Why Engines Fail

A study of engine failure accidents shows pilot error leads the pack, with mechanical woes far behind


Except for those who fly gliders, most pilots live in fear of an engine failure. Some are so wary they fly multi-engine airplanes for no reason other than to give them more options should an engine decide to imitate a brick. Pilots of singles, of course, have no choices. When the engine crumps, its time to hit the softest thing you can find as softly as you can manage.

But the fear of imminent engine failure is in some ways misguided. Annual inspections, preflights and runups are all designed to catch little problems before they become big ones. In addition, the relatively simple engine design helps make them more reliable.

On the other hand, there are forces at work against engine reliability. Lack of use and outside storage are two of the biggies. Sometimes even the pilot is an excessive burden on the engine – not by being overweight but by ham-fisting the controls or otherwise operating the machinery improperly.

For the most part, however, airplane engines tend to be fairly reliable when given the care and feeding they demand. They may need maintenance, but at least they keep the wind blowing the right way. Usually.

In a study of 300 general aviation accidents by Bruce Edsten of the Louisville, Ky., Flight Standards District Office, about 20 percent of them involved a loss of engine power. Note that Edsten studied only accidents, not those occasions where the pilot dead-sticked to an uneventful landing or where a twin landed on one engine.

At any event, the study shows that many loss of power accidents fall clearly into the realm of pilot error. Knowing that improves your odds, as truly catastrophic engine failures are rare creatures indeed.

1) Fuel Exhaustion/Starvation
Its no secret that engines wont run without fuel, yet 20 percent of the engine-out accidents were the result of fuel exhaustion (The airplane was out of gas) and fuel starvation (There was gas in at least one tank but it wasnt accessible to the engine).

Into this category fall the obvious enemies, including optimistic fuel planning, neglecting to check fuel before takeoff and having fuel siphon out of a tank due to a loose or missing cap.

It also includes fuel mismanagement. In some airplanes you can pump fuel overboard if youre running on one particular tank while another particular tank is full. Twins have crossfeeding systems to worry about. And then theres the whopper, exhausting one wing tank and going down while the other wing tank is topped off. While this is traditionally associated with Cessna pilots transitioning to a low-wing design, it happens across the board to pilots with a variety of training histories.

Starvation can also occur when the fuel tank vent gets clogged with debris or insects. Fuel cant flow out of the tank because air cant get in to replace it.

Good preflight planning, a careful preflight inspection and proper in-flight decision-making can prevent nearly all fuel exhaustion/starvation accidents.

2) Fuel Contamination
Close on the heels of not feeding your engine is feeding it something it doesnt like. About 11 percent of the accidents studied involved contaminated fuel.

Tops on the contamination list is water. Though many pilots claim after the fact to have sumped the tanks, an inspection often shows a quantity of water that would have been impossible to overlook.

Contamination can also include jet-A added to an engine designed to burn avgas. Often this mistake is the result of poor training by the refueler.If the cowling of your airplane says Turbo Centurion or Turbo Mooney 231 or otherwise mentions the word turbo, inexperienced or poorly trained refuelers can interpret it to mean the airplane is turbine-powered, rather than a turbocharged piston engine.

Pumping jet-A into the tanks of a piston airplane results at best in a nasty cleaning job. If the fuel has run through the engine, you could be looking at melted pistons or burned valves. Run it that way for long and the engine will be a worthless pile of metal by the time the airplane touches the ground.

3) Carb Ice
About 8 percent of the verified engine failures were due to carburetor ice, often when the pilot ignored the need for carb heat, but sometimes when the carb heat was used properly.

Aircraft certification requirements state that a carb-equipped engine must be able to take 30-degree incoming air and deliver it to the carburetor at 120 degrees, which is hot enough to melt carb ice and prevent more from forming. However, the certification requirements allow the manufacturer to use 75 percent power to heat the air.

In a typical carb icing scenario, the pilot applies carb heat upon power reduction, but the engine is not producing enough heat to melt existing ice or prevent additional ice from collecting. However, there are other reasons carb ice stops engines.

In about half of the carb icing accidents, Edsten says, the carb heat didnt work. Broken cables, bad linkages and deteriorated carb heat boxes can foil the pilots best efforts at supplying hot air to the choking engine.

Other times, the pilot applies carb heat and hears the engine sputter as the ice melts and the water works its way through the engine. The sputter causes the pilot to recall the sometimes sage advice, If the problem gets worse, undo the last thing you did.

Unfortunately, in the case of an ice-clogged carburetor, things will get worse before they get better – sometimes much worse. But its the only way.

Note that as a general rule Continental engines are more susceptible to carb ice than Lycomings because of the position of the carb. Other factors include how tightly cowled the engine is, whether cowl flaps are used, and how hot the engine is operating in the first place.

4) Fuel System Problems
Three of the top four causes of engine stoppage accidents involve getting fuel to the engine. A variety of fuel system problems were responsible for just under 8 percent of the accidents studied.

These problems came in a number of colors. Fuel injectors can be clogged with debris such as particles from deteriorating bladders or wet wing sealant. Fuel selector switches sometimes jam in a position where the engine is not feeding from any tank. Primer switches can be left open or primer system seals can deteriorate, flooding the engine.

Fuel system corrosion in neglected airplanes can build up until the fuel filter or carb bowl is choked.

