Alternate air system failures

Some years ago Vans introduced an air induction bypass vent in the bottom of the filtered air box (FAB) on all models. The idea was to provide an option to get unfiltered air from the lower cowl to the engine in the event of an induction blockage at, or prior to, the filter in either carburetted or FI engines. The introduction of the bypass vent reportedly followed a couple of serious RV accidents involving induction blockages.

Vans first attempt was a magnetically held hinged flap vent. It was not one of his better ideas and there were many failures reported. Then a couple of years ago he scrapped the magnetic vent and replaced it with a pivotting vent which is still being supplied today on all FWF kits.

I note however that on August 30, 2006, John Ross reported that his new pivotting bypass vent had totally failed after only 50 hours of operation on his RV8. I have attached a photo supplied by John which clearly shows the classic aluminium stress crack which caused the vent to depart the FAB. Bear in mind that this material is .063" thick.

This is a critical component. No-one would want any parts from this component failing and being sucked up into the engine.

It is now 18 months since John's incident and it would be interesting to know if any other builders have experienced failures of the pivotting bypass vent (or operational problems of any kind).

For those with fuel injected engines in particular (no carbie heat bypass) it would seem to make sense to have an alternate air source, particularly if flying IFR. But if the bypass mechanism is dodgy the cure may be worse than the ailment.

Any feedback out there ???

Left click on photo for enlargement.

I wouldn't expect this part to crack there if it were built to plans and in the closed position. On the other hand, it seems vulnerable to cracking if it were left in the open position. Do we have any info on the particulars of how this happened?

Captain Avgas said:

But if the bypass mechanism is dodgy the cure may be worse than the ailment.

Click to expand...

I agree completely. I had one of the early failures of the old magnetic door. When the fix for the fix came out, I made a plate to cover up the whole, screwed it down and never looked back.

Between this and the infamous fuel pick up service bulletin, I will never again take a Van's SB for granted and will give serious consideration to whether future SBs should be complied with.

My $.02.

I'm not planning on installing mine. How many other airplanes (Cessnas, etc) have a filter bypass?

All of the fuel injected single engine Pipers that I can think of have an alternate air system

lostpilot28 said:

I'm not planning on installing mine. How many other airplanes (Cessnas, etc) have a filter bypass?

Click to expand...

All certified airplanes have an alternate air source. I have the horizontal Airflow Performance FI with their intake with an alternate air door. Don

lostpilot28 said:

I'm not planning on installing mine. How many other airplanes (Cessnas, etc) have a filter bypass?

Click to expand...

It's for when that little birdie or clump of slush jams up the ram air inlet tunnel along with the carb heat entrance. I don't believe Cessnas are plummed this way. I changed mine to the new method, and prefer keeping it.

L.Adamson

Doh! I knew you guys were going to show me how little I really know about certified airplanes. So are those mainly just FI engines? I don't recall ever seeing a bypass knob on the few types that I've flown. If they were there, no one showed me!

When to use the filter bypass?

lostpilot28 said:

Doh! I knew you guys were going to show me how little I really know about certified airplanes. So are those mainly just FI engines? I don't recall ever seeing a bypass knob on the few types that I've flown. If they were there, no one showed me!

Click to expand...

Sonny,

When applying carb heat on the Cessnas, the filter is bypassed by default. Our RVs only provide warm air (carb heat), still through a filter though. Any obstruction (bird, garbage bag) will block the regular and heated air flow. That is why there is an additional filter bypass.

One of Vans support personnel says that he had heard of one individual whose engine stopped because the filter box ingested a plastic bag while in flight. I think that event prompted the filter bypass SB.



My problem with the filter bypass system is :I take it that the first step to take of engine problems is to apply carb heat to resolve the issue (assuming you are not running low on fuel). If no improvement after a short while (30 seconds?), or if engine is stopped, then apply the filter bypass and restart engine if required.

Anyone have another procedure to recommend? There will definitely be a section for this in my operating manual.

Not all.

yakdriver said:

All certified airplanes have an alternate air source. I have the horizontal Airflow Performance FI with their intake with an alternate air door. Don

Click to expand...

