The devil is in the details…
We always dreamed of a turbine-powered pressurized Bonanza that would fly for about the same cost as a piston-powered Bonanza. Never happened but there are two single-engine turboprop “personal” airplanes out there right now and if you want to know how the price compares with a Bonanza, it doesn’t. Turboprops are pricey, even with one engine. I’m talking about the Socata TBM airplanes and the Piper PA46-500TP Meridian, the latest version of which is known as the M600. Some would put the Pilatus PC-12 in there as well, but it is a much bigger airplane and is more widely used in utility, medical, charter and airline flying than by affluent couples flying from up north to Naples, Florida.
One of the lures of the personal turboprop single is that it doesn’t require an FAA type rating to fly. (Turboprop twins don’t either until they top 12,500 pounds MTOW). Insurance companies will require initial and probably recurrent training but that is likely less demanding than type rating training though the schools would probably dispute that fact.
The turboprop singles are easy enough to fly but there is a lot more to operating them than, for example, a Bonanza. I’ll give you an example of why this is true.
A commercial-rated pilot and his wife were cruising along at FL280 in their new TBM 900, en route from Rochester, New York, to Naples, Florida. The pilot was no novice with 7,100 total hours and 4,190 hours in TBMs over the last 20 years. He had recently completed a five-day transition course to the 900 that included 36 hours of training. The pilot also led the TBM Owners and Pilots Association which has excellent safety programs. To say that he was familiar with the airplane would be an understatement.
About an hour and 40 minutes into the flight, the pilot told the controller of an abnormal indication and requested a descent to FL180. The controller cleared the pilot to FL250 and told him to turn left 30 degrees. The pilot acknowledged and again requested a lower altitude.
The pilot didn’t request emergency handling nor did he specify the nature of this problem. The controller, however, deduced that he had a pressurization problem and immediately coordinated with another facility to clear nearby lower airspace. The controller was next able to clear the TBM down to FL200, with another heading change, but the pilot did not acknowledge this or any future transmissions.
The airplane remained level at FL250 and about 30 minutes later military airplanes were dispatched to intercept and examine the airplane. Everything looked normal except for the fact that the occupants appeared to be unconscious or asleep and neither was wearing an oxygen mask. A photo showed that the appearance of the emergency door fit into the fuselage was consistent with a depressurized cabin.
The military airplanes stayed with the TBM until it neared Cuban airspace at which time they broke off the escort. The airplane remained at FL250 and on track until five hours and 48 minutes had elapsed since takeoff. At that time the fuel was obviously all gone and the airplane descended into the Caribbean Sea, north of Jamaica.
In the probable cause finding the NTSB faulted the design of the cabin pressurization system and the design of the checklist and nothing else. No mention of the pilot. I had more questions than answers when I finished reading the report.
The pressurization system had a feature that automatically shut off the bleed air that pumps up the cabin if there is an overheat condition with the bleed air. I was flying a turboprop Aero Commander 690 one day years ago when such a thing happened. That airplane did not have an automatic overheat shutoff but it was quite obvious that we needed to shut it off manually and quickly or we would be roasted. It was hot in there.
The automatic system in the TBM was prone to false alarms and unnecessary shutdowns. It appears that this was such a case, based on recovered wreckage.
A warning about the shut off and depressurization was annunciated and there had to have been other warnings. The NTSB said the cabin would fully depressurize in four minutes which meant the cabin altitude would increase by more than 5,000 feet per minute for that length of time. That would send a strong and unmistakable message through your ears.
The checklist was not adamant about first donning the oxygen masks in case of a cabin depressurization but that would not have helped in this case. The pilot had not turned the cockpit oxygen switch on. This, and checking the oxygen masks, is a preflight checklist item for the airplane.
The pressurization system and related emergencies got about 90 minutes of attention in the 36-hour training course and the NTSB seemed to infer that this was inadequate. It is pretty cut and dried in case of a depressurization at high altitude, though: just follow the checklist with a high degree of urgency.
The pilot had not had recent altitude chamber training which should be required for pilots flying these airplanes.
This pilot was unresponsive just two minutes and 30 seconds after the first report of an abnormal indication. If he had chamber training, he would have known this was sure to happen. The last time I did the chamber, they had us take off our masks at 18,000 feet and at 25,000 feet. At 18, things faded slowly. At 25 the lights dimmed rapidly. In my case, flying a P210 that didn’t have a supplemental oxygen system, I did not often fly above FL190 because of what I had seen in the chamber.
This pilot was likely flying at FL280 because that is where the TBM 900 flies fastest and pilots buy airplanes like this to go fast. If a pilot is going to fly that high, a complete understanding of hypoxia is required as is the need to be locked and loaded for an emergency descent if the pressurization fails.
If you are skeptical about the urgency of this, I have a suggestion. Turn a light with a dimmer to full bright in an otherwise dark room. Punch the stopwatch in your iPhone and as it counts up to two minutes and 30 seconds, dim the light and try to make it come out even at dark and two minutes and 30 seconds. That’s what happens to your brain and vision without oxygen or pressurization at FL250.
Given that, you don’t really need a checklist to know to don the mask, call Mayday and make an emergency descent to 10,000 feet if the airplane depressurizes at FL280. That is just one example of how flying life does become more complicated and demanding when you fly higher and faster in your new turboprop single.
