Most of us prefer to believe that things happen for a reason, or at the very least, that events have definite causes. That’s hardly unique to aviation, but the assumption of a predictable universe takes on particular importance when you’re counting on reproducible physical laws to keep you alive in a situation for which nature left your body unequipped. Soaring thousands of feet above the mountaintops probably qualifies, as does crossing below the polar ice cap in a submarine, or even scuba diving.
A corollary of believing that things happen for definite reasons is believing that things don’t happen for no reason—or, more to the immediate point, don’t stop happening without good cause. This makes it a little uncomfortable to peek into one particular corner of the flight-training safety record. Over the past 10 years, more instructional accidents have been caused by engines that quit in flight for no apparent reason than by confirmed failures of some hard piece of the engine or prop. In fact, the number of accidents due to unexplained power losses is more than one-third higher than the number due to broken crankshafts, cracked cylinders, burst oil lines, and the like. Even adding in every accident blamed on failures of electrical accessories (usually magnetos) or fuel-system components (pumps, carburetors, B-nuts, etc.) barely brings the total up to even. From 2001 through 2010, 92 fixed-wing instructional accidents came about after engines stopped running for clear and specific reasons, while 82 were due to losses of power that were never satisfactorily explained.
Of course, sometimes the lack of explanation seems more pro forma than a real mystery. After a Cessna 172 lost power in the base leg of the traffic pattern at the end of a cross-country student solo, the pilot recalled having applied carburetor heat before reducing power. Clean fuel was in the tanks, and, “Examination of the engine did not reveal any mechanical malfunctions.” Presumably that included the carb heat itself. Still, with temperatures and humidity conducive to serious icing, one might wonder whether that could have been the one time the pilot forgot to turn on the carb heat until after the engine began to stumble.
Others, though, are genuinely perplexing. A Piper Arrow was damaged in a forced landing after gradually losing power immediately after takeoff. During the investigation, the engine was hooked up to a fuel can and run at “moderate power settings” for about three minutes before the fuel flow gauge began to fluctuate and the engine abruptly stopped. After restarting, it ran for two more minutes before quitting again. Teardown and inspection of the fuel pump, lines, servo, divider, and injector nozzles found everything in good working order, performing according to specifications. The engine of a Cessna 172RG seized after losing most of its oil. The bottom of the airplane was covered in the stuff, but detailed examination of the engine failed to find any actual leak.
However, in the great majority of these accidents the engines show no sign of damage, and the stoppage proves impossible to replicate. This statement from the report on a forced landing in a Piper Tomahawk is entirely typical: “The engine was started and ran smoothly for 5 minutes at various power settings.” Fuel mismanagement will have been ruled out based on the position(s) of the selector valve(s), and often on the design of the fuel system itself.
Of course, from the pilot’s point of view a loss of power is almost always unexplained at the moment it happens—unless the reason is something as obvious as a fuel selector set to an empty tank or a mag switch that’s somehow been turned off. What counts is recognizing and managing the emergency in whatever limited time your altitude provides. Quick reliance on the old mnemonic of ABC—pitch for best-glide Airspeed, identify the Best field available for the forced landing, and, if time permits, run the applicable Checklists—is key to maximizing the chance of walking away in one piece. From the operator’s standpoint, though, it’s a little disquieting to think that even meticulously maintained engines have been known to quit for no good reason at all—just about as often as because something actually breaks.