So if you’re at sea level, the altimeter is 29.92 and it’s 25 degrees C, DA could be calculated by adding pressure altitude (zero, in this case) to 120 times the result of 25 degrees C (actual) minus 15 degrees C (standard at sea level). Crunching the numbers gives a DA of 1,200 feet.

3. Density Effects.
Wouldn’t it be nice to know what each degree temperature change does to takeoff performance (other conditions remaining the same)? Rule of thumb No. 3 steps in to answer this challenge. For each degree Celsius of divergence from standard, the takeoff roll changes by roughly 1%. According to the Cessna 172P manual, a takeoff at sea level with standard conditions would require a roll of 890 feet. Up the temperature five degrees, and the roll jumps to 925 feet, just under a 5% boost.

2. Abort! Abort!
If you haven’t heard of rule of thumb No. 2, you need to take some time to get cozy with it now. It states that an aircraft should achieve 70% of its flying speed by the time it has consumed 50% of the runway or an abort is in order. This halfway point is so important that there’s now a sign available to mark it, which has a “1/2” on it (see AIM Figure 7-5-1). You may be wondering why you need more than half your speed when you’ve only used half your runway. This is due to the fact that acceleration doesn’t occur in a linear fashion. You can actually calculate the percentage of liftoff speed required for any given distance of runway with the formula 10 times the square root of the percentage of runway used.

1. Grain-Of-Salt Rule.
This is probably one of the best rules of thumb out there. It reiterates the importance of skepticism by pilots in regard to what’s in the performance section of aircraft manuals. According to the Piper Warrior II manual, the performance charts “do not make any allowance for varying degrees of pilot proficiency or mechanical deterioration of the aircraft.” Thus, performance data reflects the best-case scenario and realistically is underestimated. Considering this premise, rule No. 1 rightfully declares that all performance data should have at least a 20% safety margin tacked on as insurance. If the performance required is so tight it doesn’t allow for this leeway, it may be best to rethink the situation.

CONCLUSION
It doesn’t take a rocket scientist to figure out that rules of thumb aren’t meant to replace performance charts or good judgment. They can, however, help pilots understand the influences of different performance factors on their aircraft, which should, by default, help augment safety. Whether helping measure the distance remaining to a checkpoint or preventing the continuation of a takeoff gone awry, rules of thumb can be excellent additions to the arsenal in a pilot’s mental flight bag.

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There’s a common misconception that critical pilot errors occur only during flight. A surprising number of accidents result from inadequate preflight, like using checklists and forgetting to fasten the cockpit door (above). They’re small mistakes that could escalate into a series of big ones.

One of the most disturbing statistics about general aviation accidents is that more than 75% of them are made because of pilot error. Considering that it’s unlikely that pilots are going away anytime soon, the solution comes in the form of prevention. Saying this is easy, but actually making progress toward this goal is rather problematic. The first step toward eliminating pilot error is to examine the enemy. Just what types of errors are pilots committing and why? Then, armed with this information, pilots can make a concerted effort to avoid such mistakes through a fusion of training, planning and keen attention.

1 Weather. The more a pilot knows about it, the better. While thunderstorms, icing and winds claim their share of airplanes, the real weather gadfly are those serene, innocent-looking clouds and their cousin, fog. Clouds and fog aren’t inherently dangerous; it’s just that when pilots fly into them, they don’t know how to fly on instruments. They fly into a cloud, lose control and crash. Accident gurus call this flying “VFR into IFR.” Well over 80% of such accidents are fatal.

Even though these accidents are referred to as “inadvertent flight into instrument meteorological conditions (IMC),” only 24% of them are inadvertent. The remaining cases show pilots who continue into poor weather. Why is this? Overconfidence is one. Some believe that they don’t need to stay current or that their hour under the hood is good enough. Social pressure also plays a role. Passengers want to get there and pressure the pilot to continue. Last, there’s “get-there-itis.” Pilots are mission-oriented, sometimes too much so.

2 CFIT. Another common pilot error that often involves weather is controlled flight into terrain (CFIT). A simplified definition of CFIT is “flying a perfectly good airplane into the ground.” If a pilot is in a cloud or in fog, he or she can’t see the ground. If the pilot isn’t doing a good job of keeping up with the terrain, an unpleasant meeting with the ground is more likely. Another time when CFIT can be a factor independent of weather is at night. Pilots seem to have a knack of flying into trees and hills after the sun goes down. Again, if pilots allow themselves to be lulled into neglecting to constantly compare their present altitude to that of surrounding terrain, the outcome is likely to be nasty. If you can’t see the terrain, you must be able to point to your position on a sectional, en route chart, approach plate, etc., or you shouldn’t be flying.

3 Poor Communication. Another boo-boo pilots seem to have an affinity for involves deficient communication. This difficulty of communicating comes in several forms. When dealing with air traffic control (ATC), pilots tend to hear what they want to hear. Good pilots anticipate what is coming next, including ATC instructions; however, this profound skill can trick the mind into “hearing” what is expected regardless of what actually filters into one’s headset. Also, misunderstandings between ATC and pilots happen all the time. This plays into the most knotty communication quandary of all: the lack of communication. It’s silly that a pilot would rather keep quiet than ask for help or clarification. If there is any question on what was said, ask for elucidation. It’s amazing how shy pilots can be when it comes to this simple task. Don’t fall into this trap. It’s better to find out you’ve misheard something immediately rather than finding out your license is going to be suspended later.

4 Low-Level Maneuvering. If you ever hear the words “watch this” from a pilot, look out! Pilots are notorious show-offs. How many times have you heard about the pilot who performs an impromptu air show for friends and significant others? A few low-level maneuvers later, and the plane is falling out of the sky. Some air show. The problem isn’t just that pilots are flying low to the ground; it’s this combination of flying too slow and in too tight of a turn that causes crashes. Of course, adherence to the minimum safe altitudes laid out in the FARs is a much smarter practice. If you do actually find a legitimate reason to fly close to the ground, fly the plane like you do when you’re close to the ground at other times, like during landing. Monitor your speed and your bank angle. You certainly wouldn’t try a 60-degree bank turn with no flaps at a very slow speed when turning base to final, so why do it over your parents’ or friend’s house?

5 Inadequate Preflight Inspections. It’s amazing how many pilots mess up preflight inspections. A cursory walk around simply to “kick the tires” so you can hurry up and “light the fires” is beckoning for trouble. Take your time during your preflight. If you find yourself inspecting in haste, slow down. Take a comprehensive look at everything, with checklist in hand, to make sure you don’t miss anything. When you finish, scrutinize the details. Take one last waltz around the airplane, looking for anything that jumps out as being amiss. Perhaps a door isn’t flush with the fuselage or there is still a red, waving flag-looking apparatus on the pitot tube. It might sound funny, but there was actually an occasion when a pilot neglected to unhook a tail tiedown, which was connected to a concrete block. The pilot wondered why the plane required so much power to taxi and why it had an inexplicably aft center of gravity in flight. Luckily for this pilot, he was able to live to tell his story.