Fortunately there are not a lot of accidents like this in these airplanes. In fact, on balance, the pilots flying the TBM and Meridian have a good safety record. Because of the relatively small fleets, and more overseas and special use flying in the case of the TBM, I had to do some estimating and interpolating to come up with a rate for it but I did find that the TBM had an apparent fatal accident rate of .51 fatal accidents per 100,000 hours. That is just a bit better than the .56 I calculated for the Cessna 172 which has best record in the piston fleet. (I have never calculated a rate for the Diamond airplanes.)
The Meridian is interesting because the airframe was derived from the piston-powered Malibu/Mirage. (The latest version of the turboprop, the M600 is substantially different, with an all-new wing.) From an operational standpoint, the piston and turboprop airplanes would be close to equal in complexity as well as operating environment.
Initial and recurrent training would be an insurance requirement for the Meridian and probably for the piston airplane as well.
Given all that you might think the fatal accident rates would be similar. They are not. The fatal accident rate for the turboprop Meridian appears, at .88 per 100,000 hours, to be over twice as good as the piston airplane which I last calculated to be 2.04.
The fact that these two single-engine turboprops have substantially better safety records than piston-powered prop airplanes is quite contrary to some other things in the safety record.
The turboprops are flown mostly by pilots who have become ever more affluent and moved to the airplanes from high-performance piston singles and twins. In other words, the same folks only with a bigger flying budget. Historically, more performance has generally meant a higher accident rate. Pilots don’t do as well in retractable singles as they did in fixed-gear and the fatal accident rate in twins is, in most years, higher than singles.
Expanding the altitude capability has also resulted in a higher accident rate, with one caveat. Just the addition of turbocharging doesn’t seem to reflect a lot of added risk because most pilots don’t fly the turbocharged airplanes high too often. Adding pressurization has tended to make things a lot worse. Fresh out of the box, the P210 and Malibu suffered greatly in the hands of pilot who apparently thought they could fly through hell for love just because the airplane was pressurized.
Some of the pressurized piston accidents were caused by engine problems in the Malibu and engine and systems problems in the P210. (See the post Logbooks: A Long and Wonderful Flight, With Beginning Turbulence.) Most of the accidents, though, were related to bad decisions and poor technique on the part of the pilot.
The turboprops didn’t suffer from this, though. The accident rates I gave earlier cover the complete operational history of the airplanes in the United States and accident rates tend to improve, at least slightly, as the airplanes and pilots mature. They started off okay and are probably doing ever better.
What stands out in the turboprop fatal accidents?
The mechanical failure of the PT-6 engine that powers both airplanes is not on the list. I found one failure that was attributed to poor maintenance and a couple of power problems with undetermined causes. On one Meridian, the cowling was apparently not properly latched and came open after takeoff followed by a crash landing. There have been power failures followed by successful forced landings.
The PT-6 does have a wonderful record of reliability when only the core of the engine is concerned but a lot else goes on to keep the fire burning and the turbine turning and PT-6 failures that do occur are often related to the systems and accessories that are necessary parts of the engine. This is another way of saying that they can and do fail, if not often, and the airplanes have only one of them.
If one thing stands out in the fatal accidents it is the loss of control in instrument meteorological conditions. This often comes soon after takeoff which would suggest that the pilot didn’t get the ducks in a row before taking off. When a pilot loses control in IMC within a short period of time after takeoff it is obvious that he was neither ready for nor up to the task at hand. Even a slight distraction could lead to trouble.
The pilot of the TBM with the pressurization problem was well into the flight but he had apparently not taken care of the preflight checklist items on the oxygen system and that simple omission put him in a bad place even though the NTSB faulted the design of the system as a probable cause of the accident.
With the exception of the first Piper Meridians, these airplanes have capable autopilots that are well matched to the airplane. When looking at the accidents involving losses of control in IMC it is obvious that either a lot of hand-flying is done, or at least that some pilots don’t have a good understanding of how to get the most value out of an autopilot.
There have also been a few low-speed losses of control, on both go-arounds and missed approaches. The same rules apply here as in all airplanes.
I mentioned that at first the pilots of pressurized piston singles got into trouble while trying to fly through hell for love. This has happened infrequently in the personal turboprops. One pilot pressed on, climbing into icing conditions that got the best of him but, right up to the minute he lost control at altitude, there were good options available. While the airplanes have good on-board weather avoidance information there have been a couple of losses in or around thunderstorms but that is about it.
From studying everything that has gone on with the TBM and Meridian and with knowledge of the high performance piston fleet, I get the feeling that the lower fatal accident rate in the turboprops has to be attributable to better training. Better reliability could be a factor and, though it is contrary to what has happened in other areas of private aviation, the enhanced performance capabilities of these airplanes may have also made a contribution to safer operation. And, who knows, maybe the pilots fly with a better respect for the demands of such spirited airplanes.
These personal turboprop singles are being joined in the fleet by a single-engine personal jet, the Cirrus Vision SF50. Setting aside the performance and price differences, it will be interesting to see how the jet fares in the hands of owner-pilots. If anything, they will be better-trained because a type rating will be required to fly the jet.
One final thought about the turboprops, the TBM specifically. We landed at Sun Valley one day and parked next to a TBM. When my wife, Ann, got out of our P210 she walked over and checked it out. After her evaluation she told me that while she liked our P210 she just might like the TBM a bit better.