Regular readers may recall that I’ve logged my share of hours over various bodies of water, mostly the Atlantic and Pacific Oceans, the Gulf of Mexico, the Persian Gulf, the Red Sea and several others you may never have heard of. I often hadn’t heard of them, either, until someone hired me to fly across one.
No, this isn’t about ferry flying. Some of my friends who know I’ve survived 13 engine failures in 12 airplanes (yes, one was a double-failure in a Cessna Crusader over the Congo, Africa) are a little amazed at the fact that I’ve never had to swim away from a sinking airplane. They’re also nonplussed that I still regard piston aircraft engines with ultimate trust and respect.
Indeed, most of my flying has been in multi-engine and single-engine retractables, lots of Mooneys, Pipers, Aerostars, Cessnas, Beeches, Commanders and the like.
I’ve also delivered my share of single and twin turbine machines, but most of those have been powered by Pratt & Whitney PT6A engines, arguably the world’s most reliable aircraft propulsion systems, extremely unlikely to fail under any reasonable (and some unreasonable) circumstances. There have also been a handful of jets, limited by the fact that most jet buyers have their own crews.
Accordingly, all 13 of those engine problems have been in piston-powered machines. Fortunately, only one has resulted in a busted airplane, a Piper Lance over the rocky Ogaden Desert of southern Ethiopia, near the border with Somalia. Equally providential, the pilot/owner was in the right seat, saw the number-two cylinder fail on the EDM-800 engine analyzer, and agreed there was nothing I could have done to save the airplane. We both walked away and were rescued five days later, but that’s another story.
The point is that aircraft piston engines are almost unbelievably reliable if treated properly, or sometimes, even if they aren’t. That should come as no surprise to pilots who fly general aviation aircraft on a regular basis.
These days, most of my flying is fairly conventional, hopping around the Southwestern U.S. in my airplane or doing short, post-annual test flights in Cessna 400s. My airplane’s engine has 1,500 hours on the tach, and I’ll be surprised if it doesn’t provide the full 2,000-hour TBO.
The reason I have such confidence is that aircraft engines are specifically constructed for dependability first and power and efficiency second and third. We can argue all day as to whether aircraft engines should be so expensive, but there’s little question the additional cost buys a level of reliability several times better than any other form of internal combustion power. In my book, that makes them worth the money.
The differences between aircraft powerplants and automotive engines couldn’t be more pronounced. Most aviation piston mills operate at roughly a third the max rpm developed in auto engines, partially because most aircraft utilize direct drive to the propeller. Props aren’t very efficient at tip speeds approaching Mach .80, which usually equates to 2,700 rpm with standard 76-inch blades. Similarly, aircraft piston engines are generally limited to about 350 hp, whereas these days it’s not uncommon to find auto engines cranking out 500 or more ponies.
We typically fly at 65 to 75 percent power, but cars can cruise at legal speed (in the U.S.) on 10-15 percent of their rated power. The difference is related to the fact that automobiles don’t need to expend huge amounts of horsepower just to keep the wheels turning. The tires provide all the “lift” that’s necessary. Aircraft must dedicate a major percentage of their power just to support their own weight. Whatever is left over propels the airplane forward.
Most autos rarely see full power operation, but airplanes climb at 100 percent on virtually every takeoff for at least a few minutes, and turbo models may maintain full power to 25,000 feet.
Cars don’t have stipulated TBOs on engine operation, though they do have recommended service intervals. When they need service, you simply take them to the shop. Aircraft engines enjoy recommended TBOs, which may demand a complete rebuild and sometimes require service during the mandated annual inspection. Part 121 aircraft—airliners—must have an inspection every 100 hours.
(I digress, but I know of one 737 that used to log a full 100 hours of duty time every week. That’s right, 52 100-hour inspections a year. It was an Air Nauru Boeing operated, crewed and maintained under contract by Qantas of Australia and flown all over the South Pacific six days a week, 17 hours a day. It received its 100-hour inspection on the seventh day in Sydney. Like virtually everything operated by Qantas, it was an immaculate, flawlessly maintained airplane.)
