Rick Koch:
Yesterday, I sneaked out of the office early to go fly. It was a nice Denver afternoon, about 40 degF. I primed the engine and it turned over a few times, tried to fire a couple times, but no start (common). I gave a couple extra primes and it turned over a few more times then started. All pretty typical for me. But what was not typical was at the instant the engine actually started, I heard a very substantial “thunk” – a single, quick “jolt”.
It’s always hard to put a descriptive word or phrase to an unfamiliar sound, plus you risk coloring people’s view of what it might be by choosing the wrong words. Anyway, in my mechanically-challenged mind, I wonder if I might have been slow in backing off the key start switch and the starter was still engaged
and got smacked (?)… or could the starter itself have been a little sluggish in disengaging, and got “thunked”? Are either of these possible? Any better, more informed ideas?
The engine ran as smoothly as ever so I flew for about an hour. Back on the ground, I did not try to restart. I had such a great flight I didn’t actually think about it anymore until I was driving home. I’m hoping to get
out again today and fly. Anything I should check for or do this time when starting?
While there are perhaps some other less likely causes, the most likely cause is that one of the mount cushion spacers bottomed out against the inner diameter of the mount cushion bonded washers. The spacers have a thin rubber coating that slightly reduces the shock, to prevent damage, but it still gives a pretty good bump to the engine mount and airframe. That’s why you don’t want to continue to operate with sagging engine mount cushions.
This is a very common occurrence with the high-compression IO-360, even with near-new mounts. The IO-360 has gel-filled spacers in two of the four mounts, to make this less likely, but it happens nonetheless. It is less common on the O-360, but it isn’t unusual. It is caused by a sudden strong “impulse” in the engine. It will normally only happen during start-up and shut-down. On start-up, it is the first firing impulse that can cause it. On shut-down, it most often happens if the engine momentarily “diesels”, or if it momentarily starts to run again before finally winding down. It can easily happen if you turn off the ignition on a running engine, then turn it back on before the engine stops turning. This will also quite likely damage the exhaust system, when the accumulated fuel vapor lights off.
As far as the starter Bendix goes, a really sharp ear can detect the Bendix getting thrown aft on the shaft against its stop ring, as the engine fires, but it is more easily heard than felt. In my opinion the only starter-related issue, that you would be likely to feel as a strong jolt, would be “kickback” during starting. This occurs when the spark retard system fails to work properly, and the engine fires too soon. This can kick the engine in reverse rotation, and can break starter parts or the flywheel. When this occurs, the engine never actually starts and continues to run; it just kicks strongly backward one time, and dies.
Anyway, just take a look at your mount cushions and see whether they are still in good shape. Also take a look at your accessories, such as the vacuum pump, and make sure that there are no signs that moving parts are contacting fixed parts. Also make sure that there aren’t any moving parts that have been tightly tie-wrapped to fixed parts. For example, if a hose is connected to the engine, it should not be tied off to an airframe part unless there are at least eight inches or so of free length to absorb engine vibration. I have actually found things like vacuum pump inlet fittings that were tie-wrapped to the engine mount.