6 Inadequate Preflight Planning. Renowned classical novelist Miguel de Cervantes wisely said “forewarned forearmed.” Those who are prepared are equipped to deal with the tasks at hand. Typically, the level of preflight preparation is proportional to how smoothly the flight goes. Think about a time when you rushed your flight planning and how it came back to haunt you later. Often, pilots take off with no planning whatsoever. That’s when they have a tendency to get tangled in temporary flight restrictions or nasty weather. Countless pilots neglect to check density altitude, even though they’re planning a departure from a short strip with a field elevation of 6,000 feet on a 100-degree F day. Weight and balance also is something that often is dismissed. But how can you know for sure you’re in limits if you don’t even bother to check?

7 Failure to Use a Checklist. Lots of pilots get into the mindset that flying is like riding a bike—something you can do easily out of memory. While it’s true that 99% of the time, you’ll remember to do everything required of the checklist, it’s that remaining 1% of the time when you forget to do something that will bite. You can make sure you complete everything you need to all the time if you consistently use a checklist. Sure, you can do cockpit flows or whatever other technique you like, but back up your actions with a checklist. And don’t just blindly read it. As you go through each item, verify that the handle is in the right position or something has actually been accomplished. Just think of the number of gear-up accidents that could have been avoided if the pilots actually ran the before-landing checklist (hint: all of them!).

8 Failure to Perform the “I’M SAFE” Checklist. Another common error of pilots is forgetting to use the “I’M SAFE” checklist. For those who have forgotten what the letters stand for, here’s a reminder: Illness, Medication, Stress, Alcohol, Fatigue and Emotion (some say E is for Eating). Sick pilots have no place in a cockpit. Can you fly with a cold? Maybe, but you’re more susceptible to spatial disorientation, you could have a painful run-in with a blocked eustachian tube or just feel so blah you make stupid mistakes. And don’t be tempted to hide your illness with medication and then go flying. There are lots of over-the-counter medications that can make you a zombie. Of course, illegal medications shouldn’t be in anyone’s blood, let alone a pilot’s. You’ve got to make a choice—fly or take drugs—you can’t do both.

Stress is commonplace in our fast-paced world, but there is a point at which it becomes so intense that it’s a distraction. If you’ve got to go to divorce or bankruptcy court in the morning, it’s probably a good idea to reschedule today’s flight. When your mind is outside the cockpit, you’re bound to make mistakes. And there certainly is no time that your mind is farther outside the cockpit than if you’ve been drinking. The effects of alcohol obviously are detrimental to good cockpit decision making, and alcohol can affect your flying ability, even though you don’t have any booze left in your blood. Hangovers are essentially just like any other illness; if you have one, don’t fly.

Fatigue is a somewhat underrated no-go item. Many of us have flown when we’re not at our peak performance level. Alas, fatigue goes hand in hand with red eyes and transoceanic flights. But there are things that pilots can do to mitigate fatigue. Being well rested by planning ahead makes a big difference. If you know you’ve got a 5 a.m. flight, you need to go to bed early. It’s a no-brainer, but pilots weaken their minds through a lack of sleep all the time. Emotion, just like stress, is something that everyone has to deal with, but there are times when this, too, is at a level that is intolerable in a cockpit. If a loved one just died, cancel your flight. Your mind won’t be in the cockpit, so keep the rest of your body out of it, too. Finally, make sure you’ve eaten something and stay well hydrated. A physiologically sound pilot makes better decisions than a hungry, thirsty one.

9 Running Out of Fuel. It truly is unbelievable how many pilots run out of fuel every year. It’s interesting to note that most of these incidents occur not because, say, the fueler didn’t put enough gas on board. Instead, pilots try to push it just a little bit too far, running out of gas just short of their destination. That darned “get-there-itis” bug tends to afflict pilots all too often when it comes to fuel. Who wants to make an extra stop, anyway? But that 30-minute fuel stop is better than the one you’ll have to make when your tanks go dry.

The problem with fuel management is pilot mentality. Pilots think of fuel in terms of distance, particularly if, during their planning, they determined the flight could be made with the amount of fuel on board. Instead, fuel should be thought of in terms of time. The best way to implement this philosophy is to determine how much fuel will be available once you’re airborne, in hours and minutes. Of course, an allotment of fuel should be set aside for time to divert, then a little more for reserve. Upon departure, a countdown timer should be started. When the clock expires, you land. No ifs, ands or buts about it. This alleviates the problem of changed groundspeed due to wind and helps give pilots a mental excuse to land short of the destination.

10 Mismanagement of Technology. Scientist and novelist C.P. Snow once said that “technology is a peculiar thing. It brings you great gifts in one hand and stabs you in the back with the other.” The mismanagement of technology is a pilot error that has come under particular scrutiny lately, as glass instrumentation has quickly been invading the cockpits of general aviation aircraft. There is much debate concerning whether modern cockpits augment or diminish safety. But the fancy equipment is not to blame; it’s the pilots who don’t manage their resources properly that cause exigency. What often happens is that pilots don’t take the time to learn the equipment thoroughly. When the glass does something a pilot hasn’t seen before or something needs to be changed quickly, too much concentration is focused on the avionics. What suffers is situational awareness and, more alarmingly, aircraft control.

The accident data says it all. According to the statistics, pilots have the cards stacked against them. But they don’t have to sit idle. Alternatively, pilots can be proactive to reduce risks. They can immunize themselves against common mistakes. Keeping a careful watch, pilots can intercept error chains before they go too far. As President George Washington wisely said, “timely disbursements to prepare for danger frequently prevent much greater disbursement to repel it.” With each bit of extra effort, pilots will, no doubt, increase the safety of flight.

The post Top 10 Pilot Errors appeared first on Plane & Pilot Magazine.

Earlier this year, the NTSB released the findings of a special study that they conducted comparing glass-cockpit aircraft and similar conventional, or “round dial,”-equipped aircraft. The purpose of the analysis was to determine if the “transition to glass-cockpit avionics in light aircraft would improve the safety of their operation.” It’s likely that this study stemmed from the GAMA report that in 2006, 90% of all new piston aircraft were equipped with flat-screen avionics. The investigation also may have been prompted by anecdotal evidence that flying glass is somehow harder or more dangerous than conventional flying—as if Avidyne and Garmin glass displays could be likened to rogue computers such as the HAL 9000 of 2001: Space Odyssey. But after much hoopla, media retort and blog discussions, essentially the NTSB report tells us nothing we haven’t known all along. What’s problematic, though, is that they present their findings in a skewed way leading one to believe, at least in part, the nonsensical notions that glass is more dangerous than round-dial aircraft.

It’s important to look at how the study was conducted to confirm the soundness of the methodology used. We’ve all heard compelling results of studies or polls that later turn out to be proven wrong or erroneous. This often is caused by researchers using poor technique when inquiring into the subject at hand, or worse, making claims that go beyond what can logically be construed from collected data. Furthermore, if a study is comparing apples to oranges, it’s invalid.

A recent NTSB study concluded that glass-cockpit aircraft were no safer than conventional instrument aircraft. Their recommendation for training on specific equipment is necessary to realize the safety potential of glass cockpits.

