My FAB (slightly modified) tried to ruin my airplane

andyrv · #1 07-19-2009, 08:54 PM

After about 50 hours my alternate air (built to plans, vertical induction fuel injection) door pulled the rivets right through the FAB fiberglass. No big deal since no loose parts were available to be sucked into the engine. I built a U shaped backing plate inside the FAB so the rivets would go through metal on both sides of the fiberglass. The part inside the filter was too large to be ingested into the engine. Today due to vibrational characteristics all of the rivets pulled through the outside door plate breaking the inside plate. As you would expect this part was large enough to cover most of the intake opening. All rivets are intact so no metal went into my engine.

At 1500 agl in climb out today almost total power loss. Boost pump on, switch tanks, check various mixture positions. No luck. Reduce throttle and got smooth engine. No more troubleshooting. Flew back to the field at about 40% throttle. Got the plane on the ground (luckily the grass strip I was flying from was only 3 miles). Nice landing.

http://i32.tinypic.com/x5dw0.jpg

My current plan is to build a full size aluminum plate on both the inside and outside the bottom of the FAB. The filter would actually sit on the inside plate and no plates or loose parts inside the filter circumference. The outside alt air door plate will also be drilled for rivets around the entire circumference (as you can see in the pictures the rivets do not go around the entire circumference) to make it more solid. Get engine dynamically balanced. Expensive part is to get repairs or replacement of the fuel servo since the 4 venturi air tubes were damaged by the loose plate striking them.

Andy

L.Adamson · #2 07-19-2009, 09:26 PM

I didn't modify mine with a plate inside the FAB, and left the bottom plate full size. But during my condition inspection at 86 hrs, I found that some of the blind rivets had loosend, along with some pro-seal I had smeared around it.

At first, I thought one blind rivet was completely missing, but it was actually left out for a drain hole. I have a drain hole on each side of the filter.

What is obvious though, is that there is a lot of shaking down there. I re-riveted, and will keep an eye on it.

L.Adamson --- RV6A

gciampa · #3 07-19-2009, 10:16 PM

I had the same issue with the FAB. Ended up making an aluminum backing plate inside that could be used as an attach point for solid rivets.

I would recommend calling Vans and asking for a SB, along with a change to the plans. I know this has been previously reported, but, to my knowledge, there is not an SB for this very serious condition.

It would be B.A.D. to have parts from the FAB or air bypass assembly being sucked into the engine.

This should be a special interest item as part of routine maintenance. I think.

greenmatter · #4 07-19-2009, 10:18 PM

Quote:

Originally Posted by gciampa

I had the same issue with the FAB. Ended up making an aluminum backing plate inside that could be used as a solid attach point for solid rivets.

I would recommend calling Vans and asking for a SB, along with a change to the plans. I know this has been previously reported, but, to my knowledge, there is not an SB for this very serious condition.

It would be B.A.D. to have parts from the FAB or air bypass assembly being sucked into the engine.

This should be a special interest item as part of routine maintenance. I think.

i agree with this

RV8RIVETER · #5 07-20-2009, 07:11 AM

Might be a good idea to glass over the plate so that there is no way for anything to come loose.

DanH · #6 07-20-2009, 08:03 AM

<<Might be a good idea to glass over the plate so that there is no way for anything to come loose.>>

Excellent idea.

The coefficient of thermal expansion for steel and glass are pretty close. I'd make the interior reinforcement from steel sheet, with nutplates, then glass over the assembly to encapsulate everything. Attach the exterior ring with screws.

allbee · #7 07-20-2009, 08:35 AM

I had the same problem. What I did is put two more nutplates around the circle. I used #6 nutplates and screws. I have so far 65hrs on this mod and working fine.

allbee · #8 07-20-2009, 09:21 AM

this is my theory on why this happens. Sure the bottom gets soft from the oil and fuel from the engine. What I think is happening. You pull back on the throttle. You are decending and the speed is up. Lots of air is coming in and nowhere to go. Pushes on the FAB from the inside. Weak spot, those chinsy pop rivets at the door. Pushes out the rivets and ...

AlexPeterson · #9 07-20-2009, 11:38 AM

I struggle with the premise of needing the mechanism at all. If you are in snow, pull the alternate air/carb heat. Seems like the mischief potential of having parts that might get sucked into the engine is high as compared to forgetting to pull alt air/carb heat to avoid a snow blocked air filter. Just my opinion.

N941WR · #10 07-20-2009, 11:44 AM

Quote:

Originally Posted by AlexPeterson

I struggle with the premise of needing the mechanism at all. If you are in snow, pull the alternate air/carb heat. Seems like the mischief potential of having parts that might get sucked into the engine is high as compared to forgetting to pull alt air/carb heat to avoid a snow blocked air filter. Just my opinion.

Alex,

I agree with you. I'm VFR only and never put that door in my FAB. During the rebuild (my FAB got mushed in my taxi incident) I will again build the FAB without the alternate air door.

However, if I were IFR, then I would install it, along with a heated pitot, and a bunch of other things.