The accident occurred on a cloudy, cold February afternoon in 2021. The National Transportation Safety Board (NTSB) has released its final report, and it contains some clues to the pilot’s thinking. There are no surprising mechanical or meteorological findings. No unexpected revelations. Instead, it was as it initially appeared—a normally functioning airplane flown below the published approach minimums out of the clouds and into the ground.

Cessna 441s are workhorses—this one powered by two 715 hp turboprop engines—and they are popular with charter operators. This 1978 model Conquest II had two pilots in the cockpit. One was a professional 18,000-hour airline transport pilot (ATP), the other a 770-hour pilot with a commercial certificate who had recently retired. It’s unknown who was in what seat, or who was flying at the time of the accident. What we do know is the more experienced pilot had been thinking about the instrument approach at their home airport for hours.

At 9:24 a.m., the ATP-rated pilot called Leidos Flight Service for a weather briefing. The plan was to fly from Belvidere, Tennessee, to Bowman Field Airport (KLOU) in Louisville, Kentucky, on to Thomasville Regional Airport (KTVI) and then return. It was “severe clear” at the destination, but closer to home a cold front was passing overhead. Right away the briefer talked about possible icing, as conditions were conducive for ice to form on wings and propellers in a cloud layer aloft. The briefer said, “The only trials and tribulations you have this morning [are] going to be punching through that layer as quickly as possible, minimizing the time in the clouds.” Asked if he had anti-ice or deice equipment on the Cessna, the pilot replied, “Yep, uh-huh. But I don’t like to use it.” The briefer calculated the icing layer was about 3,000 feet thick, and the pilot wouldn’t be in it long if he climbed at a good rate. The forecast for hat afternoon was for improving weather.

When heading back to home base, the pilots found the weather had not cleared. When they started the approach, the ceiling was 800 feet overcast, visibility 9 sm, with the ground temperature right at freezing, light rime icing conditions in the clouds, and tops of the clouds at about 4,000 feet. But for a Cessna 441, that’s well above the minimums published on the RNAV GPS RWY 36 straight-in approach of 400 feet and 1¼ sm. The final approach track has several altitudes, crossing the fixes at YOKUS at 4,000 feet, and WETSO at 3,000 feet, and with the LNAV/VNAV minimum altitude of 1,367 feet. The runway elevation is 979 feet.

The Cessna correctly crossed YOKUS at 4,000 feet and started a descent. It did not stop as prescribed at 3,000 feet but continued gently descending. At 2,300 feet, the radar data ends, at 2,100 the ADS-B data ends. The airplane hit trees close to the WETSO intersection at an elevation of 1,880 feet. It rolled inverted, hit the ground, and caught fire.

There was no distress call, and no medical or other unusual factors. The NTSB concluded the probable cause to be “the pilot’s failure to follow the published instrument approach procedure by prematurely descending the airplane below the final approach fix altitude to fly under the low ceiling conditions, which resulted in controlled flight into terrain.” It added, “the pilot likely attempted to fly the airplane under the weather to visually acquire the runway.” This might not be as rare as we’d like to think. While staying at published altitudes is a basic safety rule for instrument flying, a 2020 Embry-Riddle Aeronautical University peer-reviewed research study found compliance approaching the runway to be remarkably poor.

In fact, 96.4 percent of the 114 pilots descended below their stated personal minimums on a simulated ILS approach by an average of 303 feet. And 81.5 percent descended below the published federal minimums (by an average of 43 feet). The researchers noted, “These values are highly concerning.” The authors concluded that “pilots are knowingly or unknowingly accepting additional risk during a very critical phase of flight… A simulated (i.e., cash bonus) manipulation designed to mimic external pressures had no effect on pilots’ lowest altitude flown.”

The accident pilot had a possible motivation to descend below instrument altitudes. It’s not discussed by the NTSB, but this incident mirrors a fatal airline accident from December 1, 1993, at what is now called Range Regional Airport (KHIB) in Hibbing, Minnesota. A 19-seat twin-turboprop was on the localizer back course approach to Runway 13. Like other similar aircraft, the Jetstream 3100 was susceptible to tailplane icing. So a technique had evolved among line pilots to minimize their exposure to icing conditions. The NTSB report said the pilot’s “probable intention was to descend at higher than normal rates of speed to minimize the time in icing conditions.”