The NTSB made some stretches that compromise their findings. The premises of the NTSB conclusions rely heavily on data they accumulated through surveys of pilots of both glass and conventional general aviation aircraft in 2006 and 2007. These surveys asked for summaries of how the aircraft were used and the number of hours they were flown. However, only 24.7% of conventional aircraft users and 33% of glass users responded. According to the University of Texas, a minimum of 50% response rate is necessary for sound data when using mail surveys and a minimum of 40% is necessary if the survey is done via e-mail. So probably the most critical component used to calculate the accident rate used in the study, the number of hours flown, was deficient.

Furthermore, the NTSB made no effort to ensure the compatibility of accident circumstances. Simply, they compared apples to oranges. Glass airplanes are generally flown farther than conventional aircraft, deal with different weather conditions, are typically flown by a different pilot cohort, and tend to be used less for instructional flights than round-dial types. Is it fair to compare fatality rates of aircraft flown in IMC versus those flown in VMC, then say, “Aha! Those airplanes that are flown in IMC are more dangerous”? Of course not. Numerous previous studies have shown that flying in IMC is inherently more deadly than in VMC.

The NTSB study looked at 2,848 conventional and 5,516 glass aircraft that were built from 2002 to 2006. There was a wide range of aircraft included in the study, but the big names were Cessna, Cirrus, Diamond and Piper. Then the accident rates of these aircraft from 2002 to 2008 were analyzed. A total of 141 accidents occurred in conventional aircraft and 125 in glass. So which is safer? On the surface, the conclusion is obvious—glass: There were 125 accidents among 5,516 glass airplanes versus 141 accidents among 2,848 conventional aircraft. Unfortunately, it’s not fair simply to just count accidents nor is it fair to calculate an accident-per-airframe ratio. Why? Because if 5,000 of those glass airplanes were sitting in the hangar the whole time, we might change our opinion on their safety. So, accident rates per 100,000 hours are calculated to level the playing field.

Keeping in mind that although incomplete hour estimates were used to come up with the following numbers, they do provide some insight into the safety of glass versus conventional airplanes (also note that the response rate was better for glass users, therefore, in theory, those numbers should be a better reflection of reality than conventional numbers). Between 2006 and 2007, the accident rate (per 100,000 flight hours) in glass was 3.71 and for conventional it was 3.77. So glass airplanes (or at least this group of them) were, overall, safer. However, the fatality rate says something different. Among glass aircraft, the fatality rate was 1.03 (per 100,000 flight hours), but the conventional cohort had a rate of 0.43. Before passing judgment, it’s important to determine if they’re comparing apples to apples.

The many advantages of glass come at a cost other than money: time. Time is required for training in order to develop proficiency.

The NTSB identified several attributes concerning the way glass airplanes are flown, and by whom, that differ from round-dial aircraft. One is that glass airplanes are significantly more likely to be flown on an IFR flight plan and thus are logically more likely to encounter instrument meteorological conditions. Another is that glass airplanes are significantly more likely to be conducted on personal or business trips, while conventional tend to fly on a lot more instructional flights than glass types. The highest certificate held in glass is most likely to be a private, while in a conventional aircraft included in the study, there were many more student pilots in the mix. So from the start, you can see that the two groups that were evaluated aren’t at all comparable.

What’s the solution? Compare glass aircraft to other aircraft that are being operated in a similar manner. Looking to the 2009 Nall Report (that looks at the previous year’s stats), data can be extracted to use for a more reasonable assessment. Considering glass aircraft are used more for personal and business trips, it would be wise to compare glass accident rates to other aircraft flown on such trips. The general aviation fatality rate among personal and business flights was 1.89 overall versus that of glass which was 1.65. Not bad. The lethality of glass crashes, the likelihood you’ll die if you crash, was 31.2%. That sounds bad, but we need to put even this in perspective. Considering that glass airplanes are flown in instrument conditions more often, one would expect their lethality to be more similar to aircraft involved in accidents under those conditions than in general. The lethality for IMC accidents in 2008 was 75%. So the 31.2% for glass doesn’t look so appalling when the way the aircraft is normally flown is considered. Also, private pilots are involved in 50% of all accidents of which 52% are fatal. Student pilots, on the other hand, were involved in only 8% of accidents, of which 3% were fatal. Again, when considering most glass are flown by private pilots, one would expect a lethality rate higher than the entire accident population and more similar to that common among the type of pilots flying the aircraft.

What does all of this mean? First, the NTSB provides some great fodder for discussion. They conclude that we need better training for glass. I doubt anyone can disagree with that. But I truly believe that glass, in the hands of a competent, well-trained pilot, has the potential to make flying safer. A moving map with terrain and weather on it certainly can boost situational awareness. The NTSB report also purports that glass is somehow not as safe as similar make and model round-dial aircraft, primarily leaning on the fatality rate to make such a claim. Yet it’s hard to argue with the raw numbers that indicate glass aircraft are, in fact, safer than the average of general aviation and among aircraft flown under similar circumstances.

The post Is Glass Safer? appeared first on Plane & Pilot Magazine.

Severe weather over Florida.

To this day, I can remember a rather “interesting” experience that occurred on a long solo cross-country while I was pursuing my private pilot license (almost 20 years ago). The time to my last waypoint, the destination airport, had expired, yet the field was nowhere in sight. After a few moments of panic and rechecking of numbers, I looked down, and (aha!) there it was.

Anyone who has flown long enough can appreciate how this type of scenario can happen. Flying in unfamiliar territory and into strange new airports can be quite a task, primarily because you’ve never seen the scenery before. Try finding some obscure grass strip down south—it’s like looking for a needle in a haystack. Even with the fanciest GPS units guiding you, it can still be hard to identify certain things from the air. But it doesn’t have to be that way anymore. Now, before you even get in the airplane, you can use sectional chart overlays on Google Earth to show you exactly what to expect on a flight. Best of all, it’s free!

Getting Started
Download Google Earth at earth.google.com. (Because it’s frequently updated, be sure that you have the latest version.) Next, log on to www.wikihow.com/Overlay-Sectional-Aeronautical-Charts-in-Google-Earth. (Type this address exactly as it appears here; if you lowercase the “A” in “Aeronautical” or capitalize the “i” in “in,” the link won’t work.) Scroll down to “Sources and Citations,” and click on “Sectional data download.” Once the sectional data has downloaded, you should see a folder labeled “Aero Charts” under “Places” in the Google Earth sidebar.

An uncontrolled airport near Crawford, Colo. Google Earth allows pilots to visualize terrain, buildings and weather, among other things, over sectional charts. Additionally, they can familiarize themselves with unfamiliar airports, checkpoints and destinations with this invaluable flight-planning tool.

Within the aero chart folder there are four options. There’s an “About” item so you can learn more on the overlay feature. The last three options are the most interesting: sectional charts, terminal area charts and 3D airspace. If you have the latest sectional data and an updated version of Google Earth, when you click on “Sectionals,” every sectional chart will be superimposed on the correct spot. The same applies to terminal area charts. Three-dimensional airspace can be added so that you can view what Class B or C airspace will look like from any angle or altitude. (Keep in mind that none of these charts or overlays can be considered current. They should not be used for purposes of actual navigation!)

A 3D view of Miami Class B airspace.

