Alternate air system failures

[Captain Avgas]

In the end I opted to use Tony Tromp's concept (with a mod) which works great. Easy to open and close from the panel. All with 1/8" set rivets.

My big change to Tony's design is that I used a full sheet of 0.032" aluminium inside the Fab as a backing plate. Therefore no chance of small sections fatiguing and breaking off inside the FAB. I extended the sheet to just beyond the filter to prevent the filter from wearing into the bottom of the fibreglass FAB (as has been often reported). Therefore it serves a dual function. I've attached a couple of pix. You can left click on them to enlarge.

Note that I've orientated the sliding mechanism off at an angle to the FAB so that the cable does not conflict with the nose gear (RV7A).

Incidentally, Van's alternate air apparatus is a very poor design (to put it mildly).

 

[foka4]

Fresh perspective?

I'm just a few weeks away from installing my FAB, so I've started contemplating how I will incorporate an alternate air source.

I have some specific questions that I hope members of the group can address:

1) How does the original magnetic door fail? Many failures are mentioned in the forums, but I can't find much detail on what the failure modes were. The Van's service bulletin states that the reason for revising the design was that the door could open at high power settings, even when no blockage existed. This doesn't imply a failure as do the forum posts.

2) Has anyone ever successfully used the manual door to recover from an in-flight intake blockage (due to to icing or FOD)? Having separate (and exclusive) alternate air and carb heat controls seems like a recipe for confusion in the cockpit when the cards are down.

3) In cases where the alternate air source has opened, as anyone ever verified that the air from inside the cowl is warm enough to prevent carb ice?

Given all the reports of trouble with the latest manual design from Van's, I wonder if the community would be better served by curing the mechanical shortcomings of the original automatic design. The simplicity and operational benefits seem to point to this concept as a superior design, and its viability has been proven in many certified aircraft.

Thanks,

Matthew

 

[JonJay]

Magnet was not strong enough...

I'm just a few weeks away from installing my FAB, so I've started contemplating how I will incorporate an alternate air source.

I have some specific questions that I hope members of the group can address:

1) How does the original magnetic door fail? Many failures are mentioned in the forums, but I can't find much detail on what the failure modes were. The Van's service bulletin states that the reason for revising the design was that the door could open at high power settings, even when no blockage existed. This doesn't imply a failure as do the forum posts.


Matthew

The Magnet was not strong enough to keep the door closed. It is a fine line between not strong enough and too strong.
I opted for no alternate air as an acceptacle risk vs what I percieved as a higher risk potential with installing the mechanical system available at the time.
I have never heard of anyone averting trajedy through use of the door. There have been a few instances cited where assumptions where made that the system would have worked had it been installed or employed.

 

[sprucemoose]

The Al piece that held the magnet in place was riveted to the fiberglass, and the rivets worked through and caused the whole thing to come apart. At that point, the door was free to open at any time.

 

[s24789]

Alter

My airplane partner was preflighting and as usual looked at the alternate air system. A couple of the pulled rivets had fatigued. Luckily no pieces made it into the engine. We are no considering the slide design or just glassing it over. We really would like to keep the alternate air in some fashion but will jot risk a 20 k engine.
Phelps