The Jetstream crew started the approach a little high, above the clouds, and descended at 2,200 feet per minute once on the final course. This high rate of descent inside the final approach fix was against written company procedures, partly because, when leveling out, it leaves little time or space for correcting errors. The airplane quickly descended below the minimum altitude and crashed into woods 4 miles from the airport—at about the same relative runway position as the Cessna 441.

In both accidents, the pilots were trying to manage the threat of airframe icing in the clouds with anti-ice or deice equipment they didn’t completely trust. They were trying to fly safely. And while minimizing time spent in cold, wet clouds is a valid general strategy, rapid descents inside the final approach fix is a dangerous practice. In both cases, no actual airframe icing was observed by investigators.

In trying to avoid icing, the pilots ignored basic instrument flying rules. Good pilots work hard to minimize threats, but sometimes risk management can be like holding too tight to a balloon. Push hard enough in one place, and it blows out somewhere else.

Editor’s Note: This story originally appeared in the October 2023 issue of Plane & Pilot magazine. 

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Nevertheless, the common vernacular is VFR or IFR, so we’ll continue in that vein. Even so, there are degrees of operational difficulty that require modifying the terms. To define simple VFR, we generally regard a cloud ceiling of at least 3,000 feet above ground level, or flight visibility of 5 nautical miles or more, to present little concern for control or navigation. If either of those parameters has a lesser value, the weather is termed “marginal VFR,” as long as it isn’t below what’s stipulated to require adherence to instrument flight rules. Operating in MVFR is a cause for concern, as one may encounter pockets of IFR weather hiding in the murkiness. 

Going further down the scale, IFR conditions are generally regarded as a ceiling of less than 1,000 feet or visibility below 3 statute miles as reported on the ground, which is pretty challenging stuff. Even where it’s legal, visual flight in such conditions is risky at best, capable of turning deadly within minutes. Special training, extra equipment, and adherence to specific procedures are the only way to survive what is essentially “blind flying.” 

And then there’s “low IFR,” or LIFR, denoting really bad weather of less than 500 feet of ceiling or under a mile of visibility. This winter, the Central U.S. experienced a week of widespread conditions with less than a half-mile of visibility and less than 200 feet of ceiling, barely adequate for the sharpest airline crews to operate. Coastal or river valley airports often report low-IFR situations when wide-open VFR prevails a relatively few miles away.

Occasionally, “special VFR,” or SVFR, operations allow a visual-flight alternative to IFR flying, a means of dealing with a local ceiling or visibility that’s just slightly below the 1,000 feet and 3 miles required for controlled airspace. If cleared for a SVFR entry or departure, the pilot will be told to “maintain special VFR,” meaning to stay clear of clouds and keep no less than 1 mile of visibility, effectively turning the Class E or D airspace into Class G. It’s still VFR, but barely.

Unfortunately, reported weather doesn’t always match the actual conditions, making neat categorizations difficult to recognize. On one such day, when a neighboring automated weather station reported 10 miles of visibility and 1,100 feet of ceiling, I sallied forth to relieve ground-bound boredom. The local ceiling turned out to be 700 feet, making for a short flight around the pattern back to the safety of the hangar. As a wise old pilot once reminded me, “the weather is what it is, not what it’s supposed to be.”

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The pilot asking was experienced enough to plan a trip thoroughly but was still learning with every new encounter. Because he wasn’t sure of the significance of “WS020/32020KT” in the Terminal Aerodrome Forecast, he wanted me to give him an up-or-down judgment about going.

Ordinarily, I avoid playing dispatcher when I haven’t been “plugged in” to the weather situation. But I did know a front was moving in, and the time of day was conducive to temperature inversion. Those two factors logically gave rise to the likelihood of low-level wind shear, so I spoke to that effect on his trip. He opted not to go out of an abundance of caution.