Many of general aviation’s engines are rated for 2,000 hours between overhauls, including my 200 hp retractable. A 2,000-hour TBO could represent well over 300,000 miles of travel. There are very few auto engines that could last long enough to cover that distance (though my wife’s beloved Toyota 4Runner did just that before we traded it a year ago).
Consider for a moment if auto engines were built the same as aircraft engines. Back in the late 1960s, I bought one of the very first Porsche 911s, and while the car was a wonderful driver, it spent a disproportionate amount of time in the shop. In those days, the whole idea of an auto engine in an airplane seemed almost incomprehensible, though that first Porsche had dramatic similarities between its six-cylinder, horizontally opposed, air-cooled mill and a typical aircraft engine.
Porsche apparently recognized the same thing. A Porsche aircraft engine became less illusory and more reality in the early 1980s, when the company decided to build a dedicated engine for the aviation market, the PFM 3200. PFM stood for Porsche Flug Motor, and the engine was basically a highly modified 911 mill, geared down from its automotive 6,000 rpm to a sedate 2,700 revs and rated for 217 hp.
Mooney Aircraft adapted the PFM to fit the Mooney Bravo fuselage. The new airplane was dubbed the M20L, and it featured single-lever control for throttle/rpm/mixture, electronic ignition and altitude-compensating fuel injection, and was adaptable to full aerobatic fuel and oil systems. Such technical sophistication was remarkable for the early 1980s.
The PFM engine was first installed in a Mooney in 1985 and flown around the world on a promotional tour. I first saw the PFM 3200 mounted on a test stand at Porsche’s Stuttgart factory in 1983. Engineers were subjecting the PFM to attitude tests, rolling the stand to 70 degrees left or right of level and pitching it up and down 60 degrees, extremes Porsche obviously hoped it would never have to worry about in a ground-bound 911 sports car.
Mooney began deliveries of the M20L Porsche/Mooney in 1988. The PFM was a beautiful airplane, but it was underpowered, 150 pounds heavier and five to 10 knots slower than a 201, and $50,000 more expensive. Despite all its technology, only 41 PFM Mooneys were sold, and the model was discontinued in 1990. Former Mooney Vice President of Engineering Roy LoPresti told me the main disincentive to better performance was the M20L’s extra weight, higher form drag associated with the large cowling and the increased cooling drag.
Porsche also tried to interest Cessna in the PFM 3200 engine, possibly for application to the Skylane, but, after a few hours of testing, Cessna passed.
In addition to the stresses of simply propelling the aircraft, aviation engines must endure a wide range of operating temperatures on many flights. A Piper Mirage departing Palm Springs in summer may need to tolerate +40°C temperatures on takeoff, then cruise at 20,000 feet in skies frozen to -20°C, then land in Phoenix an hour later at +40°C again.
Another major difference between flight power and ground power is that there’s no single controlling government agency that oversees every move a car manufacturer makes. Aircraft/engine manufacturers are faced with huge and often arbitrary FAA certification expenses that must be amortized over a few dozen or, at most, a few hundred engines, whereas companies that produce automotive powerplants may develop new products with minimal government interference, then spread the cost over thousands or even millions of units.
By definition, this discourages innovation in aircraft propulsion systems. There are literally hundreds of auto engine manufacturers, but new aircraft engine production is dominated by Continental, Lycoming and, to a lesser extent, Rotax.
When you consider the mission profile of aircraft engines, you have to be at least a little reverent that the safety record is so good. We often think nothing of launching on a 1,000-mile cross-country in an airplane. Conversely, the most complex challenge many autos face is a half-mile run to the store for a quart of milk, a trip with plenty of places to pull over to the side of the road if your engine were to act up, a luxury we don’t get in a single-engine plane.
As of January 1, 2016, Senior Editor Bill Cox has logged 15,100 flight hours in 321 types of aircraft. He also holds 28 world city-to-city speed records, has made 211 international delivery flights, and owns and flies a LoPresti Mooney. You can email Bill at firstname.lastname@example.org.