At all times the expected respective lane-associated engine instrument “off” readings and operation of the red lane “off” lights were as expected. I shall only mention the results I did not expect.

I’m at the airport to transition to a 912is-equipped Bristell LSA, monitoring the engine runup through two G3X touch MFD.

Run engine up to 4,000 rpm per manual and checklist and turned lane A switch off. No change or drop/rise. RPM is steady at 4,000 and it didn’t waver at all. Turned switch back on and it stayed at 4,000 rpm. Tried it again just to see with exactly same result. The manual doesn’t say it HAS to drop but on the third try it immediately dropped about 50 rpm and then instantly (without touching the lane A switch) there was a soft mechanical “clunk” sound and the rpm immediately bounced back to 4,000 and remained there. Hmmm. Repeated the lane A check several times and half the time there was no drop (although red light and engine instrument readings performed as you’d expect, going off and on as expected) and half the time there was the 50-75 rpm drop and then the soft mechanical “clunk” with rpm bounce-back to 4,000.

Tried lane B and the results differed. Each time lane B was turned off it dropped about 50 or sop rpm and stayed there until I turned land B switch back on. With lane B there was always a drop, and never a drop and bounce-back, and never a mechanical clunk sound (that accompanied lane A bounce back).

The Rotax 912is operator’s manual says at runup setting of 4,000 rpm “Engine speed should not drop/increase more than 250 rpm.” Does that mean any drop or no drop is okay within 250 rpm? Why does lane A drop half the time? What’s with the bounce back? And really, what’s with the “clunk” mechanical sound? What’s with the “normal” drop with lane B? Why does lane B drop and stay there until the lane B switch it turned back on and yet lane A bounces instantly back with the switch off? Why is there no “clunk” with lane B?

My first flight will be a long haul from Lancaster, PA across densely-wooded rural Maine with few airports and very few open places for a forced landing. Then it’s across the country all the way to rural central Missouri. When I last flew regularly in the 1980s and early 1990s I lost engines and don’t want to experience that again (I was thankfully in multi-engine aircraft then). 

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