For light aircraft, a wind shear note in a TAF is not necessarily a hazardous warning, but a flag that means something is out there, requiring us to be ready to deal with it. It shouldn’t be confused with wind shear alerts generated by LLWAS systems at major airports, which indicate serious, real-time threats. Airliners are maxed out on spare performance capability when taking off and landing, so a loss of airspeed due to a wind shear encounter can be life-threatening.

Piston-engine airplanes flying through changing wind conditions can cope with turbulence and airspeed fluctuations using throttle jockeying and pitch-attitude adjustments. That’s why we fly our approaches at 1.3 VS0 as a speed reference, with half the reported gust spread tacked on for safety, and we’ll lift off and climb out faster in rough air.

So, what’s the issue with those “WS” forecasts? They are telling us there’s a possibility of a change into the noted wind direction and speed at or below the height shown–2000 feet AGL in the opening paragraph. It can be from a frontal passage when the surface wind shifts and the low-level atmosphere gets stirred up by the change. You may take off to the south, then suddenly find yourself battling a northerly wind as you climb out on course. Hopefully, you’ve considered this wind change in your fuel planning.

Any turbulence encountered in the wind shear will generally be short-lived unless your flight profile calls for staying low or there’s perpendicular-oriented terrain upstream. As you climb out, your GPS track will notably shift, requiring a heading change, but the ride will smooth up. In the absence of hills, the rough air comes from the point at which two layers of air are moving in different directions, giving us some piloting to do as we climb or descend through the friction level. 

We used to hear of “low-level jet streams” encountered on clear, cool nights with calm air at the surface. We would find them during climbout, the airplane suddenly bucking and bouncing and then calming down as altitude increased, but with the ground lights moving sideways. I’ve seen 40 knots of wind at 3000 feet AGL, as the cold surface air layer provided a slick cushion for the flow of warm air aloft moving across it. Once up into the inversion, there would be no turbulence, just a massive heading correction.

Therefore, we need to pay attention to the TAF wind-shear notation because it means changes are afoot, but in the absence of other indications, it’s not a reason to scrub a trip. Just be prepared.

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Smoke lowers visibility, not only at the surface, but aloft as well. It is not unusual for smoke to lower flight and surface visibility to less than 1 statute mile, making flying VFR impossible and dangerous, especially at night and in mountainous terrain. Even under IFR, visibility may be in the low IFR flight category and below published minimums for some airports. In fact, the FAA implemented a ground stop for flights bound for New York’s LaGuardia airport because of smoke and reduced visibility on Wednesday.

So, what’s a pilot to do? If you don’t have to fly, that’s likely the best option. If you do decide to make the flight, it’s best to be on an instrument flight plan. Also, if you have oxygen on board, consider using it even below 10,000 feet. In fact, wearing an oxygen mask is always a good approach.

All smoke contains some carbon monoxide, carbon dioxide, and other particulate matter. Hemoglobin bonds with carbon monoxide 200 times more readily than it bonds with oxygen, and often produces hypemic hypoxia. Depending on what is actually burning at the surface, smoke can contain a variety of different chemicals, including aldehydes, acid gasses, sulfur dioxide, nitrogen oxides, polycyclic aromatic hydrocarbons (PAHs), benzene, toluene, styrene, metals, and dioxins. None of these are good to breathe, especially if you have health issues (also consider your passenger’s health).

The smoke starts as eddies in the planetary boundary layer (PBL). This is the layer of air that is directly influenced by the earth’s surface. But then some of that air gets mixed above the PBL into the free atmosphere, and encounters stronger horizontal winds. Smoke from these fires can travel thousands of miles. In fact, some of the smoke from the Canadian fires has reached as far south as the Carolinas and northern Georgia, albeit in low concentrations of particles.

In the early morning hours, the atmosphere around the regions where the fires are burning is often fairly stable near the surface. That will trap some of the smoke, keeping it close to the surface. The fires burn so hot that they often produce convective updrafts along with “clouds” called pyrocumulus, pyrocumulus congestus flammagenitus, and cumulonimbus flammagenitus (with lightning) that carry the smoke high into the flight levels. These “clouds” don’t produce any rain, but those updrafts can contain severe turbulence that manifests as strong surface winds, which can exacerbate a large conflagration.

