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Old 19th Aug 2020, 11:37
  #129 (permalink)  
Pilot DAR
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Join Date: Aug 2006
Location: Ontario, Canada
Age: 60
Posts: 4,851
Yes, I have seem several mag switches worn to the point where the key could be pulled out in any selected position, this mode of failure can be easily checked with the engine stopped. Yes!!! I have personally experienced (as the guy hand propping the plane) a mag switch, which when selected off, left the mags live. Happily, my habit of hand propping as though the mags were always live, and the engine could unexpectedly start, prevented an unhappy event that day. After the engine (C150) was wrongly running, and I glared at my trusted buddy in the cockpit - he replied with a surprised look, holding up the keys, which I could see through the running prop arc!

So, though I agree with BPF's observation about the risks to the exhaust of a careless live mag check, I always do these checks - with great care. I am certain to turn the key off only at the slowest possible idle RPM, and for the most brief period. Doing it that way, I've never had an exhaust backfire, but as BPF says, a live mag check conducted at power can certainly damage the exhaust. For those who would like a refresher; if the engine is turning, with the mixture rich, it's pumping fuel through. If it's running while it's turning, the fuel is obviously being burned - no problem. If you (during a successful live mag check) prevent the fuel being burned, it gets pumped through anyway. It'll accumulate in the exhaust after a few strokes, and may be reignited when the mags are turned on again. If the engine was turning fast, and the period of no ignition was many strokes, it'll pump quite a bit of fuel. That cold create a damaging backfire. This also applies in flight, if you must reduce power beyond throttle to idle, move the mixture to cut off, rather than turning the key off, if you intend to return the power in flight.

If the engine has turned over many strokes with the fuel on, and ignition off, if on the ground, it would be wise to let the engine stop, then sit for many minutes before restarting, so fuel may evaporate from the exhaust. If in flight, select the mixture to cut off, and let the engine windmill so as to blow through any fuel accumulated in the exhaust. You'll know you did it wrong if you hear a bang when you return ignition.

There is hardly any damage you could do to an exhaust system which won't be a few thousand dollars to repair. And, any exhaust work introduces the risk that a cylinder stud is damaged during removal, then you're having to change a cylinder as well, which will be many more thousands of dollars. So, it's worth not abusing the engine this way.
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