RV-9 w/ Subie accident cause?

NTSB lists an "Evenson RV-9A" as having an accident this summer and it appears the engine might have failed due to high manifold pressure in relation to the supercharger. I'm curious if anyone has heard what the possible / probable cause of this engine failure was?

Thanks.

Doug Lomheim
RV-9A; 13B FWF
OK City, OK

His electrically operated manifold pressure relief valve failed, overboosted the engine, causing an internal failure.

John has it correct. The small servo motor failed while the waste gate was in the fully closed position. I learned later the safety backup 'blow off' valve was designed to relieve pressure at 50" MAP. The most I saw was 40". But that was high enough, with the stock ECU, to damage the engine.

BTW, I still like the normally aspirated 4 cylinder Egg engine.

Roger Evenson, Tucson
7A building, with an ECI 360 this time.

revenson said:

John has it correct. The small servo motor failed while the waste gate was in the fully closed position. I learned later the safety backup 'blow off' valve was designed to relieve pressure at 50" MAP. The most I saw was 40". But that was high enough, with the stock ECU, to damage the engine.

BTW, I still like the normally aspirated 4 cylinder Egg engine.

Roger Evenson, Tucson
7A building, with an ECI 360 this time.

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And to complete the information: The servo, the same one you all use in your trim systems, is rated for over 1000 times the force needed to move this tiny valve. The servo failed after the builder attempt to adjust it's travel. A fuse blew and was replaced before the incident. The adjustment was not done correctly and the servo failed due to a over-center / jammed valve.. It is as simple as that. Builder did the best he new how and the whole thing is very unfortunate.

Jan Eggenfellner

Hi Jan: Nice to see you online here. But I want to clarify a bit.

Yes, the earlier jam may have contributed to a weakened servo motor, but that jam was on the 'fully open' side of the wastegate travel. The adjustment after that was done correctly, and did not result in jamming in either the 'closed' or 'open' position. There was no further problem until the servo motor failure.

After the accident, I very carefully inspected the waste gate to determine whether it had jammed closed. It was mostly closed, but not at the end of its travel. It was NOT physically jammed. The servo motor would not operate.

I also want to make clear that if the 7A project I bought to replace my 9A did not include an ECI engine, and if Eggenfellner Aircraft was still producing the older, 2.5 liter engines, I would be now be rebuilding with another Eggenfellner Subaru. I still have much confidence in that engine and Jan's firewall forward package.

Roger, Tucson, AZ

janeggenfellner said:

The servo, the same one you all use in your trim systems, is rated for over 1000 times the force needed to move this tiny valve.

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Is there intended to be a backup, or fail-safe mode if this servo fails, or is it accepted that a servo failure should result in engine failure? If this servo fails in a trim system, this is not a big deal. You simply have a bit of force to hold until you land. But the consequences of this servo failing in an engine installation appear to be much more significant.

What is the temperature in the area where this servo mounts, and how does that compare to its designed operating environment in a trim system?

Kevin Horton said:

What is the temperature in the area where this servo mounts, and how does that compare to its designed operating environment in a trim system?

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You took the words right out of my mouth. Exactly what I was wondering after reading this post.

Here's RAC's answer:

http://www.rayallencompany.com/RACinfo/FAQs.html#Anchor-Can-36340

"Can I use a RAC servo inside the engine cowling?

We know of our servos that are used successfully inside cowlings, but we are hesitant to recommend it. Although the DuPont Minlon? brand nylon we use is the same basic type that is used in BMW valve covers, the strength of the gears inside get weaker as temperatures climb above 180 degrees F. Try to keep the servo in a ventilated location under the cowl away from exhaust pipes or better yet, behind the firewall. If your heart is set on using them for cowl flaps, remember that cowl flaps take a terrific air load beating so keep them as small as possible."


I see on the RAC site that Seawind uses the RAC servo for cowl flaps - probably a reasonable install yet even RAC is sort of discouraging it on the site. But not sure about the wastgate scenario and it would largely depend on how remote of an install it is. My curiosity is piqued. Would love to see an install pic. Wastegates have to be well over 180 degrees - clearly it is not mounted to the wastegate, but what is the proximity and what is the temp in that area? Been measured before under a tight RV cowl?

The recommended installation per Jan is to put the servos inside the cabin and not in the engine compartment.

Thanks Rod

That makes perfect sense - assume it would then be remoted to the wastegate via a cable.