However, later in the day as the sun starts to heat the ground, that smoky air higher up moves downwind and it gets mixed down to the surface via turbulent mixing that occurs in the prime heating of the day. And that’s how you get some of that smoke to show back up near the surface at distances hundreds of miles from the origins of the fires.

Look for FU in the terminal aerodrome forecast (TAF) or in METARs. FU is an abbreviation for smoke from the French word fumée, as you can see below from this TAF for the Syracuse Hancock International Airport (KSYR). Notice the visibility is forecast to be as low as 3/4-statute miles in the early afternoon. This is in the low IFR flight category.

In fact, Syracuse was reporting 1/2-statute mile visibility earlier in the morning, as shown below. The bad news is that obscurations, such as dust, smoke, and blowing sand are not automatically reported by ASOS or AWOS. At airports with a trained observer, they can augment the observation by adding FU to the METAR, as occurred at Syracuse.

KSYR 071354Z 29010KT 1/2SM R28/P6000FT FU BKN027 OVC150 15/04 A2969 RMK AO2 SLP048 FU OVC150 T01500044

What about using model output statistics or MOS? These forecasts are made available in some of the popular heavyweight apps. Here’s more bad news. Unfortunately, MOS doesn’t account for smoke in the visibility forecast. The TAF is much more reliable when smoke is expected in the terminal area. Those are issued by highly trained forecasters who can account for the effects of smoke. The Localized Aviation MOS Program (LAMP) does advect observational data such that in the first few forecast hours, you will see the LAMP pick up on lowered visibility reported at airports, but beyond those few hours, it will quickly tend to discount the effects of smoke since it doesn’t really have data to support this phenomenon. Certainly, this area of research will eventually integrate smoke into the MOS forecasts in some future release.

I like to use the smoke forecast from the High Resolution Rapid Refresh (HRRR) model, which you can find here. HRRR-Smoke uses infrared (IR) satellite data to start. We know that fires create heat anomalies and that will show up nicely on IR satellite data. So, it’s not just about the smoke. Using this information means that the model is determining where the source of the fires are located. Once that information is known, it relies on changes in temperature, wind, water vapor, and precipitation to predict where the smoke will eventually end up in the atmosphere. Keep in mind that this is an experimental forecast.

The HRRR-Smoke is refreshed hourly and produces a forecast out to 18 hours, but for runtime hours divisible by six (00Z, 06Z, 12Z, and 18Z), it can provide a forecast with lead times out to 48 hours. There are four different loops that can be used to include near-surface smoke, 1,000-foot agl smoke, 6,000-foot agl smoke, and vertically integrated smoke. It’s a good idea to look at all four.

For departing or approaching an airport, the biggest concern is the conditions that might occur when landing or taking off. Near-surface smoke gives you smoke concentrations at about 8 meters (26 feet) above the ground. As shown below, this is indicated on a pale-blue to deep-purple color scale at the bottom of the forecast map. As you might expect, the northeast, especially New York and Pennsylvania) is currently covered in a smoky haze—purple and red are really bad, while light blue indicates relatively low concentrations (measured in micrograms per cubic meter of air). You can see below that smoke near the surface has traveled as far south as South Carolina and Georgia with fairly high concentrations.

Instead of measuring smoke around 8 meters off the ground, vertically integrated smoke is modeling what a 25-kilometer-high column of air looks like over any given location. The best way to think of this is as the smoke you can see covering the entire sky vs. the smoke near the surface you can smell. As you can see below, smoke covers most of the country east of the Mississippi River. The scale is a bit different in the magnitude of the numbers, but warmer purple-red colors are still very bad, and the cooler pale blue colors represent much lower concentrations.

Even with these forecasts, on any given day it’s extremely difficult to get a sense of what flying conditions will be like at cruise altitude. How high do you need to climb to be on top of the smoke, assuming you can fly high enough to even get on top? There isn’t a good answer. Certainly, in regions where high concentrations exist using the vertically integrated smoke forecast, expect smoke from the surface well up into the flight levels. In other regions, smoke can often top out at 15,000 feet or so, with higher concentrations below. It just depends on the current conditions, including wind direction and atmospheric stability.

