Several manufacturers have jumped into what is likely to be a lucrative general aviation market for these microturbines. While the concept of turbine engines remains consistent with all the players, subtle differences between the offerings are present. Before we get into some of the available microturbines, let’s look at what makes an engine a “turbine” and what differentiates the micros from what we know as the “standard” turbine engines widely used today.

The Basics

The truth is all turbine engines are relatively simple. This also adds to their reliability, as there are far fewer moving parts than in an internal combustion engine, and those parts are, unlike piston engines, rotary instead of reciprocating. Regardless of how they’re producing propulsion, whether it’s driving a shaft with a propeller (turboprop), powering the aircraft with a high-speed exhaust (turbojet), or running an internal fan (turbofan), they share a similar design.

All have a core that consists of a compressor—which does just as the name implies—plus a burner, which ignites aforementioned highly compressed and very hot air, and the turbine, which is the point of the first two parts. In the turboprop world, the hot exhaust gas that exits at the main turbine passes through an additional turbine before entering a nozzle. Most of the energy generated by this exhaust at the nozzle is then utilized to turn the secondary turbine. The secondary turbine is connected to a driveshaft and gearbox, which is then directly connected to a propeller, turboprop, or a turbofan.

• READ MORE: A 200-hp Turboprop for Homebuilts

Another benefit of turbine engines is the fuel that feeds their appetites (jet-A) is more readily available worldwide and typically at a lesser cost than its counterpart, avgas, which is short for “aviation gasoline” used in the piston world. In fact, turbine engines are like Mikey from the old Life Cereal TV commercials—they pretty much will consume anything within reason to satisfy their fueling hunger. With that in mind, in addition to jet-A, it’s expected that many of the microturbine engines will also be happy with other forms of kerosene, including over-the-road pump diesel fuel, especially in emergency situations. Manufacturers’ additional testing of these microturbines will ultimately determine what type of alternative fuels, if any, can be used on a permanent basis.

TurbAero

Australian company TurbAero is developing a small, lightweight 200-horsepower engine, which will initially target those experimental aircraft that utilize conventional opposed-piston engines, usually the Lycoming IO-320 or IO-360 varieties (think Van’s Aircraft RVs and the like). TurbAero’s design utilizes a process called “recuperative technology,” taking the air that normally would enter the combustion chamber—in this case at 200 degrees Celsius—and, utilizing a heat exchanger, preheats the air by extracting waste heat from the exhaust gasses—and then uses that to preheat the compressed air, thereby reducing the amount of raw fuel required.

This preheating, or recuperative technology, translates to an engine that can run more efficiently, burn less fuel, and operate with a good balance between performance, size, weight, and cost. A good balancing act, indeed. And, lest you think this is some newfangled technology, it’s actually a proven process that has been utilized on land-based vehicles, such as the M1 Abrams series tanks on the battlefield, with TurbAero resizing the recuperator, right-sizing it for the TA200TP 200-horsepower microturbine with the intent on bringing it to market sometime in 2025.

How did TurbAero get into the microturbine game? As is often the case, the company came about by pure happenstance when its CEO, David Limmer, was looking for a turboprop engine for his own personal experimental aircraft in 2008. Not able to find one, the next best solution was to build one of his own, completing it in 2015. It was during this time Limmer recognized the potential for a range of small turboprop engines for aviation applications, specifically the experimental/builder’s market. Three years later, with funding in place and partnering with his brother, Andrew, TurbAero was formed. After attending numerous aviation symposiums worldwide and talking with various kit airplane manufacturers and many hobbyists, TurbAero settled on development of the 200-horsepower TA200TP engine.

With additional refinements going into the fifth year of development, the company is poised to transition to the next phase, planned for the first quarter of 2024 and culminating in putting the engine through its paces on the test stand. Refinement will follow based on the results of the testing.