Within the aero chart folder there are four options. There’s an “About” item so you can learn more on the overlay feature. The last three options are the most interesting: sectional charts, terminal area charts and 3D airspace. If you have the latest sectional data and an updated version of Google Earth, when you click on “Sectionals,” every sectional chart will be superimposed on the correct spot. The same applies to terminal area charts. Three-dimensional airspace can be added so that you can view what Class B or C airspace will look like from any angle or altitude. (Keep in mind that none of these charts or overlays can be considered current. They should not be used for purposes of actual navigation!)

Plan Your Flight
Now that the overlay data is loaded on your computer, let’s look at how it can make your life tremendously easier when planning a flight. Zoom in on Tampa, Fla. If you’re not sure where that is, just type “Tampa” into the “Fly To” box in the Google Earth sidebar. Next, click on “Sectionals,” which will overlay the whole gamut of sectional charts on top of the Google Earth satellite imagery. You should see Tampa International Airport, Vandenberg Airport, MacDill AFB and Peter O Knight Airport. As long as you have “Roads” selected under “Layers,” major roads and their names/designations will still appear through the sectional chart. At the bottom of the “Places” box (underneath the sectional chart names), you’ll see a sliding scale. This allows you to “fade” the sectional overlay. Try it out. It will allow you to compare what the sectional shows with what you can really expect to see from the air. Never been to Peter O Knight before? Take a look and check it out. Once you’ve seen what it and the surrounding areas look like, it will be rather hard to miss.

Mount Rainier in Pierce County, Wash.

But it gets even better. You can adjust your vantage point in Google Earth to match the direction from which you’re flying and even approximate the view from your proposed cruise altitude. Using Peter O Knight, let’s say you’re planning to arrive from the east. Use the navigation tool at the top right of the screen (the one with the eyeball on it) to rotate the image so that west is at the top of the screen or straight ahead (and north is facing to the right, as indicated on the eyeball tool). Next, use the shift and down arrow keys to adjust the “altitude” of your view. (For keyboard help, click on “Help,” then “Keyboard Shortcuts.”) You’ll also need to use the tool with the hand on it to create the right perspective of an arrival from the east. You can see your current eye height in the bottom right corner of the Google Earth screen. Set it to around 4,500 feet. Back yourself up to U.S. 41 (just to the east). Now you can see exactly what an arrival will look like. Shift back and forth between Google Earth and the overlaid sectional to compare notes. You can even add in 3D buildings (under “Layers”), which will show you what downtown will really look like. Interesting, eh?

Let’s try something harder. Type “Bishop Harbor” into the “Fly To” search box. Using the same sectional overlay, look just northeast and find Manatee Airport, which is a typical grass strip. Using the overlay fade slider, check out how it blends into its surroundings. Also note how the sectional doesn’t depict the lakes and factory just south of the field. Once armed with this visual data, it would be hard to miss Manatee. Use the different navigation tools to check out what it looks like from different directions and altitudes.

Ever wonder if you’ll be able to see a checkpoint from the air? Take the guesswork out of it by looking at Google Earth before banking on a particular location. For example, look just southeast of Manatee Airport and see what you think about using the city of Parrish. With Google Earth, it will certainly be a lot easier to find because you’ll know exactly what to look for! Such a preflight check is also a good technique to use when evaluating road intersections, whether you can see a railroad clearly, or what a little stream or other body of water looks like from the sky. By taking the time to perform this simple reconnaissance, you can practically eliminate the chance of getting lost at the hands of crummy checkpoints.

A 3D building view of the Las Vegas Strip.

Another neat feature: 3D terrain is built into the sectional overlay. Type “Red Lodge” into the “Fly To” box. Zoom out a little bit, and be sure that “Terrain” is selected in “Layers.” Take a look to the west while tilting your eye altitude down to check out the surrounding mountains. Welcome to Montana! Can you imagine flying into this airport at night for the first time without looking at it from this perspective? Cycle between the sectional and Google Earth to gain even more perspective. Of course, the use of Google Earth to evaluate terrain shouldn’t be limited to the immediate terminal area or restricted to VFR flights. Want to know why a departure procedure calls for a particular climb gradient? Check it out on Google Earth. Why are minimum IFR altitudes so high along a particular route? Ask Google Earth for some help.

And just when you thought it couldn’t get any better, Google Earth has yet another surprise. See where it says “Weather” under “Layers”? Click on it. If you expand the menu (hit the box with the plus on it), you can select to display weather conditions, satellite views and radar returns. Of course, this is overlaid on not only Google Earth, but also on the sectional chart, if you so desire. Pick somewhere rainy, and try it out. Zoom in on the location, select the appropriate sectional and call up the weather overlay. You’ll probably never fly without it again!

There’s little doubt that Google Earth can be of great assistance in the flight-planning process. It’s an invaluable tool to use when choosing a suitable route because it helps you find reasonable checkpoints. It can also keep you out of trouble with terrain in all types of operations. And it makes finding airports a cinch. All it takes is a few minutes of downloading to arm yourself with one of the most powerful flight-planning tools available. Best of all, if you use Google Earth, you’ll be one of the few pilots at the hangar who doesn’t have an embarrassing “getting lost” story to tell.

The post Google Earth: The Ultimate Preflight Tool appeared first on Plane & Pilot Magazine.

It’s a pristine, fair-weather day, so you can’t resist the urge to hit the sky for some pattern work. After a few rounds, your circuits begin to get a bit messy, which you attribute to a slowly escalating wind. It’s time to call it quits. On base to final, the darned wind is blowing even harder than before, causing you to overshoot. You crank over toward the runway and pull back. But to your surprise, the plane quickly rolls more than you expected and now you’re looking at the runway, but it’s upside-down. You’ve just become a stall/spin statistic. Sadly, 10% of all general aviation accidents result from stalls; nearly 15% of such accidents have fatal outcomes. So how can the nasty surprise of a stall be avoided? The answer is much simpler than you’d think.

Unless you’re a nonpilot, you’ve been introduced to the term “angle of attack” (AOA). The definition can be regurgitated by just about every pilot: It’s the angle between the relative wind and the chord line of the wing. While the concept of a chord line is pretty easy to understand— it’s simply a line drawn between the leading and trailing edges of a wing—a true understanding of relative wind is not quite so straightforward.

Relative wind approaches the wing parallel, but opposite to, flight path. This is an important notion to remember. According to Aerodynamics for Naval Aviators, “regardless of the condition of flight, the instantaneous flight path of the surface determines the direction of the oncoming relative wind.” Thus, no matter the pitch angle of the airplane, the flight path at any particular moment establishes the direction from which relative wind will meet the wing.

This definition makes clear one critical item every pilot should know: Pitch angle isn’t coincident with AOA. In fact, you can stall at any pitch angle. Don’t think it’s true? Then why doesn’t an airplane stall while doing a loop even though it’s transitioning through straight-up flight? Why does an airplane continue to spin even when it’s directly facing the ground? The wing doesn’t care which way the airplane’s nose is pointing; it only cares from which direction the wind is blowing.

Relative wind can come from a direction completely different than where the nose of the aircraft is pointing. Take, for example, slow flight. You slow the airplane down to a point at which you are rather nose high, but are still traveling horizontally. The relative wind, acting opposite to flight path, would be coming from straight ahead. However, the wing is canted upward at a high angle, thus a significant AOA exists. Pull back on the yoke a little harder, and the airplane will stall.