The best strategy is to look for pilot weather reports that often will point out where the top of the smoke is located. As with all pilot reports, when the smoke is really bad, pilots will often avoid flying through the area. If you do happen to fly on a smoky day, please take a few minutes to document what you experienced and submit a report.

How long will this be with us? That depends on two factors. First, can most of these fires be contained? This largely depends on Mother Nature; without ample rainfall, honestly, there’s little hope. The second factor depends on how long the stagnant air will remain in place. Given the blocking omega weather pattern that has set up across the entire U.S. and southern Canada over the last few weeks, it might be with us for a couple more weeks.

Editor’s note: This story originally appeared on flyingmag.com

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If you plan an altitude where the temperature aloft is zero degrees Celsius or less, airframe icing becomes exceedingly more likely while flying in visible moisture. Therefore, the freezing level is one key variable that you need to determine during your preflight analysis to better quantify your risk of airframe ice.

Let’s clarify something right from the beginning. The FAA likes to use the term “freezing level” in all of its documentation. This is kind of a misnomer, given that water in the liquid state doesn’t necessarily freeze just because the static air temperature is below freezing. We must be concerned about the presence of supercooled water, which leads to airframe icing. On the contrary, water in the solid state (i.e., snow) must melt even if the static air temperature is a hair warmer than zero degrees Celsius. Meteorologists prefer to use the more accurate term of “melting level.” But pilots are stuck with “freezing level” for the foreseeable future.

A Rarely Standard Lapse Rate

One approach that some instructors teach is to use the standard lapse rate to calculate or estimate the lowest freezing level. That is, they use the current surface temperature at the airport and then subtract 2 degrees Celsius for every 1,000-foot gain in altitude. For example, if the surface temperature is 10 degrees Celsius and you are departing from an airport at sea level elevation, then the freezing level should be 5,000 feet. That method seems easy enough, but it’s a bad idea to do this. The standard lapse rate should only be applied to performance tables in the pilot operating handbook as a method to determine the departure from standard.

When Mother Nature is at her worst behavior, the atmosphere is rarely standard. In fact, during the late morning and afternoon, the environmental lapse rate is more often than not greater than standard near the surface. This is the layer of air that is directly influenced by the presence of the earth’s surface and, therefore, is what meteorologists refer to as the planetary boundary layer (PBL). The lapse rate in the PBL is often closer to the dry adiabatic rate (DALR) of 3 degrees Celsius for every 1,000-foot gain in altitude. Under these more typical conditions, using the standard lapse rate will cause you to calculate a freezing level that is higher than the actual lapse rate suggests. Therefore, if you expected the freezing level to be at 5,000 feet, you might be surprised during your climb to encounter supercooled liquid water beginning at 3,500 feet instead of 5,000 feet.

You might say, “That’s crazy—no sane pilot would do this.” Well, even the FAA fell into this line of thinking. In 2005, a student and a flight instructor ended up with a hard landing at Paine Field (KPAE) in Everett, Washington, after accreting airframe ice in a Cessna 172. They departed a nearby airport, Boeing Field (KBFI), to go out and shoot a few practice approaches in actual instrument conditions. This is certainly a noble effort when it is safe to do so. After the first missed approach, the instructor noticed ice accreting on the airframe and directed the student to return to Paine Field to land. During this landing, the aircraft ran off the runway, which resulted in an accident and subsequent FAA and National Transportation Safety Board (NTSB) investigation.

The FAA later determined the instructor, who was pilot in command, busted FAR 91.9 (a), which prohibits pilots from operating an aircraft without complying with its operating limitations. In this case, there’s a placard in the aircraft that states: “Flight into known icing conditions is prohibited.” The instructor was also cited with careless or reckless operation under FAR 91.13 (a). So, a certificate action was taken that included a 90-day suspension. The instructor requested an evidentiary hearing and later appealed the ruling to the NTSB. However, the NTSB agreed with the case the FAA presented.