One of the current design refinements is optimizing the engine for operation and efficiency at lower altitudes. Like all turbines, these engines are most at home at the higher flight levels. However, TurbAero realizes that to be successful in the experimental market, it’s critical to provide a reasonable level of efficiency at the 10,000 to 12,000-foot range instead of the rarified air in the flight levels at which turbines are generally accustomed to operating. Considering the lower per-gallon price of jet fuel, along with the typically lower cost of maintenance compared to a reciprocating piston engine, and given the potential for higher reliability, the overall cost and benefit to operating the microturbine should likely fall within a range that most aviators in the experimental world would find justifiable, even with a planned price to market at $85,000. With the ultimate goal of bringing these microturbines to the certified market—in all likelihood, probably not for another five to 10 years—the question of overhaul recommendations comes to mind.

Most microturbine manufacturers are looking at TBOs of other certified turbines and seeking to replicate those in the marketplace, with the 3,000-hour TBO as the benchmark for future certification. In the meantime, since these turbines are designed for the experimental market, TBO is not a requirement now, although it’s likely these engine makers will look at safety and provide recommendations of their own long before their microturbine is spooling up in the certified world.

• READ MORE: TurbAero Small Turboprop at EAA AirVenture Oshkosh

In that regard, TurbAero is integrating a data acquisition function into the FADEC (full authority digital engine control) on their TA200TP engine. This purportedly will allow users of this engine to benefit from a health and usage monitoring system, designed to identify potential failure points before the failure occurs. It’s a grand prospect that could save maintenance costs at the very least and bent airframes at the very worst if it comes to fruition.

PBS Aerospace

Another turbine engine manufacturer, PBS Aerospace, brings its experience manufacturing propulsion systems for the defense sector, with expertise in tactical UAVs, target drones, and missiles, and enters the general aviation experimental market with a small, 43-pound micro turbojet engine that’s used in some SubSonex jets, as well as the BD-5J jet.

According to vice president of customer support Frank Jones, the flexibility of the company’s TJ100 turbine engine will allow builders to utilize various versions of it, with some having a fuel-oil mix that will allow straight-up rocket-style launching without skipping a beat.

While PBS suggests a 300-hour time between hot sections at the moment, as additional experience and time gets put into these engines, the manufacturer may increase its suggested time between hot sections. Again, in the experimental world, these are simply suggestions, albeit strong ones. However, because there are so few hours being put on these microturbines, manufacturers like PBS are being ultraconservative in recommendations for overhauling/hot sectioning their turbines.

At this time, PBS turbines in need of maintenance or hot sections require the engine to be returned to the company’s facility in the Czech Republic. However, as of this writing, since none of PBS’ TJ100 engines in the field have accumulated operational hours approaching the 300-hour mark, no overhauls have yet been necessary. Jones says the firm hopes to have facilities in the U.S. available for overhaul purposes when the time comes. In the meantime, as mentioned, other manufacturers are anticipating reaching similar recommended TBOs as their big brother counterparts, meaning 3,000-plus hours before the requisite hot section and overhaul. Pricing of the PBS TJ100 is in the $80,000 range, depending on accessories and other options.

Heron Engines

For those who don’t want to wait for the development of those microturbines still in the testing stage and earlier, there’s one manufacturer—Crete, Greece-based Heron Engines—with a 130-horsepower version ready for preorders. Although you may be sacrificing some available horsepower compared to others that are developing 200 hp versions, the Heron microturbines, weighing in at a svelte 82 pounds, with gearbox and developed after decades of work designing and building UAVs from founding member Alex Vrontoulakis’ father, are available for preorder today. Vrontoulakis is now partnered with Alex Fatseas and since 2018 they have been committed to bringing their microturbines to market, with the company promising to make them affordable (low $40,000s), easy to maintain, with projected overhaul costs in the $7,000 range.

Solar Turbines

You wouldn’t normally think a turbine engine used primarily as a helicopter auxiliary power unit (APU) or ground power generator would be a good fit as a primary source of propulsion. But in the case of the Solar T62, the manufacturer says, you would be wrong. This compact engine continues to see some use in the experimental helicopter world, where it can be found in kits made by Rotor X Aircraft Manufacturing (formerly RotorWay International) and Mosquito Aviation (the single-seat Mosquito homebuilt helicopter). However, you might need to look hard for one of these as they are few and far between, and mostly found on auction sites such as eBay. When you can find one on the market for sale, you can expect to shell out about $15,000 for the T62 turbine—and of course, we’re talking used.