I know my Aeronca did not. Carb heat dumped into the same airbox and any obstruction was still in the way.
I did not install any bypass in my 6. The odds of the mechanism causing a bigger problem, like injesting a loose rivet, out wieghed the odds of having a blockage. That was my choice, not meant to be an argument one way or another.

Let me rephrase. Certified aircraft with fuel injection have an alternate air system. Carb equipped aircraft have a carb heat system that can be filtered or unfiltered but most can also operate as an alternate air source. Don

Great timing. I just installed mine tonight!!!!!!!!!!!

Jekyll

One in a Million?

yakdriver said:

Let me rephrase. Certified aircraft with fuel injection have an alternate air system. Carb equipped aircraft have a carb heat system that can be filtered or unfiltered but most can also operate as an alternate air source. Don

Click to expand...

Bingo well put. The down side of vans new cable alternate air door, small parts coming loose and getting sucked up, into the engine. I have a FAB 360 and got the kit, but have no plan to install it. Van's service letter says installation is optional, especially if you don't fly in freezing precipitation: snow, sleet, freezing rain.

True story: I know of one Mooney driver that kept the RAM air alternate air inlet open in IMC, wet snow. It choked the engine and it shut down. He glided from the low teens to warmer temps and VFR before the engine started. The Mooney RAM air feature is unfiltered. Apparently the wet snow blocked and re-froze in the TB.

Chance of blockage in VFR, no precipitation, is One-in-a-million? The plastic bag covering the scoop or bird going into the airbox and blocking could happen.

True story: Bird up the air box did happened to a RV! There is an old article with pictures in the RVator to prove it. The incident involved Vans original unfiltered air box (RV-3, RV-4 days). Van's FAB designs did not come along until the late 80's, RV-6 time frame. The original unfiltered airbox was just a straight shot from the scoop into the carb, with a 90 degree turn at the carb. No filter or carb heat. The pilot took off and the engine started to run rough at reduce power. He landed. There was no sign of trouble upon first inspection on the ground, until the cowl/air box where removed. Upon further inspection, looking up into the Carb, feathers and bird parts where discovered, jammed in the carbs venturi! It was not enough to stop the engine I recal. True story! This may have been the motivation for Van to make the new air box. I helped two guys convert their RV's to FAB airboxes, which includes a new scoop. I'm not sure how the newer FAB designs would do in this case. Depends on how big the bird was and how far into the airbox it got.

Snow and ice are the biggest threat to the FAB boxes, I think. If you are not flying in freezing weather (and really should not be), than the second Alt air is not critical. If you have a Carb do put the Carb heat / Alt air in, in my opinion. As weak as the carb heat may be, it's better than nothing. Carb ice is rare in RV's but not unheard of. If the conditions are right a long let down at low power could doom you. One RV'er learned the hard way. Keep the power (and heat) up.
http://www.ntsb.gov/ntsb/brief.asp?ev_id=20030110X00044&key=1
http://www.ntsb.gov/ntsb/brief.asp?ev_id=20021104X05426&key=1

szicree said:

I wouldn't expect this part to crack there if it were built to plans and in the closed position. On the other hand, it seems vulnerable to cracking if it were left in the open position. Do we have any info on the particulars of how this happened?

Click to expand...

Let's get back to the original question. Any known new FAB door failures. I too would like to know the particulars in this failure: was he operating this door as a pre flight item on his checklist? How much pressure did he have pushing on that tab. It doesn't take much to hold this door closed. I think we need to know some more particulars.

Looking at how it sits and is supported on the opposite side with the extrusion, I just see this as surprising. You clamp the cable down on the bottom of the FAB itself so the cable tension pushing on that tab should move with engine.

The part in the picture looks like it was blue anodized?

If that is correct did it come from Vans like that?

The one on my acft is untreated.

The reason I ask is anodizing that part could make it more likely to fail as it did in the picture.

The accident below is a good reason for alt air. It provides a secondary path for induction air.

http://www.ntsb.gov/ntsb/brief2.asp?ev_id=20010308X00560&ntsbno=LAX01LA110&akey=1

szicree said:

I wouldn't expect this part to crack there if it were built to plans and in the closed position. On the other hand, it seems vulnerable to cracking if it were left in the open position. Do we have any info on the particulars of how this happened?