These cases are similar to the fuel starvation accidents in that there is fuel aboard the airplane but it cant get into the cylinders. However these problems tend to be maintenance issues instead of operator error. Even so, a good preflight should catch most of these malfunctions before they get to the point of causing the engine to quit.

5) Cylinder/Valve Failure
When someone thinks engine failure, these are usually the kinds of problems that come to mind, but they represent only about 7 percent of engine outage accidents. Failures of the cylinders and valves may appear to be maintenance-related, but in fact they are often the result of improper engine operation by the pilot.

The kinds of errors that lead to premature cylinder/valve failure include overboosting a turbocharged engine, exceeding the engines rated power time limits, taking off without warming up a cold engine, and operating at low density altitudes that result in the engine developing more power than it was designed to handle.

But the major cause appears to be improper leaning. Lean too little and the engine can spit unburned gas through the exhaust valves, either burning them or leaving deposits on them. Either way, the exhaust valves then fail to seal well and the engine begins its trip down the road to early top overhaul.

Run too lean, particularly in a high-power cruise climb, for example, and the engine will run so hot that youre just asking for something to break.Also included in this category would be the notion of shock cooling, which some experts swear by and others swear at.

6) Miscellaneous Blunders
About 6 percent of engine failures were the result of a variety of gaffes. Were talking things like tools rattling around inside engine compartments after maintenance people accidentally left them there and improper installation of seals and other components.

These problems are impossible to predict and only sometimes able to be found during a preflight inspection.

Having a reliable shop do your maintenance work is part of the solution, but face it, everyone occasionally makes mistakes. Encourage your shop to do things right, even if it grounds your airplane for longer than youd like. Conduct thorough preflight inspections, especially right after maintenance of any kind. Make test flights (close to the airport) after maintenance of any kind before embarking on a trip.

7) Ignition Problems
The engine wont run without a spark, and ignition problems accounted for about 5 percent of engine failure accidents. Because of redundant magnetos, most of the problems were caused by the spark plugs – notably plugs fouled by oil or lead deposits.

Remember that many spark plugs were really designed for 80/87 fuel, and 100LL has seven to eight times as much lead in it. If alternative plugs are available for your engine, it may be wise to ask your mechanic about using them.

Mag checks before every takeoff should be mandatory, but some pilots skip the runup if theyve landed only minutes before. Mag checks not only help ascertain the health of the magnetos, points and wiring harness, they isolate one plug in each cylinder so that a fouled plug is obvious.

For many airplanes, ground leaning is necessary to prevent plug fouling, particularly in hot weather or at high altitude. Many pilots lean to a point just shy of roughness on the ground, knowing that if they advance the throttle for takeoff without checking mixture rich the engine wont develop enough power to get them into trouble.

8) Induction System Blockages
Engines need to breathe, too. Just under 3 percent of the accidents studied resulted from a blockage of the induction system that choked the engine.

Some of the causes were the result of obvious errors in the pilots preflight inspections. Cowl plugs are sometimes left in place despite the fluttering Remove Before Flight banners. Bird nests can be sucked in.

Other potential causes include alternate induction air doors that stick shut and are then unavailable when the main induction inlet gets clogged by ice or debris.

If a clogged induction system seems like an obvious problem that youd spot on your preflight inspection, consider that about 85 percent of the people who participate in the preflight inspection contest at Sun-N-Fun miss an orange rag stuffed into the induction inlet of a Piper Cherokee.

9) Lubrication Leaks
Just over 2 percent of the accidents were caused by oil leaks.

This may be a result of leaving the oil dipstick out and the oil blowing out the filler/inspection tube. It may be the result of a turbocharger failure that pumps the oil overboard. It might stem from leaky oil lines, a failed oil cooler or an improperly installed oil filter.

In any case, the failures are generally detectable by monitoring oil pressure and temperature gauges while the engine is running. Falling pressure and climbing temperature can mean only one thing: Put the airplane on the ground.

10) Major Internal Failure
When engines go bump in the flight, major internal failure is the cause only 2 percent of the time. These are the catastrophic failures that punch holes in the engine case, seize the engine, and otherwise make it perfectly clear that this engine has given up.

Internal failure includes broken crankshafts, camshafts and connecting rods, and melted bearings.

There is little the pilot can do to detect when a connecting rod will suddenly decide its work is done. Engine oil analysis may sometimes detect problems with the internal components before they fail, but often the failure is sudden.

Nor should pilots take refuge in that ethereal time between overhauls figure. Catastrophic failure can happen as easily to an engine thats 500 SMOH as 2000 SMOH.

Mystery Cases
The sources of engine problems cited above total about 72 percent of engine-failure accidents. In fully 28 percent of them, the investigators could find no reason why the engine lost power.

Failed components can be smashed or burned beyond recognition. Fuel system clogs can be lost amid the dirt and soot and chaos that results when metal strikes ground.

Lacking the sophisticated recording devices found on airliners, general aviation airplanes are frequently reluctant to give up their secrets.

But enough of the cases are left that pilots can spot ways to improve their chances of landing with the propellers still turning.

Fuel, spark and air. Make sure the engine can get those three things and the chances of ending your flight with a surprise power-off glide fall into a realm just about everyone can live with.

Also With This Article
Click here to view “Engine Diagnostics.”
Click here to view “Tricks and Traps.”


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