PISTON to TURBINE: When is it the Right Move? Part I

There’s definitely an allure to a turbine. Do they exude status? Are they really safer and smoother? Is the performance that much better? Or is it just another hill to conquer?

I’m a flight instructor in the PA46 world, and I get to talk to a lot of people who are ready to make their dream of burning jet fuel a reality. I love the PA46 market for many reasons, the biggest being the type of person who ends up gravitating to this special niche of aviation. It attracts those who would rather fly themselves and who also demand the best in avionics, performance and style. About 50 percent of the airplanes in the PA46 market are piston versions and 50 percent are turbine versions. So, I help a lot of people make the move from piston to turbine and hear all the reasons why they make the move. I also provide initial training to many turbine wannabes and often see what makes the move from piston to turbine a huge success or a dismal failure.

Why a Turbine?
Well, the answer is different for many people, but it usually emanates from a negative piston event. To realistically consider a turbine upgrade, you’ve got to have solid experience in the piston world. Most prospective pilots don’t even think about turbine aspirations until they have at least 500 hours in the logbook, and most have more than a thousand. I’ve seen some pilots move to a turbine in less than 500 hours, but in today’s extremely tight insurance market, most can’t make the move with such few hours and still be insured.

One of the really negative aspects of the PA46 market is that there are plenty of people who have more money than aviation experience. Simply put, they can buy more airplane than they can responsibly operate. Experience is important. Some prove it every year by causing a nose gear collapse on a turbine takeoff, crashing the airplane on landing by using beta/reverse improperly, or by just “being behind the airplane.”

So, most of the time, a turbine wannabe is going to have filled up the first logbook. And, with a filled logbook usually comes a harrowing piston story. Most newbies in aviation have an “it can’t happen to me” attitude when it comes to engine troubles. But, anyone with a filled logbook will probably have an inflight story about a valve spring breaking, a turbocharger blow up in flight, a seriously high EGT that scares them, or a complete engine failure. With that experience comes a freshly matured aviation perspective that is far more conducive to one day becoming an old pilot.

As for those big turbocharged pistons, are they as reliable as the moderately sized normally aspirated piston engines? I’ve got an IO-470 in my 1961 Bonanza, and I can operate that sweet engine very cool, very smooth, and once I go above 4,000 ft MSL, the engine is now running with less and less internal pressures. A Lycoming TSIO-540 in a Piper Mirage is working hard most of its life because it has turbochargers. During the climb, cruise, and even sometimes in descent, that engine is producing manifold pressures that are above 29.92 inches. I think less is asked of the normally aspirated engine, but turbine wannabes usually migrate from an airplane with a big turbocharged Continental or Lycoming engine starting with a “5,” meaning engines where a lot is being asked of them in flight. Anything with “TSIO” as the first letters of the engine data plate are really engines designed to create about 250 normally aspirated horsepower, but they then bolt on all sorts of devices to allow them to create much more horsepower. With big piston engines, more is asked, and asking more demands a more adept aviation management touch.

So, that pilot with two logbooks ends up with a vibration in the engine, an engine hiccup, or starts logging unwanted glider time, and they survive to tell the story. Then, I get a phone call. So has started the turbine dreams of many a pilot.

And let’s not forget about spouses. While many pilots believe they are the head of their household, the spouses are sometimes the neck, turning the head in whatever direction desired. The spouses don’t like any engine noises other than a hum. Did your engine hiccup? Just ask your spouse; they are aware. They are like a wonderful watchdog that will let you know when anything weird is approaching. They may not be able to tell you what the amber or red light means, but they know when that light normally comes on and normally doesn’t. So, when that noise occurs or the light comes on, the spouse is sometimes the one to say, “I think I’m okay with that turbine upgrade. It is safer, right?” Many a pilot has those turbine dreams answered because the spouse got onboard with the turbine plan.

How Important is a Good Engine?
You already know the answer: It is everything! Every single-engine pilot should be familiar with the mantra, “Put all your eggs into one basket and watch that basket.” Engine health is critical. But so is the type of engine. To explain this better, let’s take a step backward in time to illustrate a point.

I own a 1940 LP-65 Porterfield. It is a historical tandem trainer airplane built before WWII (which means I love it already. I love old airplanes) that is affectionately called “the skinny bird” because it is so narrow. It is a fine airplane in every sense of the word, except for one thing: It has an engine that is not reliable – a Lycoming O-145. This was Lycoming’s first horizontally opposed engine, one they made just after they stopped making sewing machines and automobile engines. It is a true historical engine, and it is not very reliable. There are more of these engines that are now the foundation for an FBO coffee table than are still pulling airplanes through the sky.

The O-145 is rated at 65 HP, but that is a stretch. There’s not a VSI in my Porterfield, but I bet the rate of climb is less than 200 fpm with two people on board. With one person, it probably climbs out at a whopping 300 fpm. It is just an atrociously underpowered airplane. If anything happened to the engine on takeoff or climb out that was minor, just something that would reduce the power a little such as loss of a cylinder, a broken valve spring, or carb ice, I seriously doubt the engine would get me back to the runway.

So, although I love the Porterfield, you fly the Porterfield differently than other airplanes. You are super-vigilant about knowing the wind; you listen to the engine carefully while still over the runway; you turn 30 degrees off runway centerline so you have a chance of making “the impossible turn” possible; you stay near the airport until you have enough altitude to return to the airport; and we never take the Porterfield more than 100 nm from my home airport. Simply put, you fly the Porterfield with a different mindset than you fly an airplane with a strong and reliable engine.