Click to expand...

Steve, I suspect that you are right. It always seemed highly likely to me that this cover failed because it was flown in the open position.

However the question that might be relevant is whether the current design provides enough detente to keep the cover closed. I have measured this device and it only requires a half an inch of pivotting movement for the cover to pop open....that's not much....and it assumes the cover was 100% engaged in the first place. There's a whole lotta shakin' goin' on down there...if things can come loose, they will.

It is empirically observable from the John Ross photo that if the cover comes open it WILL crack and fail with time. The only question is how long it will take to crack....will it take 10 hours of engine run time, or 10 minutes.

I have emailed John Ross and asked him if he would be prepared to comment further on his experience.

I have also had an exchange with Ken Scott at Vans. In his last comment to me he stated: "FWIW, helped take our RV-10 apart for annual last night and specifically looked at bypass door. After 500 hours it looks brand new...no cracks, elongated holes, etc."

I think this is a matter that would weigh more heavily on those who are opting for fuel injection because without the alternate air device they have no redundancy whatsoever in respect of the air induction system. Those with carburettors have the carbie heat device which is a bypass of sorts and they can apply some heat to help prevent frozen particles from blocking the filter.

The question in my mind now is whether this device has a propensity to pop open. I suspect that if the cover moves just enough to disengage it might not be obvious from the cabin.....it might not be noticed until the next under cowl inspection.

I wonder if any flyers with this device have done an inspection and found the cover to be not properly engaged in its detente.

I'm certainly thinking that I wouldn't want to place the pull knob for this device anywhere near the cabin heat knob and have a passenger pull on it by mistake unbeknown to me....and some time down the track the device disintegrates causing my engine to ingest metal.

asav8tor said:

The accident below is a good reason for alt air. It provides a secondary path for induction air.

http://www.ntsb.gov/ntsb/brief2.asp?ev_id=20010308X00560&ntsbno=LAX01LA110&akey=1

Click to expand...

As I remember, it is this accident, that's the reason for the alternate air door. I know it was California, over mountainous terrain, and two people. I don't see any others on the NTSB reports that fit the criteria & time frame. It's exactly the reason, I installed and am keeping mine.

L.Adamson --- RV6A

Captain Avgas said:

The question in my mind now is whether this device has a propensity to pop open. I suspect that if the cover moves just enough to disengage it might not be obvious from the cabin.....it might not be noticed until the next under cowl inspection.

I wonder if any flyers with this device have done an inspection and found the cover to be not properly engaged in its detente.

I'm certainly thinking that I wouldn't want to place the pull knob for this device anywhere near the cabin heat knob and have a passenger pull on it by mistake unbeknown to me....and some time down the track the device disintegrates causing my engine to ingest metal.

Click to expand...

Unless someone else checks first; tomorrow, I will pull the fiberglass air inlet on the cowl, since mine is seperate from the bottom cowl (was originally two piece, and screwed on); and then move the door a bit, and check for noticeable movement on the knob.

My air door knob is on the far right side, and has a big red "For emergency use" lable next to it.

L.Adamson -- RV6A

I respect your opinion, but....

L.Adamson said:

As I remember, it is this accident, that's the reason for the alternate air door. I know it was California, over mountainous terrain, and two people. I don't see any others on the NTSB reports that fit the criteria & time frame. It's exactly the reason, I installed and am keeping mine.

L.Adamson --- RV6A

Click to expand...

This was a Carburated engine and there is no mention of what caused the accident, only an assumption of icing? , perhaps I missed it? In fact, it seems to indicate that a alternate air device was present on the airbox, so I am confused as to the reference here.
"The air box design includes a pilot-actuated alternate air door for simultaneously blocking off outside inlet air and opening an alternate heated air source. The alternate air source can be either heated air directly from the warm air plenum of the lower engine compartment, or from an exhaust manifold heat muff, at the discretion of the aircraft manufacturer (builder). This design permits the pilot to supply either outside ram/pressure recovery air to the carburetor, or to completely block the outside ram air and instead supply heated air from inside the engine cowling, or from an exhaust stack heat muff, to the carburetor. The heated air is ducted to the front side (upstream) of the air filter."

