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My FAB (slightly modified) tried to ruin my airplane

A

andyrv

Well Known Member
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.



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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
 
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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
 
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.
 
gciampa said:
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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.
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i agree with this
 
Might be a good idea to glass over the plate so that there is no way for anything to come loose.
 
<<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.
 
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.
 
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 ...
 
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.
 
AlexPeterson said:
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.
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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.
 
FAB and cowl fire

I know some body who had the alternate door. There were some fuel leaked out of the door on the bottom of his lower cowl. When he started his engine, there was a small fire on the bottom cowl. Fortunately, he did not have major damage. I was debating if I keep the door at all. But I going to have it IFR equiped, so I decided to keep it. Just be carefule about the potential fire when starting the engine
Peter
 
I have had the thought of removing mine. It would take putting fiberglass over the hole, which is big. :( I wish I would have known of this problem before I did the FAB. Oh and the option of pulling the heat knob, no can't do that, I have fuel injection.
 
Just to be clear, if I were building, I would install the classic door outside the filter for carb heat/alt air. I just don't see the need for one inside the filter. That is where the peril comes from.
 
AlexPeterson said:
Just to be clear, if I were building, I would install the classic door outside the filter for carb heat/alt air. I just don't see the need for one inside the filter. That is where the peril comes from.
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What about snow/ice/bird pushed up against the filter?

L.Adamson --- RV6A
 
I think fabricating a door on the very back of the FAB would be a much better idea. If that stupid thing did fall apart, the parts would be stopped by the filter. If a bird or something went inside, than it would pack against the front of the filter. A door on the back side would open things back up.
 
L.Adamson said:
What about snow/ice/bird pushed up against the filter?

L.Adamson --- RV6A
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That is what the alternate air/carb heat is for - however, it needs to be pulled prior to plugging the filter with snow. As for a bird - tough to wrap itself all around the filter. If a bird can stick itself somehow in my induction system and kill the engine, I'll tip my hat to it, and remember this post.
 
AlexPeterson said:
That is what the alternate air/carb heat is for - however, it needs to be pulled prior to plugging the filter with snow. As for a bird - tough to wrap itself all around the filter. If a bird can stick itself somehow in my induction system and kill the engine, I'll tip my hat to it, and remember this post.
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Not so tough to warp itself around the filter as the feathers will do that.

BTW, has anyone ever taken a bird in the intake?

As for making the door pull outward to open, that will not work. If the main intake is plugged the suction will be such that you won't get it open, if the engine is still running. If it is not running, then maybe.

When I was racing I would put a plastic grocery bag over my K&N cone filter before cleaning the engine. (This kept the cleaner from removing the oil on the filter.) Once I forgot to remove the bag before starting that little 1.8L four banger. The engine would fire and then die, fire and die. After a few tries I checked to see what was wrong. The bag was still on the filter.

The suction from that engine compressed the K&N filter enough that it had to be replaced.

The reason I tell that story is so you get an idea of the forces at work in your intake.
 
Straighten me out, please

This thread has certainly convinced me to modify my FAB. See if I'm on the right track. I plan to build an interior metal ring, drill out the pop rivets from the exterior ring, and install platenuts on the interior for #6 flathead screws. So far so good? Also, someone suggested encapsulating the whole interior ring assembly with glass..Does everyone think that is necessary..? Seems to me that if you dripping oil/fuel, it is more likely that some of the glass would de-laminate and be sucked into the engine, as opposed to all the the screws coming loose and the interior ring becoming a problem..Any thoughts on this, or did I miss the point (as usual!)..
 
bkc3921 said:
This thread has certainly convinced me to modify my FAB. See if I'm on the right track. I plan to build an interior metal ring, drill out the pop rivets from the exterior ring, and install platenuts on the interior for #6 flathead screws. So far so good? Also, someone suggested encapsulating the whole interior ring assembly with glass..Does everyone think that is necessary..? Seems to me that if you dripping oil/fuel, it is more likely that some of the glass would de-laminate and be sucked into the engine, as opposed to all the the screws coming loose and the interior ring becoming a problem..Any thoughts on this, or did I miss the point (as usual!)..
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You are pretty close to what I ended up doing with mine. I felt that the pivot was held together with a #8 platenut and screw. It never fell apart. So I got rid of the remaining revits and put in the #6 platenuts (two more) around the circle, no extra backing, didn't want it going into the engine, countersunk the plate on the outside for the screw and the rivets to hold the platenuts in. so far, everything is working out fine, over 60hrs on this with no problems.
 
The FAB is "the" weak link in the FWF part of the vertical induction RV's, I've said that for awhile.

Looking over Van's plans for the alt air door, every customer I've worked with has decided not to install it, and I support their decisions. Even much better thought-out designs have problems, and require careful attention during 50 hour and condition inspections. I'd avoid the full aluminum plates, and simply glass in the aluminum plate as mentioned earlier. I'd avoid steel, the weight doesn't help anything, and thermal expansion is going to be the least of your worries with this setup.

The last box I put together, I avoided pulled rivets as much as I could, because they have a tendency to work their way out, and crack the matrix of the glass when they're pulled. I use AN470A rivets (soft) in glass, and I OVER squeeze them so that they have a larger surface area holding, and don't take as much pressure to squeeze (as such, don't crack the matrix of the glass as much). I also FAY SEAL the box with proseal when I put it together... helps the whole thing vibrate together instead of the pieces moving in relation to one another.

If I ever build a vertical induction RV for myself, it will NOT use the Vans box... There has to be a better way.
 
Plate and glass

I think a plate and glass is a good idea... I´ve made it this way, and put it on the outside. This way there is nothing on the inside, but the end of the screws.

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Store-bought, or home-made?

Is this alternate air mod something you buy from Vans, or is everyone "rolling their own"? Couldn't find it on Vans website. Thanks.
 
FAB bypass

The bypass kit comes from Van's. This kit is the one referenced in the bulletin to replace the older magnetic version.
 
What about the nuts

We do everything we can to eliminate the risk that something from the FAB get suck into the engine - but what about the nuts that hold the plate to the carburetor..? What is the best way to eliminate the risk for them to get loose..? What nuts is best to use?
 
FAB plate mounting

My setup uses bolts with drilled heads and safety wire on them to hold the top plate to the injection servo.

Andy
 
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