I went to the field this morning and flew my avistar. I was watching a guy fly and his engine quit and he brought it in no problem. His spinner nut was gone but the prop was still attached. He said that it must have backfired and ran backwards before it quit. I have never heard of that on here anywhere. Does this happen much?

I have only had the engine running backwards during startup never had i happen during flight though

I have seen an engine backfire and begin to run backward in flight... for it to happen the engine has to be WAY out of tune, compensating for an air leak or some other problem that really needs correction.

It is rare, but it can happen.

If your engine starts backward... rapidly cycle the throttle a couple of times and it will usually kick itself to running forward.

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there is one other time it can kick itself into running backward in air... be doing a rapid throttle transition just as it runs out of fuel... it can run backward for a few revolutions before it dies.

I've seen this a couple times before. People may do a range check prior to flying but they don't always think to check prop nuts. So it's quite possible that his spinner nut was loose prior to his taking off.

I have seen this with 4 stroke engines, but not 2 stroke engines.

Yes, the nut had to be a bit loose or a backfire wouldn't have spun it off. but without a backfire.. it COULDN"T spin off in flight. The normal rotation of the engine acts to tighten the nut. (which is why electric starters can so often spin the nut right off.)

It was a 4 stroke. I just thought it was weird that this happened. I've never heard anyone mention it on this site.