I certainly don’t take issue with the outcome of this judgment, but one thing struck me as being a bit strange. The FAA argued that the instructor should have been aware of the lowest freezing level and, therefore, the potential for airframe ice at higher altitudes. To my chagrin, they suggested the instructor should have been aware of the surface temperature at the airport of 2 degrees Celsius—based on their briefing prior to departure—and then, they should have used the standard lapse rate to determine that the temperature would be at or below freezing in the clouds aloft. Ugh! Well, this just happened to be convenient for the FAA since the lapse rate near the surface was close to standard on that day and time. Therefore, it worked out, coincidentally, to favor the FAA’s case against this instructor. The lapse rate, as stated previously, varies and may not be the accurate measure to use when figuring the altitudes where icing might be likely.

Temperature Inversions

At the other extreme is a common situation when there is a formidable surface-based temperature inversion. In this situation, the surface temperature can be a chilly 7 degrees Celsius (45 degrees Fahrenheit) with a freezing level at more than 12,000 feet msl. This is quite common in regions around a warm front. As warm air overruns cold air at the surface, this creates a negative lapse rate (called an inversion) or a scenario where the temperature increases along with altitude before it begins to resume a more normal positive lapse rate in the free atmosphere aloft. When such an inversion exists, using the standard lapse rate may leave you with the impression that the freezing level is quite low when, in fact, it might be a very reasonable day to fly from an icing perspective if you remain below 12,000 feet.

In this scenario, it is quite common when an aviation accident occurs for the casual observer to quickly conclude that the aircraft encountered icing conditions. If it’s that chilly at the surface, then the freezing level must be just a few thousand feet up, right? That’s what pilots generally thought when a Beechcraft B58 Baron went down after departing the Spirit of St. Louis Airport (KSUS) on the winter evening of January 8, 2022. The flight departed at 7:10 p.m. CST headed westbound toward Denver. They were on an IFR flight plan and were cleared to climb to a cruise altitude of 8,000 feet. Shortly after reaching cruise, the Baron appeared to depart controlled flight with a rapid descent under unknown circumstances. It subsequently impacted the terrain 2.5 miles south of New Melle, Missouri (12 miles west of the Spirit of St. Louis Airport), killing the pilot and another occupant.

The temperature at the surface was 7 degrees Celsius with a dew point temperature of 6 degrees Celsius. There was much speculation and debate within the internet aviation community that airframe icing may have played a role in this fatal accident. This is certainly understandable. The elevation of KSUS is 463 feet msl, and using a standard lapse rate, the altitude of the lowest freezing level should be approximately 4,000 feet as shown here.

1,463 feet –> +5 degrees Celsius
2,463 feet –> +3 degrees Celsius
3,463 feet –> +1 degrees Celsius
4,463 feet –> -1 degree Celsius

Using the standard lapse rate in this way leads to an incorrect freezing level. This is echoed in the one-hour forecast (above), which suggests the lowest freezing level west of the Spirit of St. Louis Airport was between 11,000 feet and 13,000 feet. Using the standard lapse rate instead of the low freezing level forecast cre-ates an error of 7,000 to 9,000 feet in this case.

Shortly after the accident, the NTSB was quick to point out in a press conference that the freezing level was 12,000 feet and icing was unlikely since the aircraft remained below this level. But that didn’t make the internet community all that happy. Some still concluded that the NTSB was premature in its comments and that the causal factor would ultimately be associated with airframe icing, citing the standard lapse rate in their argument.

In fact, one YouTube personality suggested this flight likely encountered freezing drizzle, although no precipitation was reported at KSUS at the time of the accident. The NTSB will release its findings very soon, but the temperature profile on that evening included a healthy surface-based inversion and clearly the standard lapse rate would lead to a much lower freezing level.

Warm Air Overrun

With a little weather forensics, it was easy to discover that this was the classic case of warm air overrunning cold air—the result of the northerly movement of a warm front through the accident area as shown above. In fact, the temperature at approximately 5,000 feet msl was 9 degrees Celsius as shown below. It is not possible to accrete ice at those static air temperatures. Yes, there was freezing rain reported at the surface about 100 nm to the north-northeast at the Abraham Lincoln Capital Airport (KSPI) in Springfield, Illinois, where it was much colder, and the surface temperature was a chilly 1 degree Celsius.

