Daleandee
Final Approach
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Dale Andee
As my instructor from my one tandem skydive said, “wear your helmet, you want to be awake for the fire”.That’s a helluva one to walk away from. Paul Bertorelli might be onto something about helmets…
Yeah, but: There are more causes to a loss of engine power than mechanical failure of the engine.
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Ron Wanttaja
Don't have statistics for the overall fleet, but have looked at the Cessna 172. Its overall rate of loss-of-power accidents is about half that of homebuilts. Major factor is that non-certified engines are involved in almost half of the homebuilt accidents.How do these statistics compare to certified GA aircraft?
" ... put an early (2007) 100 HP engine on it for some baseline info on carb setups/temp and timing for improvements. While performing ignition switches in flight, the sudden "jolt" on the engine (from shutting down ignition and turning on backup at high power setting) caused the cam gear key to shear and slip. At teardown we realized it was an earlier design we had tested but never flight proven (being an early test engine - 2007) Now this engine had about 20 hours on it prior to this failure and seemed fine until this particular testing."
Over on Homebuilt Airplanes Bill Clapp gave a reason for the engine failure in his Sabrewing:
FWIW ...
Sound like a keyway failed.
The keyway setup wasn't engineered for "performing ignition switches in flight, the sudden "jolt" on the engine (from shutting down ignition and turning on backup at high power setting) caus[ing] the cam gear key to shear and slip."I get that but he talks about, "it was an earlier design we had tested but never flight proven" and I have no idea what that refers to. To my knowledge there was no change in keyways by GM but William Wynne (FlyCorvair) does have procedure that he insist on with regards to attaching the cam gear to the cam. According to him things like this can happen if the assembly is done incorrectly. Not everyone follows his procedure. There are other differences in the approach to building these conversions between various vendors so I'd like to have more clarity as to exactly what was done that failed and why (if it can be known) it did fail.
On Homebuilt Airplanes I asked for more clarification so hopefully he'll come back and explain. Camshaft keyway failures in Corvair engines, while not unheard of, are extremely rare and generally occur in older, well worn, engines. I fly one of these things (mine was built by Dan Weseman to William Wynne specifications) so I get a bit curious when things like this are reported but the information is not real clear.
The keyway setup wasn't engineered for "performing ignition switches in flight, the sudden "jolt" on the engine (from shutting down ignition and turning on backup at high power setting) caus[ing] the cam gear key to shear and slip."
I would assume the modification done by Wynne might involve enlarging the keyway and key stock, a higher shear strength key material, heat treating of the gear, etc.
Early Testing Hubs had .002" inerference fit on the cam gear and non-hardened keys. We changed this to .004" interference fit, hardened keys and tig welded "anti-slip". We never had an early test hub shear a key or slip timing but the math indicated these changes would be prudent. Looks like the changes were warranted but this engine had many hours on it so I was not concerned until it happened.
Ours are new and interference fit onto hub. Now the interference is tighter plus we tig weld the gear to the hub along the joint to prevent possible shear. It is a good change we made early but out of caution not out of any failure evidence.