This is the same inverse shift in mindset that you gain when moving from a piston to a turbine. The turbine relaxes the mind from troubles related to loss of power, either a complete loss of power or a reduction in power. If you frequently fly over mountains, fly at night, fly low IFR, fly over water, or fly any other scenario where an engine failure could spell death or destruction, then the turbine has tremendous appeal. If I fly a North Atlantic crossing in a piston, I’ve got the immersion suit on up to my waist and I’m vigilantly scanning for trouble. If I’m in a turbine, I’m far less concerned. The shift in mindset is what many turbine wannabes want, and I don’t blame them.

Is the Turbine Really Better?
Yes! If I can afford to sit behind a PT6, I’m sitting behind a PT6. In every way that matters, the PT6 is a better engine. Strong statement? Yes. But, search the heart and mind of anyone that operates both piston and turbine and this truth will come out. If money were no object, you too would sit behind a turbine, assuming fuel availability and performance parameters are equal. If we are being honest, the turbine is better.

I mean, a piston engine is very old technology. The engines of today are not substantially different than they were 80 years ago. We still use magnetos, still have lead in our fuel, and still have catastrophic engine failures. Yes, there’s been some advancements in technology, some major ones. But, not many advancements in engines that develop more than 350 horsepower. So, if you dream of more performance and more reliability and you want to fly your own airplane and go far, you will have turbine dreams.

The horizontally opposed piston engines that dominate the owner-flown cross country market have a gob of parts that move in different directions, they have four separate strokes happening in the same small space every second, and have complex systems that are required to keep the engine developing more horsepower than originally intended. It is a testament to the fabulous engineering of these machines and the broad cadre of wonderful maintenance providers that keep these engines running that we don’t have more accidents than we have today. It really is old technology.

The turbine engine? They too are a wonderful technology, but it is much simpler. The turbine has fewer parts and those parts all move in a continuous direction. Things turn in a turbine engine, and things push and pull in a piston engine.

The key to a turbine is that the four stages of developing energy (intake, compression, power, and exhaust) happen in separate areas of the engine. So, the intake does nothing but intake, the compression section does nothing but compress air – you get the point. And the area where combustion occurs, the “hot section,” is where the power is generated. Because heat translates into wear with any engine, the hot section of a turbine gets lots of attention at maintenance events. “Nozzle inspections,” “borescopes of the hot section,” and “hot section inspections” are all new terms that replace “top overhaul,” “cylinder replacement,” “valve guide wear,” and “corrosion on the cam” that happen with a big piston engine. With the turbine conversion comes a whole new vocabulary, a whole new language that must be learned.

Whereas a piston engine in a typical owner-flown cross-country airplane is an engine designed for less horsepower, and then we ask more of that engine to increase the horsepower, most turbine engines in the owner-flown world are engines designed to create far more horsepower than is actually used in the owner flown application. Most turbine engines are “derated,” which means the torque is limited to a number that can be managed by the airframe. For example, the PT6-42A found in a Meridian is nearly identical to the engine found in a King Air 200, except that the Meridian is derated to 500HP and the King Air is derated to 750HP. The PT6-42 could develop more than 1,000HP if an airplane were strong enough to “let the horses run free.”

So, the PT6-42A in a Meridian is never really challenged at all. At most, it is developing only half the horsepower it could create. There’s a lot of margin built into a turbine. That makes me feel good about sitting behind a turbine.

Are Turbine Engines More Reliable?
Yes, with a huge caveat. As I’ve illustrated, the turbine engine certainly fails less than the piston. But the statistics of the turbine and piston world are not altogether different. Why? I think there are two reasons why: pilot experience and power rollback.

Engine failures in piston engines usually don’t cause fatal accidents. Most piston engine failures result in a controlled landing. Now, that doesn’t mean that the airplane flies again, but it does mean the pilot lives and gets to go flying again. Engine failures are not the nemesis of fatal accidents. The stall/spin scenario is the fatal scenario that every pilot should know to understand and avoid. An engine failure can certainly lead to a stall/spin accident if the pilot flubs the engine failure, but the engine failure was not the deadly event.

In the PA46 world, I’d like to report that the turbines have a much better fatal accident safety record than the piston versions, but that is not the case. It seems that the turbine pilot causes fatal accidents just as often as the piston pilot. But, it is not because of an engine failure, it is because of the pilot mishandling the airplane and causing a stall/spin accident.

The other event that causes accidents in the turbine world is the power rollback. In a turbine, there is the potential that the power can “roll back” to idle and the engine won’t respond to power lever movements. It seems that the turbine engine airplanes certainly have fewer engine failures, but they seem to have a lot of power rollbacks that are mishandled by the pilot.

A single-engine turbine pilot will always have a Manual Override (MOR) switch or lever to control the engine in a power rollback scenario, but it also seems that the MOR is either fumbled or not even used when the rollback occurs. Every turbine pilot, either in a single or multi-engine turbine, should fully understand the power rollback scenario. Embarrassingly for the turbine community and the CFI cadre that train the turbine community, it appears there is a serious lack of understanding of the power rollback.

So, is the turbine more reliable? Yes! But, the turbine pilot had better understand the power rollback because the rollback is a real threat and nullifies the huge reliability advantage of a turbine over a piston.

Stay tuned for Part II in the May issue with more insights and considerations surrounding turbine performance, operation and maintenance.