Piston Engine Failure Modes


As a pilot, you’re almost certainly going to spend some time flying behind (and eventually beside) some piston engines.  There are some details that are really useful to know about how they operate (and eventually break).


Some fundamental knowledge about engines:

An internal combustion engine (be it 2-stroke or 4-stroke) is inherently an air pump. It must be able to suck air in, and blow air out. If it cannot do this, it will not operate for very long.

This is why you have alternate air (carb heat) on the intake.  On the exhaust, well, you hope the complicated baffling in your exhaust doesn't fail and plug up the outlet of your
muffler. All my aircraft have straight pipe exhausts for a reason - if it isn't there, it can't fail.

Now for the engine internals. We hope that as you fly along, the big pieces stay connected and in place. On very rare occasions, the crankshaft will break, or a connecting rod will break, or a cylinder head will come off.  Fortunately this kind of failure is pretty rare.  And you can't do much about it, as a pilot.

If the engine internals stay in place, that means that the piston and valves are probably doing what they are supposed to be doing, at the right time, in a sealed combustion chamber.  This is good.

When we do a leak down test at annual, we put 80 psi in the cylinder at TDC (piston up, valves closed) and we see where it leaks.

Most common is air coming out crankcase breather tube, which indicates worn piston rings. This is expected as the engine wears, and is a pretty harmless source, because it does not have a catastrophic failure mode. You can fly with broken rings. Your oil consumption will increase, your oil temperature will increase, and your oil will get dark fast, but the engine will actually run pretty well. This is easily spotted with a greasy
top spark plug. Time for a new set of piston rings, and you hope there is enough meat in the cylinder to just re-hone it, otherwise you will need a new cylinder.

Hissing out the air filter indicates leakage past the intake valve.  This is not terribly common. It lives a pretty easy existence, as it is cooled by the incoming fuel-air mixture. Sometimes it can stick because of crud on the stem and guide, but that's easily repaired.

Hissing out the exhaust – put your ear next to it – must be taken very seriously, because it indicates leakage past the exhaust valve. It's easy to get false alerts because the exhaust valve stem and guide frequently get dirty and will stick and hang open, but if the valve is closed and leaks, you have to do something about it NOW.  What could be happening is a crack on the edge of the exhaust valve face, and it can spread across the face, and when that part of the face breaks off, your engine will run unbelievably rough. This is not a good failure mode.  It is to be avoided at all costs.

Another failure mode of the exhaust valve is "necking". Due to the very hot exhaust gases, the stem of the exhaust valve just above the face gets gradually narrower and
narrower and the face can break right off the stem. Again, a very rough engine results when this happens.


Bottom line is that you must very carefully monitor the health of your exhaust valves.  They live a very hot, hard life and if they fail it will really ruin your day.

Your AME can look at your exhaust valves during the annual inspection with something called a borescope to inspect the valve face and the stem and make sure nothing bad is going to happen there.

And of course, your engine requires spark (you have two magnetos) and fuel.  You probably already know that, but you wouldn’t simply believe the number of aircraft accidents attributed to fuel exhaustion (excess air in the tank) or fuel starvation, which is when an empty fuel tank is selected, but there is another tank with fuel in it.  Oops.




May 2013