Engine Reliability

Though I'm certainly happy that circumstances have allowed me to become an aviator and somehow manage to buy my own airplane, humble as it is, I always wince a little when I hear that comment, because I know what's probably coming next.

I can almost be guaranteed sometime in the next few minutes, I'll hear someone ask, "What do you do when the engine quits?"

I know it won't work to respond with some smart-aleck remark, such as, "Pray" or "Wish I was somewhere else," or simply suggest, "Aircraft engines don't quit." Too many folks have read spectacularized stories in their local newspapers about airplanes that fell to Earth. "A three-engine Cessna Cherokee crashed last night only 10 miles from an elementary school, where children would have been playing if it hadn't been 2:00 a.m. in midsummer. No flight plan had been filed, so the pilot obviously was lost."

Yes, engines do fail, but the probability is so small that it's statistically irrelevant, assuming you don't do anything stupid. Ignore pilot-induced engine failures---overleaning, running the engine out of fuel or oil---and the chances of a disabling failure in a properly maintained engine is essentially nil.

For better or worse, I made several dozen trips across the Pacific in the last 20 years, flying mostly single-engine Mooneys, Bonanzas and Malibus, and I'm a little staggered by the statistics of those deliveries.

One of my regular rides was a new Mooney Ovation, usually flying from Kerrville, Texas, to Sydney, Australia. On a typical Australian trip, I'd log 13 hours from Santa Barbara to Hawaii, another 11 hours to Majuro, Marshall Islands (Majuro had avgas until a few years ago), then seven hours to Honiara, Solomon Islands (the infamous Henderson Field on Guadalcanal), and another eight hours to Sydney. That's about 40 hours total at 2,500 rpm, roughly six million engine revolutions between Santa Barbara and Brisbane, 6,500 nautical miles, most often without a single cough.

Fact is, in many respects, aircraft engines are simply more reliable than automobile engines. While it's true much of aviation's basic technology is entrenched in the last century and auto engines are sometimes as modern as tomorrow, aircraft engines have some natural advantages.

First, piston aero engines operate at considerably lower rpm. Many car engines rev to as much as 7,000 revs. Aircraft direct-drive engines are designed to operate at a redline of 2,800 rpm or less, primarily a function of prop tip speed. (There are a few engines that run at slightly higher rpm---remember the old Cessna 175 and the newer Cessna 421---but those were geared engines, not well received by many in the flying public. The complexity required was often regarded as counter to an aircraft engine's simplicity.)

At tip speeds in excess of about Mach .80, propeller efficiency falls off dramatically, and a typical 78-inch prop running at 2,500 rpm for cruise is devouring sky at Mach .77---about 577 mph---at the tips. That's one reason longer propellers are sometimes less efficient, rather than more efficient, than shorter ones. A longer prop usually increases drag disproportionate to its additional thrust. In other words, you may see better climb but worse cruise.

A three-blade tractor is a sometime solution, but this has the disadvantage of adding weight, and the same rule usually applies. The extra blade often means more drag---again, better climb but worse cruise.

Accordingly, most prop manufacturers concentrate their development efforts on mid-sized, two-blade designs rather than on props of longer diameter or those fitted with an extra blade. Hartzell's new semi-scimitar props are excellent examples of prop development that doesn't demand higher rpm, larger diameter or more blades, yet delivers more thrust and higher speeds.

Aircraft power requirements offer an interesting paradox, since aircraft engines operate at high power for long periods, whereas auto engines must increase and decrease power regularly and rarely use more than 60-70% (except for the kid down the street with the new Corvette). Considering that airplanes measure engine reliability in terms of time and cars in terms of miles, aero engines are almost ridiculously reliable.

A typical Continental IO-550-powered single that cruises at 180 knots (207 mph) is rated for 2,000 hours TBO. That's 414,000 statute miles between overhauls. Can you think of any car engine that can last even half that number of miles? (Curiously, my wife's Toyota 4Runner SUV has a nearly indestructible V6 that now has 298,000 miles on the original engine.)

Piston-powered aircraft most often come up to full power for takeoff and climb, then reduce to a more moderate cruise setting, usually 75, 65 or 55%. Mechanical engineers will tell you steady-state operation puts far less stress on the engine, even if that steady state is at relatively high power. On a typical 200 hp Lycoming IO-360-A1A for example, 75% power would be equal to a theoretical 150 hp, "theoretical" because few aircraft engines deliver rated hp at the stated manifold pressure and rpm.

Conversely, a car on the freeway can cruise along at 70 mph on probably more like 15% power, perhaps 30 hp from an equivalent 200 hp engine. That's because aircraft engines must propel the aircraft forward AND support the weight of the aircraft in flight. An auto engine obviously has only to drive the car forward. The tires support the weight.

Aircraft engines have a few other advantages, as well. They're typically lighter than car mills as most have no radiator and no muffler, and the engines are nearly always four- or six-cylinder opposed, compared to eight-, 10-or 12-cylinder car engines.

If you're into diesels, a growing contingent of general-aviation airplanes offer most of the advantages with little downside. Diesel fuel is everywhere on the motorways of Europe and much of the rest of the world, but it's not that popular on America's highways and interstates. Conversely, since aircraft diesel engines can use jet fuel, finding a place to refuel isn't that big a problem for an airplane.

Aircraft engines don't have all the aces, however. While the usual consequence of an engine failure in a car is limited to an expensive repair and some inconvenience, an inflight engine problem can be considerably more serious. For that very reason, aircraft engines employ two electrical systems and compression ratios well below those used in auto power plants. Lower compression equals lower cylinder pressure equals less stress on the cylinders, rings, lifters, valves, crank, pistons and all the other greasy bits.

Aviation companies have to deal with government approval on everything they do, and the costs of negotiating with sometimes fickle and contradictory FAA FSDOs can be staggering. This alone tends to discourage any form of innovation in aircraft engines, and it's one reason some aviation innovators will go FSDO shopping when they're looking to introduce new products. Car companies have only their customers to please, plus they can amortize any development costs over thousands (or even millions) of units, rather than mere hundreds of airplanes, in aviation.

Auto engines have a major advantage when it comes to overhaul or replacement. Aircraft engines and auto engines are very much the same, only different, intended for two dissimilar purposes. Personally, I'm happy that I could completely replace my car's engine for less than $15,000. Merely overhauling my Lycoming IO-360-A1A would cost over twice that.

Wonder if an Infiniti engine would fit in a Mooney?