Other Turbine Benefits

Because 100LL is becoming more scarce by the day internationally, industry experts predict per-gallon costs are going to continue their upward spiral. Having an engine that burns jet-A will assure more savings in the future, which will be an added benefit overall.

So, what will it take to enter the rarified air of the turbine world? Like many things in aviation, time and money. Time, since we’re in the infancy stage of microturbine development. And, of course, money, since the price of admission for experimental microturbines will fall somewhere within the $40,000 to $85,000 range and potentially go higher by the time they come to market.

With the advent of these microturbine engines, it’s quite possible we’re at the inception of a new phase in propulsion for experimental aircraft and beyond. While there are numerous obstacles to overcome, not the least of which is the cost of acquisition, obtaining efficient operation at lower altitudes and proving the microturbine design is sustainable in our ever-demanding world of aviation, there’s cautious optimism that in the not-so-distant future, we will all be hearing the unmistakable sweet sounds of turbines spooling up much more frequently at airports around the world. 

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

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ATC acknowledged the request, asked the pilot to report turning final for Runway 32, and asked if they needed any assistance. The reply—“No, sir. We should be fine. Give you a call turning final.”—was the last transmission. N13VT went down while turning onto final for Runway 32, killing both pilots.

Their journey began at 6:30 a.m. on February 16, 2021, leaving Appleton, Wisconsin (KATW). Their destination was Sebastian, Florida (KX26), for planned maintenance. The airplane had issues with its retractable landing gear, and was being flown on a FAA ferry permit. The permit required the aircraft operate with the landing gear extended at all times. In addition, the permit required a copilot for the flight even though the four-seat light piston normally didn’t require more than one pilot.

The Velocity V-Twin is a two-engine pusher canard. Built as a comfortable long-haul cruiser with an advertised range of 1,100 nautical miles, the experimental category fiberglass kitplane is a stunner. Outside, it looks like something from a James Bond movie—sort of a smaller version of the Beechcraft Starship. Entering through big gull-wing doors, inside it has European sports car styling with side-sticks and flat-panel displays.

To some the airplane looks backwards. On the front of the fuselage sit canards, small controllable wings that handle pitch control. Used in many remarkable aircraft, from the Wright Flyer to the Eurofighter Typhoon, canards can offer excellent control authority. In this case, they were designed to stall before the main wing, so at high angles of attack the nose automatically bobs down, always keeping the main wing flying. 

On the back of the airplane are the propellers. Pusher aircraft allow the wing to fly in clean, undisturbed air and offer pilots wonderful unobstructed views. We don’t see a lot of pusher aircraft because there remain issues with the propellers working in the more turbulent air behind the wing, as well as troubles with engine cooling. What is certainly an advantage with the V-Twin design is the closeness of the two engines to the aircraft centerline, reducing unwanted yaw in single-engine operations. The accident airplane was built in 2020, and other than the landing gear retraction mechanism issue, had no known mechanical discrepancies.

The pilots landed at the Southern Wisconsin Regional Airport (KJVL)—a tower-controlled field with three paved runways—for fuel. The pilots pulled into the Janesville Jet Center and asked to be fueled up with 100LL. The manager remembers nothing unusual—“chitchat mostly”—about their flight down to Florida. The National Transportation Safety Board (NTSB) found no issues with the 100LL pumped aboard. 

The weather was good in Janesville. It was winter, and for sure it felt cold, just 7 degrees Fahrenheit. But that’s nothing these two pilots, both in their mid-20s, who grew up in Wisconsin, hadn’t experienced before. The unlimited visibility, complete lack of precipitation, 5,000 foot cloud ceiling, and a light 9-knot wind out of the north would have been welcome VFR conditions. 

They taxied out and took off normally. So what was the engine problem? And why couldn’t they return to land on the remaining engine?

At about 1,000 feet above the field, one of the pilots radioed the tower about an issue and requested the return to land. Post-accident examination of the airplane revealed a chaffed wire on the left-engine oil pressure sender. The NTSB report states that based on “ATC communication, the engine teardown, recovered MFD data, and POH rate-of-climb data, it appears that the flight crew may have shut down the left engine seconds after their radio call as a precautionary measure.” The damaged wiring harness caused the cockpit display to show a red “X” where the left engine oil pressure value would normally be. The left throttle, propeller, and mixture controls were found in their aft (shutdown) positions.