Even so, there was a massive temperature inversion aloft over Springfield such that the temperature at 3,100 feet msl was 8 degrees Celsius. That’s an increase of 7 degrees Celsius at 2,500 feet above the surface. The temperature didn’t go negative over Springfield until roughly 9,000 feet msl. Whether in St. Louis or Springfield, this would have created a warm-soaked aircraft in the climb.

Where There’s Freezing Rain

In a freezing rain scenario, it is common to have two (or more) freezing levels. One way this occurs is in the presence of deep saturated conditions with cold cloud top temperatures. This allows ice crystal growth and creates snow which falls into a melting layer to create rain. These drops then fall into a subfreezing layer near the surface to create supercooled large droplet (SLD) icing called freezing rain. All of this is courtesy of a surface-based temperature inversion with multiple freezing levels aloft. This is the classical freezing rain temperature profile.

But there’s also a more common case where clouds aloft are dominated by liquid when the cloud top temperature is much warmer. In this non-classical case, there may be two or more freezing levels, or the entire temperature profile may be below zero degrees Celsius. In this non-classical case, the saturated layer has a depth usually less than about 10,000 feet. This places the temperature of the cloud top to be warmer than negative-12 degrees Celsius. Warm-topped precipitation events like this—even when the entire temperature profile is below freezing—are dominated by water in the liquid state and often produce drizzle-sized drops with little or no ice crystals that are needed to develop the growth of snowflakes. This kind of non-classical temperature profile produces most of the cases of freezing rain and freezing drizzle.

If you want to avoid making a bad judgment, understand the big weather picture and then use the lowest freezing level forecast, like the one depicted above, which is found on the Aviation Weather Center’s website. This includes an analysis along with hourly forecasts up to 18 hours from the time they are issued.

This forecast is automated but is updated hourly and is generated from the Rapid Refresh (RAP) numerical weather prediction model. The vertical resolution is quite reasonable at 2,000 feet.

The official freezing level forecast is found on the same website at aviationweather.gov/gairmet. This graphical AIRMET (G-AIRMET) forecast is issued by aviation meteorologists and depicts the freezing level at 4,000-foot intervals. It also indicates where multiple freezing levels may exist, including their height. Its spatial and temporal resolution is not as good as the automated forecast, however.

If you are a weather nerd, you might try learning how to use a Skew-T log (p) diagram like the one above. One of the most interactive websites for these is found at rucsoundings.noaa.gov. The Op40 input data source used here is the same RAP model that is used to produce the lowest freezing level chart forecast. With such a diagram, you can precisely pinpoint the forecast freezing level over a particular location at a particular time or determine for yourself if multiple freezing levels exist.

And if you are a bit lazy, you can use a vertical route profile or vertical cross section that depicts the freezing level along your proposed route of flight. Many of the heavyweight apps have such a depiction, including my progressive web app, EZWxBrief. With a profile view, as shown above, it’s painless to see how the freezing level changes across your route of flight so you can quickly compare this to your proposed altitude. In fact, in addition to lines of constant temperature that include the zero-degree isotherm, you can overlay other key elements, such as clouds, icing severity, or even turbulence.

From the December 2022/January 2023 Issue 933 of FLYING

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ForeFlight, a Boeing Company, has announced its acquisition of CloudAhoy, a debriefing software provider.

According to a post on the ForeFlight blog, the acquisition was completed in response to “customer desire for more integrated digital solutions.”

• READ MORE: Did You Know ForeFlight Did This?

ForeFlight, established in 2007, is one of the most widely used weather briefing and flight planning and management tools.

CloudAhoy, created in 2011, provides post-flight debriefing, analytics, and flight operations quality assurance software products. CloudAhoy allows pilots to digitally record their flight and play it back to review their performance. The software is particularly useful in the training environment where the emphasis is on meeting and exceeding the minimum standards for certification.

The details of the merger have not been announced.

For more information, visit the ForeFlight blog.

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