Why does it stall? The aircraft stalls because you exceed something called the “critical angle of attack.” A very simple explanation of critical AOA is the angle at which the air can no longer make the journey over the top of the wing in a fashion adequate to support flight. That’s it! If you exceed the critical AOA, the wing will stall—no ifs, ands or buts about it. Even the Wright Brothers were aware of this concept. On their original Flyer, they actually had an AOA indicator. It was a stick with a string on it—rather crude, but it told the proper tale.

There are three ways that pilots can manipulate lift. The first is to change speed. More speed means more lift. The next is to change the shape or area of the wing through the use of high-lift devices, such as flaps and slats. Lastly, pilots can change AOA. Increase it and lift will go up. Decrease it and less lift will be had. This is why airplanes cruise at low AOA, but upon slowing down, increase AOA to compensate for the decrease in lift formed by the airflow speed. Amazingly, every AOA equates to a specific airspeed, once the plane is allowed to settle down. More simply, for each individual airspeed, a specific AOA is required to support flight.

Counterintuitively, there are many things that pilots believe will cause a stall but, in fact, do not. Again, stalls aren’t caused by excessive pitch angles. Pilots sometimes have a hard time grasping this one, but it’s an indispensable detail to recognize. Stalls don’t necessarily occur because of lack of speed. Though there are cases when inadequate speed leads to a violation of critical AOA, stalls can occur at any speed. Even obvious stall influences, such as increases in weight and load factor, aren’t the true cause of a stall; instead the true instigator is extreme AOA (more lift is required at higher weights, so if speed remains constant, AOA must be increased to produce the requisite lift).

Probably the smartest thing I’ve ever heard about flying is to “keep the nose of the airplane pointed in the direction in which it’s actually traveling.” If you abide by this rule, you’ll never stall. That sounds too easy, doesn’t it? But it’s true. We know that the AOA is the angle between chord line and relative wind. Though the chord line is not exactly aligned with aircraft pitch (because of angle of incidence), the chord line does move along with pitch. If you keep the chord line (pitch) aligned with the flight path, the opposing relative wind will be aligned with the wing, and AOA will remain within reasonable limits.

Although AOA is important in avoiding stalls, it can also determine other significant performance factors. There’s a specific AOA for the best glide ability of an airplane. There’s also an unambiguous AOA that provides best rate of climb, while another yields best angle of climb. And there’s even an ideal AOA for approach and landing. The Navy is big on this concept when its pilots land on carriers—AOA means life or death for the Navy pilot.

It sure sounds like AOA would be a nice thing to know—perhaps even through an instrument that shows the pilot its exact value. Unfortunately, until recently, AOA detection wasn’t an option on GA aircraft. Strangely, even some airliners don’t display AOA. Thankfully, there’s a change brewing, as easy-to-use AOA devices are becoming more readily available. In the meantime, GA flyers must avoid stalls the old-fashioned way: with a combination of airspeed and estimated flight path.

Airspeed works as an indirect AOA indicator because of the AOA-to-airspeed relationship, though this only works in unaccelerated flight.

Consequently, if the aircraft is above stall speed and the flight path is generally aligned with the pitch angle, everything is all right. If one of these items is out of wack, a correction needs to be made. Sometimes the required remedy is counterintuitive. It’s hard to force yourself to relax pressure on the yoke when staring directly at the ground, but it’s the only way to break the stall and to save your life. Another often-ignored consideration about AOA is that whenever it changes, load factor changes. You can thank Isaac Newton for enlightening us about this fact. If an alteration of aircraft direction is commanded, additional forces are applied to the aircraft, which pilots recognize via load factor.

In the aforementioned base-to-final scenario, as the airplane was in the turn, the pitch (chord) of the airplane was increased by pulling back on the yoke, but the craft “wanted” to continue traveling in the original direction for a tad bit longer. Remember relative wind is parallel and opposite to the flight path. If the plane wants to continue to the outside of the turn, but you’ve forced the chord in another direction, you’re boosting AOA. If you push it, you’ll be looking at inverted runway numbers.

I once flew with a captain who said, “I don’t understand all the fuss about AOA. We don’t need to know it.” Nothing is further from the truth. If pilots develop AOA smarts, they can be assured they’ll never stall. I can’t think of a better form of insurance. Proper situational awareness should keep a pilot up on what the actual flight path is at any given moment. That sinking feeling means relative wind is coming from the direction of the ground—not a good thing. But by simply keeping the wing and the relative wind reasonably aligned, all will be well.

The post Getting To Know AOA appeared first on Plane & Pilot Magazine.

This Article Features Photo Zoom

A friend of mine recently asked me if I actually did a weight-and-balance calculation before every flight. When I answered yes, he seemed somewhat taken aback. “Really?!” he quizzed, somewhat perplexed.

His sense of wonderment shouldn’t come as a surprise. Few pilots venture into the air without performing this task. Unfortunately, such oversights of responsibility aren’t always limited to just weight and balance. Flight planning doesn’t have to be quite so wearisome, particularly if pilots contrive a structured method to take on the feat. And no one in aviation knows the best flight-planning methods as well as the airlines. This has a lot to do with the fact that air carriers fly all over the world, day or night, and in all kinds of weather.

Individual pilots obviously don’t have hordes of dispatchers at their beck and call, nor do they have access to the superfluity of resources available to most airline flight operations departments. But general-aviation pilots can make their lives easier by mimicking the paperwork that’s generated for air carrier flights. By doing things the airline way, pilots can have a standardized set of paperwork that’s harmoniously organized and versatile enough to use on every sort of flight, from pattern work to long cross-countries.

Airline pilots get this methodical paperwork, referred to as a dispatch release, prior to each flight. Details of the route of flight, fuel load and burn, expected operational weights as well as a few other items are found on the multiple pages of the release. Final weight-and-balance numbers are presented once the actual fuel and payload become known. The release and accompanying weight and balance make a neat package that satisfies all regulatory and practical requirements.

While there obviously are differences between general-aviation (GA) and airline flying, the gist of flight planning is nonetheless the same. FAR 91.103 is specific about the requisite GA pilot preflight duties. For all flights, pilots need to confirm that the runways they intend to use are suitably long enough for takeoff and landing. The only way to realistically perform this task is to know how much the airplane weighs, which, of course, requires a weight-and-balance calculation. Clearly, a pilot also needs to know this detail in order to confirm that the aircraft is being operated within weight limitations. Bear in mind that this ought to be accomplished prior to even the simplest of flights.

For flights conducted under IFR, as well as those not in the vicinity of the airport, the flight-planning rules get even more exacting. Fuel calculations are a no-brainer necessity, as is the need to check weather reports. Alternative courses of action in case the flight can’t be completed need to be evaluated. This can be dealt with as simply as designating an appropriate alternate airport or by a more complex consideration of all possible landing sights along one’s route. Delays need to be checked, although this applies mostly to IFR flights. Of course, runway data must be examined, per regulation.

