Listening for Engine Snakes

Years ago, I enjoyed reading the Peanuts comic strip written by Charles M. Schultz — in one series, one of the characters was terrorized repeatedly by a fear of the dreaded Queen Snake. Invariably, the Queen Snake turned out to be a large stick, laying across his path to provide all of us with a smile and a laugh at the character’s fears.

STILL, THERE ARE SOME “SNAKE SOUNDS” OF WHICH YOU SHOULD BE AWARE. These “snake sounds” can warn you that you have a valve problem with your aircraft engine, and if you listen for their hiss, can help you to avoid what might prove to be otherwise exciting engine problems while you are in flight.

SOME CAVEATS: The next paragraphs describe pulling through the prop on your engine. It is essential that — whenever this is performed — you follow the directions outlined in your POH. Even more essential is that you treat the propeller as if the engine was ready to start while you are doing this and any time you manipulate the propeller.

IT IS NOT A BAD IDEA TO PULL YOUR PROP THROUGH A FULL ENGINE CYCLE, most notably if your plane has been sitting idle for a long time. This drags the crankshaft through the oil in the sump, and helps to lubricate the engine. This isn’t a perfect approach, but it allows you to listen for valve problems while you are pulling the engine through and provides you the opportunity to become familiar with normal engines sounds (assuming nothing is wrong).

LISTEN FOR ANY HISSING SOUNDS AS YOU MOVE THE PROP THROUGH. Hissing sounds are bad, as they indicate air leaking past your valves as the piston moves to the top dead center position. Leaking valves will reduce your engine performance, and may cause an inflight engine failure should the valve jam up and break off, falling into the cylinder.

If while you are pulling the prop through you happen to hear a hiss from the exhaust pipe, continue to pull the prop through. For most engines, you can hear the impulse couplings on the magnetos make their “clack” as they reach the point where they would fire the spark. If you have a 4-cylinder engine, you should hear 4 clacks to know you are done pulling the engine through, while a 6-cylinder engine owner 6 clacks would indicate you’re done.

IF YOU HEAR A HISS, the chances are better than 80% that your plane has a bad cylinder. Because you can hear the hiss, the cylinder problem will likely be a burned valve on the exhaust. This is because the exhaust pipe is open, and the intake manifold is closed and covered with a sound-muffling air filter. It will take a few minutes with your A&P mechanic and a compression tester to find the cylinder with the problem. Once tested, you will know whether the cylinder needs immediate service, or if it can be safely flown further.

BOTTOM LINE: Listening for a “hiss” isn’t part of any preflight that I know of. However, if you happen to be pulling your prop through, keeping your ears open for such sounds can be a way for you to be more proactive in your preflight activities, and in doing so, find a problem before it finds you.