I've made a point to have a few A&Ps and EAA Tech Advisor visit my shop during the build. It's always good to have another set of eyes on your work to make sure everything is ok. My Tech Advisor has mentioned to me on more than one occasion that he feels the alternate air door on the snorkel will allow dust and debris to enter. I installed the door exactly per the plans, and visually it looks tight, but when tapping on it with your finger you can hear a "snap" which indicates there is still a tiny gap between the door and its seat. Have any of you encountered something similar? That a tech counselor or some other experienced builder has expressed concern over Van's alternate air door not being tight enough?
Van's Air Force
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Alternate Air Door
- Thread starter asw20c
- Start date
Oil analysis has shown hardly any sign of dust and debris, more than 11000 RVs are flying and I suspect 90% of them have the same setup and no sign or report of any issues. How much dust do you think it will enter thru that gap, especially that most of the time it is up in the air where not as much dust exist.
Carl Froehlich
Well Known Member
How tight the door shuts is a function of tight, but not so tight that it does not open.
Side note - if there is leakage around the door the air is going out, not coming in. This assumes you have the typical 0.4” or so snorkel MP gain.
Carl
Side note - if there is leakage around the door the air is going out, not coming in. This assumes you have the typical 0.4” or so snorkel MP gain.
Carl
Desert Rat
Well Known Member
There's also supposed to be a hole drilled (#30?) in the snorkel to let any overnight rain water escape. That hole is in the lowest point of the snorkel, which is about .5" from the fuel servo inlet. Then let's not forget the fact that there's no gasket where the snorkel flange bolts to the face of the servo, just a mating face of fiberglass to aluminum held with 4 studs at the corners. I guarantee that area isn't 100% air tight.
Bottom line; there's more places for unfiltered air to get in or out than just the alternate air door. so I can't say I'm worried about it.
Bottom line; there's more places for unfiltered air to get in or out than just the alternate air door. so I can't say I'm worried about it.
Carl Froehlich
Well Known Member
Me too.
blaplante
Well Known Member
I wonder if 90% do oil analysis.
I have vertical induction servo, and (had) the alt air door on the bottom of the airbox. It was a source of elevated silicon in the oil analysis more than once. Admittedly the low location allowed it to suck in more dirt than a higher one would.
There's positive pressure in flight (to overcome the filter pressure drop). Not so much on the ground when taxiing.
Desert Rat
Well Known Member
That is interesting. I have an IO390 so I based the install on the RV14 plans. I questioned this but the install instructions and drawing for the snorkel definitely doesn't have a gasket there.
Jetmart
Well Known Member
My 14 with the 390 doesn't call for a gasket. I questioned this as well as the poor fit on the door.
The one from Aircraft Specialty seal much better. https://aircraftspecialty.com/rvproducts.html
Jetmart
Well Known Member
I don't see how that seals any better. It is still aluminum on aluminum. It certainly is a neater setup with the cable attachment. It opens a closes which the current Van's may or may not. It would be nice if there was a rubber seal between the two. Is it accurate that the snorkel at that point is under positive pressure so any air would leak out?
PilotjohnS
Well Known Member
Sorry, the upgrade doesn't look much better than original. here are my lessons learnt from the alt air door:
1)the door flange must be reinforced on the fiberglass, other wise the rivets holding it on will vibrate through the fiberglass. i installed a second flange on the inside so the fiberglass airbox is sandwiched between the two metal flanges.
2) the loop around the bolt has to be smooth with nearly 270 degrees of contact. The upgrade and the original planes shows to twist the control rod into a loop and then put the bolt through. this means only one point is pinched. this caused mine to fail and the pivot screw broke through the door.
3) the pop rivets did not come out perfectly smooth and caused my door to hang up. i switched to solid rivets, and the inside metal helped everything come out nice.
4) I needed to use AN-3 screws and nuts, not the cheap hardware in the kit to keep the pivot from backing out.
In my mind, this whole alt air door needs to be rethought. Maybe a nice CNC base, door and pivot mechanism, or a butterfly valve. At least something more immune to the vibration.
1)the door flange must be reinforced on the fiberglass, other wise the rivets holding it on will vibrate through the fiberglass. i installed a second flange on the inside so the fiberglass airbox is sandwiched between the two metal flanges.
2) the loop around the bolt has to be smooth with nearly 270 degrees of contact. The upgrade and the original planes shows to twist the control rod into a loop and then put the bolt through. this means only one point is pinched. this caused mine to fail and the pivot screw broke through the door.
3) the pop rivets did not come out perfectly smooth and caused my door to hang up. i switched to solid rivets, and the inside metal helped everything come out nice.
4) I needed to use AN-3 screws and nuts, not the cheap hardware in the kit to keep the pivot from backing out.
In my mind, this whole alt air door needs to be rethought. Maybe a nice CNC base, door and pivot mechanism, or a butterfly valve. At least something more immune to the vibration.
Figured it out (I think). Based on our conversation a few weeks ago. As I recall, you have an Avastar servo, I have a FM-150 servo. My guess is that Carl has an AFP servo as well. My gasket came with the FM-150, NOT from Lycoming or Vans.
Desert Rat
Well Known Member
That must be it.
Are there any known cases of the alternate air door being used to get an RV back on the ground safely? I realize that an event like this might not make it into any of the official incident reporting databases, but perhaps someone has heard a story. Reasons I can think of to need the door are things like:
perhaps there are others...but the risk seems low to me based on the very limited information I have now.
- plastic bag covering air filter
- ice or snow covering air filter
- bird strike covering air filter
- too much dust clogging air filter
perhaps there are others...but the risk seems low to me based on the very limited information I have now.
Desert Rat
Well Known Member
Not on an RV, but I'm aware of two instances where alternate air was used on production airplanes. One was an early model 172R where the air filter turned to mush due to flying through heavy rain and the spring loaded alt air door did its thing. That one was told to me personally by the guys who were flying; Two very experienced (think cover of flying magazine in air to air shots) factory pilots. That was probably 20 years ago.
The other was even further in history and I don't remember which model, just that somebody told me that they had a bird strike that filled the filter full of feathers and bird guts.
So yeah, it doesn't happen very often, but when it does I'd want sone sort of alt air source in my plane.
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blaplante
Well Known Member