Undoubtedly, there are several tasks to complete before heading out to the plane, all of which can seem rather overwhelming when faced without the proper strategy. Perhaps this is why many pilots don’t take the time to complete the requisite preflight tasks. Regrettably, if you neglect to perform these necessary actions and are ramp-checked or, even worse, something goes wrong, it’ll be hard to prove that you complied with any of the regulatory requirements. However, if you hand the FAA inspector a polished, methodical package of your work, it’s guaranteed that the feds will be impressed, making it less likely that you’ll face further scrutiny.

So how do you go about developing such a package, doing things the airline way? First, you need to plan your route of flight. Next, translate distances into flight times. The AOPA Air Safety Foundation Flight Planner Form serves as a terrific guide to keep the route, weather and flight plan data coordinated. Even if you’re just staying in the pattern or the local area, estimate how long you plan to stay aloft. With this flight time information in hand, you then can determine your fuel requirements.

By using a spreadsheet or word-processing program, you can create a fuel form that has an entry to type departure and arrival airports, date and aircraft N-number (see Figure 1). Next, arrange the form in a way that’s most comfortable for you to use. It’s easiest to make fuel calculations if you include lines for taxi, climb, cruise, descent, missed approach, alternate airport and reserve fuel needs. When you add up all your fuel requirements, including reserves, you end up with a number that the airlines call minimum fuel (take advantage of the spreadsheet equation functions to do the adding for you, as it will reduce the likelihood of mathematical errors). Logically, this is the minimum amount of fuel you should have on board prior to departure. Your minimum fuel should be less than the actual fuel on board. If the fuel requirements and the fuel available are close, it’s wise to plan on a fuel stop.

Now create a weight-and-balance form. Include the date and the N-number in addition to anything else you wish to incorporate. Set up the spreadsheet so that it automatically adds the weights of front-seat passengers, back-seat passengers, cargo area weights and the weight of the fuel. You can have the program multiply these weights by the appropriate CGs to get moment values. Simply have the spreadsheet add up the moments and then divide the total moment by the total weight. Presto! You’ve got your aircraft CG. All you have to do is check the POH to check that you’re within limits.

Once you know your weights and fuel burns, you can move on to runway data. Airlines use something called runway analysis to determine which runway they can use. Runway analysis books contain the maximum weight an aircraft can depart a particular runway and have different corrections to this weight based on elements like weather conditions and aircraft configuration. We GA pilots aren’t so fortunate to have such well-thought-out data and must instead assay things longhand. Yet if we create a standard form (see Figure 2) that can be used over and over, it will help cut future preflight workloads. When making such a form, include spaces for the names of all the airports you intend to visit, the date and the aircraft N-number. Provide a line or two for each runway that you intend to use. Include the runway length for later reference. To determine the expected takeoff and landing distances, consider the worst-case scenario—use the next highest weight, temperature, etc., as found in the POH. This will help take into account different pilot technique, aircraft performance reduction due to wear and tear, and so on. An even better idea is to use the distances over a 50-foot obstacle for extra padding. Finally, insert the calculated distances next to the apposite runway for comparison. If the two numbers are close, it’s probably smart to use a short-field technique or choose another runway.

With all of these computations complete, it’s time to compile everything on your own dispatch release (see Figure 3). Make the form as fancy or as simple as you’d like; it basically needs to summarize your efforts. To do this, indicate the date, N-number and route of flight. Next, make a list of required items: fuel, alternate course of action, weather, delays, runway information and CG. Next to them, put a box or circle in which you can mark to indicate that each item has been completed. By printing and assembling this document, along with the other papers, all your preflight regulatory requirements will be covered. The nice thing about it, too, is that the documents will all be together in one place, and it ends up looking quite professional.

It sounds like a lot of work, doesn’t it? But keep in mind that 90% of the work is necessary when first creating the documents. From that point forward, the time you’ll save is immense, considering that all the forms easily are recycled. Just put in the right date, pick the right N-number, and a set of updated documents are ready to go. Flight instructors and students alike can reap the benefits of having forms readily available that apply to most, if not all, of the flights they make on a regular basis. Additionally, this methodology works well for any kind of flight, from the generic one-hour training flight to treks of several hundred miles.

So, during a weather delay, create some of these airline-like forms to make your next flight easier and safer. It’s safe to say that an FAA ramp check will go a lot better with an organized set of paperwork that proves you’ve done your duties instead of fabricating and using “the dog ate my weight-and-balance” excuse. Lastly, it’s not just about fulfilling requirements; it’s about taking pride in your work. And there’s no doubt that you’ll look and feel more professional because of your efforts.

High-Tech Flight Planning
In addition to the customized forms, some people also go the electronic way and use flight-planning software on CDs or online. Going high-tech can make any calculations and prep work a breeze. The following is a list of electronic flight planners that easily can be integrated into your pre-departure routine:

CSC DUATS And Stenbock And Everson DUATS Golden Eagle FlightPrep Software—A flight-planning, filing and weather-briefing tool that features flight planning using relief of IFR charts, and NEXRAD overlay and graphical TFRs on both relief and IFR charts. Contact: DUATS, (800) 345-3828, duats@duats.com, www.duats.com.

DTC DUAT Website—Get airport information and diagrams, store briefing requests and see weather along your route of flight with route overlays—all by just logging on to the DTC DUAT Website. Contact: DTC DUAT, (800) 243-3828, helpdesk@duat.com, www.duat.com.

IMAPS Group Destination Direct—Flight-planning and moving-map software for home or in-cockpit PCs with current U.S. government DAFIF and FAA navigation data and real-time WxWorx in-cockpit weather system. Contact: IMAPS Group, (888) 227-5225.

Jeppesen FliteStar v9.0—Includes weather, TFRs, airport information, FBO data, airspace, terrain, weight and balance, and cost information for each of your flights. Contact: Jeppesen, (800) 621-5377, www.jeppesen.com.

RMS Technology Flitesoft—Provides graphic and automatic flight planning, weight-and-balance checks, weather briefings, a bargain fuel locator, a pilot logbook and automatic functions for important planning tasks. Contact: RMS Technology, (800) 533-3211, info@rmstek.com, www.rmstek.com.

Seattle Avionics Voyager Version 2.0—Features 3-D, wind-optimized SmartRouter AutoRouter, SmartWeather and Internet connectivity. Contact: Seattle Avionics, (425) 455-2209, www.seattleavionics.com.

The post Flight Planning, The Airline Way appeared first on Plane & Pilot Magazine.

The aviation industry sure loves its statistics—there’s an X% chance of this, and one aircraft is Y times safer than Z. But what if you were told that just about everything you’ve heard about aviation accident statistics isn’t true? Most pilots feel pretty good about the commonly published statistics claiming that all types of air travel are safer than driving. But if the numbers are presented in a certain way, general aviation flying can appear more dangerous than driving. Before you throw the magazine across the room and denounce such claims as ludicrous, let’s take a look at the facts.