So far, so good—a precautionary engine shutdown and immediate return to land. The plane continued to climb but at a slower rate constant with single-engine performance. They were now heading approximately south on a modified left downwind for Runway 32. The flying pilot further pitched down to a level flight attitude. Their indicated airspeed increased to about 16 knots above the maximum for flight with the landing gear extended (VLE is 140 knots for this Velocity V-Twin).

One minute and 10 seconds later, part of the right main gear door came off the airframe and struck the right propeller—an immediate traumatic event. All three blades separated about 18 inches outboard from the propeller hub, creating a total loss of right engine power. Immediately, their altitude and airspeed started to decrease—slowly, steadily, and irrevocably. The NTSB performance analysis dryly notes that from here: “with both engines inoperative, N13VT likely did not have the energy required to glide back to the airport.”

As the airplane drifted lower, recovered onboard avionics data showed a rising angle of attack, followed by increasingly frantic gyrations in pitch and roll. Through the tower window, the controller saw the airplane descend beyond trees southeast of the airport. It was in a left bank that started to tighten. As this happened, the controller observed the airplane’s nose “almost pointed down toward the ground.”

Walking his dog southeast of the airport, the last eyewitness to see N13VT aloft described the flight path “as similar to something that would be seen from a crop duster popping up over a field” with an engine “chopping at the air and working hard.” A few seconds later, it disappeared behind trees and crashed. The airplane came to rest inverted in a 3-foot-deep tributary of the Rock River, about a mile south of KJVL. There was substantial damage to both wings,  canards, and the fuselage. The aircraft was discovered upside down, mostly underwater, with its two main landing gear legs sticking up in the air. One had the gear door plate attached; the other didn’t. The pilots were found dead in the wreckage.

Their cause of death was officially reported as drowning and hypothermia, with complicating blunt-force injuries to the head. 

The NTSB found no evidence of preexisting mechanical malfunctions or anomalies that would have precluded normal operation of the engines. It was right after takeoff that the problems began.

To the pilots, this must have seemed like a nightmare worse than any simulator session. Soon after feathering the left prop and shutting down the left engine for an oil pressure problem, the right engine suddenly—violently—quit. Too far and too low to glide back to the airport, they would lose control and crash three minutes later.

Maybe a single-engine mindset would have saved them. In a single-engine airplane (or, of course, a glider) we must always be mentally ready to set down within gliding distance. Keep control and fly the airplane to the best landing spot on the best terrain presented. But multiengine pilots are usually more like systems managers, trained to operate on the remaining good engine to get to a suitable airport.

I thought it odd that only a 16-knot overspeed would break off bits of the landing gear, but the NTSB explained that mystery in its examination of the previous flight’s data. From Appleton to Janesville, the pilots flew the Velocity V-Twin well above the VLE speed of 140 knots for operation with the gear extended. In cruise, they maintained between 170 and 180 knots. Starting the descent, they reached 190, a full 50 knots above the listed maximum speed. The NTSB noted this may have weakened the gear door attachment points. And then, when single engine, the higher-than-normal sideslip angles may have helped force the door off the landing gear legs.

Two lessons are obvious from this crash, despite its crazy one-in-a-million double-engine failure: Don’t exceed aircraft limitations, and be prepared to land off-airport. 

Plus, there’s a third lesson.

Do we have to shut down an engine when the gauges show a big red “X”? An engine fire always requires a full shutdown. But if a powerplant seems to be running OK, might we be better off in some conditions letting it produce thrust for as long as possible? I fly a two-engine airliner across the North Atlantic every week, and when we’re two and a half hours away from the nearest suitable airport, it will take more than a missing oil pressure indication to shut down an engine.

For light aircraft, there have long been debates about whether two engines are really safer than one, the efficiency of pusher props, and the effectiveness of canards. This just-released NTSB report doesn’t resolve any of those disputes. But it does reinforce the reality that while a canard might stop you from stalling, it won’t keep you from crashing—and two engines won’t prevent you from losing all power.

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