Before you blindly believe the studies and the plethora of numbers associated with them, there are some things you should know and think about. It should be no surprise that aircraft manufacturers want to paint their aircraft in the most favorable light possible; after all, they’re trying to sell airplanes. And the media seems to chronically harp on the gloom and doom of flying. The point is, we have to be careful about the statistical games being played by people with different motivations. Statistics can and often are manipulated to make things look better or worse than they really are. For example, one study claimed that kids who weighed more were smarter. Wow. Maybe you should let them eat more fast food, right? Well, no. The truth behind this finding is that kids who are older (and, thus, have more years of education) weigh more. You’ve got to get the whole picture—don’t necessarily take things at face value. Such nonsense is probably what prompted British politician Ben Disraeli to note, “There are three kinds of lies: lies, damned lies and statistics.”

One big problem with comparing aviation accident statistics to driving accident statistics is that you’re really comparing apples to oranges. Aviation accident rates are normally shown as an amount per number of flight hours. According to the 2005 Nall Report, there were 7.2 GA accidents and 1.39 GA fatalities per 100,000 flight hours. But auto accident rates are based on miles, not hours. The current rate for motor vehicles is 1.3 deaths per 100 million vehicle miles. What if GA accident stats were presented in miles instead? Using an average aircraft speed of 130 mph and the estimated 23.1 million flight hours reported by the FAA in 2005, you’d be left with 16.3 deaths per 100 million vehicle miles. Not quite as flattering, huh?

Am I claiming that flying is dangerous? Of course not. We can look at yet another statistic, the chance of dying in a GA accident in 2005, which was about one in 613,000 while your chance on the road was one in 7,700. Which brings up the next point. If statistics don’t always tell the full story, as we’ve seen so far, how is a particular aircraft deemed “safe” or “unsafe”? Take, for example, the Cirrus. It’s gotten a bad rap from various media outlets while the company claims that it’s one of the safest airplanes in the sky. I’ve flown one, and it didn’t seem to be ornery at all. In fact, it flew better than many other airplanes I’ve flown. So what gives?

Perhaps the unfair comparison issue is cropping up yet again in the search for statistical analysis. While a discussion about airline fatality stats versus those in GA does come up, everyone realizes they’re two different ball games. Yet, is it fair to compare accident rates of aircraft that are designed to “go places” with those that mostly stick to training or even those that crop dust? The point is, to truly determine how safe an airplane is, you’ve got to compare it to aircraft doing the same kind of flying. Most people don’t want to purchase a more than 200-horsepower, 150-knot aircraft to do pattern work. They’d be more likely to travel, take the family on vacation or go on business trips.

Needless to say, the chances of getting into icing conditions, flying from VMC into IMC or falling victim to controlled flight into terrain are less likely in the pattern or practice area than while traveling cross-country. The point is, to really get a good idea about whether or not an aircraft is safe, it’s not enough just to look at raw GA numbers: You must consider the types and amounts of exposures a particular aircraft experiences. Even the Nall Report admits, “Meaningful comparisons are based on equal exposure to risk. However, this alone does not determine total risk. Experience, proficiency, equipment and flight conditions all have a safety impact. To compare different airplanes, pilots, types of operations, etc., we must first ‘level the playing field’ in terms of exposure to risk.”

There are several “traveling” aircraft—those primarily used to get places rather than around the pattern—that can be used as examples. The Cirrus (SR20 and 22), Bonanza B36, Columbia (300, 350 and 400) and Mooney M20R fit the bill. One way to compare aircraft is to look at how many accidents have occurred in relation to the number of aircraft built. The approximate accidents (from the NTSB database) per aircraft built are: Cirrus, 0.025; Bonanza, 0.087; Columbia, 0.005; Mooney, 0.037. While this data is interesting, it provides an incomplete picture.

What’s missing? First, each aircraft type has flown a different amount of total hours. Of each hour flown, individual operators may be conducting their flights under different scenarios, such as for business as opposed to for pleasure. Are certain airplanes more likely to encounter IMC? The aforementioned stats also neglect the fact that some aircraft have been taking to the air for many years while some are relatively new. It’s obvious that an older fleet is likely to cause pilots more problems than one coming off the factory line.

To make yet another point, how about we compare the “traveling” class with aircraft that don’t make a habit of flying coast-to-coast? The Piper PA28R, Cessna 182 and Diamond DA40 seem to fit this model. The approximate accidents per aircraft built are: Piper, 0.220; Cessna 182 (all years), 0.250; Cessna 182 (1997–2006), 0.026; Diamond, 0.009. I think these stats make a pretty good case that it’s critical to take such numbers with a grain of salt. The Piper and Cessna mentioned have had significantly higher exposure levels in terms of time and flight hours than the Diamond. Also note how the same Cessna model has different stats depending on the length of exposure (how many years they’ve been in service). So are Cessna 182s and Piper Arrows less safe than Diamond DA40s? Regardless of what those numbers look like, we can’t answer the question with the data on hand, i.e., we need more information.

So what does all this mean? Probably the most important point that can be taken away from this discussion is that airplanes are used differently; therefore, comparing one type to another or perhaps to generic GA statistics isn’t fair or useful practice. Certain airplanes give pilots the ability to fly at high altitudes, which increases the risk of unfavorable weather year-round. Considering that most GA pilots have little experience with high-altitude flight and its associated conditions, this type of exposure is of particular concern.

Many of the “traveling” aircraft allow pilots to fly at higher speeds. This requires increased situational awareness and planning, both of which have long been weak spots among pilots in general. Additionally, higher speeds increase the likelihood that the aircraft will actually be used to go places. And when pilots venture outside of the geography with which they’re most comfortable, they are exposed to unfamiliar weather systems and conditions. And don’t forget about the possibility of flying in and over exotic terrain, again creating a different kind of flight exposure.

And since “traveling” aircraft can actually cruise at decent speeds, they’re useful business and recreational travel tools. While this is usually a great thing, business meetings and vacation plans can often augment “get-there-itis” risks. Conceivably, “traveling” pilots could have a different type of philosophy on flying. Some may equate particular types of avionics or a particular horsepower to a certain level of invincibility. A pilot may be more likely to conduct a flight to a destination with lower minimums if he or she has a glass cockpit and autopilot. Or one may be more easily convinced to take a shortcut over mountainous terrain knowing that the aircraft’s service ceiling exceeds the highest peak. Truthfully, a close look at NTSB reports on accidents involving these aircraft types yields an interesting conclusion—pilots are still the primary cause of accidents.

Speaking of pilots, to make a fair statistical analysis, we must look at the types of pilots flying each make and model. According to Columbia aircraft, at least 80% of their customers fly more than 150 hours a year as compared to the 75% of all pilots who fly less than 150 hours per year. Another interesting stat is that 85% of Columbia owners have an instrument rating, but of pilots in general, only 43% can boast an instrument ticket. Do these facts have anything to do with Columbia’s excellent record? Probably. It just goes to show how important all the factors that are typically ignored in reports are key to making a reasonable determination about safety of flying and of individual aircraft.

Writer Rex Stout once said, “There are two kinds of statistics, the kind you look up and the kind you make up.” Hopefully this analysis has opened up your eyes so you can more readily spot suspicious data. So the next time someone claims that aircraft A is unsafe or flying under certain conditions is safe, take a hard look at the data. Was the comparison made between two peers under similar circumstances? Are there any factors that weren’t taken into consideration? If you can take just one thing away from this article, be sure that you don’t always believe the hype.

The post Deciphering Accident Statistics appeared first on Plane & Pilot Magazine.

The primary conflict about how lift is created centers on two white-wig-wearing historical figures—Sir Isaac Newton (1642-1727) and Dr. Daniel Bernoulli (1700-1782). Those who prefer Newton’s ideas (i.e., Newtonian lift) believe that air is forced downward behind the wing. Simultaneously, the wing is forced upward with the famous “equal and opposite reaction” described in Newton’s third law. Then there are those who favor Bernoulli’s celebrated principle that as airflow accelerates, the static pressure within the airflow drops. We all know that the air flows faster over the top of the wing than the bottom. Bernoulli’s equation therefore concludes that the wing gets “sucked” upward by the reduced pressure.

All right, so you’re tired of the guys at the hangar making fun of you for preferring one theory over another. Off you go to prove them wrong. You pull out the classic Stick and Rudder by Wolfgang Langewiesche and quickly find the statement “forget Bernoulli’s theorem.” Still unsatisfied, you reach for Aerodynamics for Naval Aviators. There you find nothing but Bernoulli and something called “circulation.” You find much of the same while inspecting Aerodynamics for Engineers by John Bertin and the more readable Illustrated Guide to Aerodynamics by Skip Smith. In fact, Newton isn’t even mentioned during the lift discussion in the latter book. To add more fuel to the fire, prestigious academic institutions seem to favor one theory over another. Portions of Harvard’s and Princeton’s Websites discuss Bernoulli as though his ideas are the only ones available.

Don’t feel sorry for Newton just yet—he has his group of supporters. The recently published Understanding Flight by David Anderson and Scott Eberhardt dedicates two pages to Bernoulli, mostly about how his theory doesn’t appropriately describe lift, followed by nearly 200 pages about Newton. The Pilot’s Handbook of Aeronautical Knowledge and Jeppesen’s Private Pilot Manual mention that both Newton and Bernoulli have to be considered to accurately define lift (frustratingly, though, an earlier edition of Jepp’s Instrument/Commercial Manual leaves Newton out of the picture entirely). Nonetheless, even among those who prefer Newton, there’s some disparity. Cranfield University, one of the U.K.’s leading aeronautical institutions, offers the suggestion that, contrary to popular belief, the wing pushes upward, providing the Newtonian action, while “the reaction to this would be for the wing to push the air down,” thus producing the requisite lift.

When I asked Dr. Sheila Widnall, professor of Aeronautics and Astronautics and Engineering Systems at MIT, about this argument, she responded, “It’s scary to think there might be controversy about this issue. This is the basis of all subsonic aircraft design.” Indeed, it is scary. Pilots at all levels are receiving conflicting information about how their favorite toys stay in the air. Equate this to some doctors thinking the heart works one way, while others believe it works in an opposing manner. Yikes!

If you’ve been standing on a soapbox supporting one theory over another, there’s no need to hide your face the next time you pass the flight-school water cooler. The truth is that, among the Newton-Bernoulli disputers, neither party is wrong. According to Dr. Jean-Jacques Chattot, professor of Mechanical and Aeronautical Engineering and director of the Center for Computational Fluid Dynamics at the University of California-Davis, the descriptions of lift advocated by Newton and Bernoulli “are actually the same thing, just from two different perspectives.” How is this possible? Take another look at the dates when Newton and Bernoulli lived.

Newton presented his laws in a publication released in 1687 (good luck reading the original as it was in Latin). Bernoulli was an avid science and mathematics scholar who followed the works of Newton. Not surprisingly, Bernoulli’s equation is actually derived from Newton’s laws. Bernoulli and Newton each correctly describe lift, but use divergent methods.

According to NASA, lift can be calculated by “adding up the pressure variation” as found by Bernoulli’s equation to “[determine] the aerodynamic force on the body.” At the same time, one could verify the value of lift by adding together the “net turning of the gas flow…from Newton’s third law of motion, a turning action of the flow will result in a reaction [aerodynamic force].” Indeed, according to Aerodynamics for Naval Aviators, to get the airflow necessary to produce Newtonian lift, an airfoil must be “able to create circulation in the airstream and develop the lifting pressure distribution on the surface,” which, of course, is described by Bernoulli.

But if both Newton and Bernoulli are correct, how did so many pilots get so off track? Well, you can pin some of the blame on educators and writers for their attempts to take something that’s rather complex (it is aeronautical engineering, after all) and spin it into something comprehensible to laypersons who aren’t calculus experts. As lift is watered down from the pocket-protector level to that of the average pilot, some things get lost or skewed in the translation. Thankfully, Newton’s laws are fairly straightforward and less confusing.

Conversely, poor Bernoulli’s concept tends to be butchered on a regular basis. Something that’s expounded in many aviation classrooms is the idea of “equal transit time,” which is an analysis of what occurs if two air molecules travel across a wing, one going over the top while the other slides along the bottom. The particle cruising over the curved upper portion of the wing has to travel farther, thus it must move faster to rendezvous with the particle from the bottom of the wing. This contention further states that the two air molecules must simultaneously rejoin one another at the trailing edge of the wing. But nothing could be further from the truth. According to significant wind-tunnel data (see Aerodynamics for Naval Aviators, www.av8n.com/how/htm/airfoils.html and www.allstar.fiu.edu/aero/airflylvl3.htm), the air molecules never meet again because the air over the top of the wing accelerates much faster than most people think.

Another common butchering of Bernoulli’s principle is the concept that a wing is a half of a Venturi tube. It’s true that Bernoulli’s equation accurately describes the pressure fluctuations that occur within a Venturi. However, no matter how you slice it, a wing simply isn’t a half of a Venturi! There’s always a tremendous amount of focus on the upper portion of the wing, but the lower surface also contributes to lift. Depending on the angle of attack, portions of the lower surface of the wing may also generate negative pressures. That, however, is hardly mentioned in most discussions of Bernoulli. Lift only occurs if there’s a positive net pressure difference between the bottom and top of the wing (commonly referred to as “high pressure on the bottom, low pressure on the top”).

Another widely held misconception about Bernoulli asserts that “fast-moving air results in low pressure.” Yet this isn’t always the case. Think about how fast air passes by the static port on the side of the fuselage. Why does it still give accurate static information as you accelerate down the runway? The reasons are somewhat complicated. Bernoulli’s equation requires you to compare “apples to apples and oranges to oranges.” The equation is actually an analysis of the total energy within a particular airflow. Therefore the total energy of air at rest (apples) isn’t equal to that within a 100-knot airflow (oranges). Actually, if you’re cruising at 100 knots, low pressure will only occur at locations where air is accelerated to a velocity higher than that of free stream flow (100 knots).

Now you’re probably thinking, “Great, they’re both right—there’s even more to learn!” Don’t get bent out of shape. Since both concepts are true, you can pick the one with which you’re most comfortable. Bernoulli’s theory tends to be harder to understand and is favored by mathematicians and engineers. Newton’s is palatable to pilots because it’s more intuitive. Regardless of which philosophy you prefer, the important conclusion is that you realize there’s more than one correct way to explain lift.

The post Bernoulli Or Newton: Who’s Right About Lift? appeared first on Plane & Pilot Magazine.