"Review of photographs taken at the accident site revealed that the accident aircraft had a carburetor air box similar to the kit sold by Van's Aircraft installed. However, it is not known as to what method of heated alternate air source and routing was utilized by the builder."


I believe this unfortunate occurance was flying into known bad conditions and not having the sense to turn around (no disrespect intended to the departed). Not sure a bypass would have saved them, certainly would not have helped an iced up carb if they did not apply carb heat in time, or at all.
I do agree, if you intend to fly this way, into IMC with possible, and even predicted, snow and ice, you better have a lot of back ups, including a alternate air bypass.
Personally, I am not qualified nor do I have that big of Kahuna's, (no offense to Kahuna). Hopefully, I wont suck a plastic bag or a bird into my intake anytime soon.

Captain Avgas said:

I'm certainly thinking that I wouldn't want to place the pull knob for this device anywhere near the cabin heat knob and have a passenger pull on it by mistake unbeknown to me....and some time down the track the device disintegrates causing my engine to ingest metal.

Click to expand...

Exactly!! I put mine on the left side of the panel, the passenger has the cabin heat, and that's it, I have everything else. I didn't want any inadvertent pulls. As to not having any inside indication the door is open, I'm thinking that is remote at best. The clamp for the push/pull cable is very close to the contact point on the cover. Any movement is going to be indicated by the black knob in the cockpit being extended. I have, in my preflight, Check Alternate Air control full in.

I checked my door yesterday, tight as can be.

L.Adamson said:

It's for when that little birdie or clump of slush jams up the ram air inlet tunnel along with the carb heat entrance. I don't believe Cessnas are plummed this way. I changed mine to the new method, and prefer keeping it.

L.Adamson

Click to expand...

Everything is about risk. In my VFR ship, I'm not so worried about slush. Heck, if I was IFR, I'm not sure I would worry about it either because I would hope not to fly in ice.

As for that little birdie, well, I don't have a shield over my canopy just in case I take one their and the chances are much greater of getting one in the face due to the size of the canopy than up that little air intake.

JM2CW

As you might have guessed, I did not install the alternate air.

Alternate Air

szicree said:

I wouldn't expect this part to crack there if it were built to plans and in the closed position. On the other hand, it seems vulnerable to cracking if it were left in the open position. Do we have any info on the particulars of how this happened?

Click to expand...

Steve:

The photo of the cracked door is mine. Although it looks to be blue anodized, it is not; the low light when the photo was taken gave it a blue tint.

The door was constructed to plans, and was in the closed position when it failed. The reason for the failure is that the door is supported by a single screw on one side. The mass of the door held in check by the single screw in a cantilivered manner caused the cracking. This is poor engineering design and practice, especially when you consider that it is attached to an extremity of a reciprocating engine. Shaking during startup and shutdown would probably be enough to crack the door over time. Mine failed at 50 hours.

I had installed the magnetic door, and when that failed I installed the replacement kit even though I did not like its design. I decided to install it and monitor it very carefully, looking at it before and after every flight. (easy to see in through the cowl outlet) When it did fail, I was lucky that no small piece of aluminum broke off and was drawn into the intake.

I removed the alternate air assembly and glassed over the hole in the airbox while I thought about a better way. I never replaced the alternate air assembly, instead I have begun an installation of an new ECI magnesuim cold air sump with a forward facing inlet. A new cowl without a scoop will be fitted, along with a new cooling plenum. This required a new 4 into 4 exhaust, and all new mounting brackets and cables for throttle and mixture.

I assisted a local RV-7 builder when we designed a new alternate air inlet door that is cable operated. The door slides on slotted UHMW plastic blocks that have a slot milled into them. The door can be opened or closed from the cockpit by actuating the cable. It is self lubricatiing and supports the door on three sides when it is closed.

Jon,
For me this is also a timely thread.
Any pictures of the UHMW door alternate air. I just fitted the snorkel on a forward induction Superior but haven't cut the hole for the alternate air. After this post I don't believe I'll install the alternate air system provided. I was thinking of using a throttle plate that rotates open and closed with the pull/push of a cable.

N941WR said:

As for that little birdie, well, I don't have a shield over my canopy just in case I take one their and the chances are much greater of getting one in the face due to the size of the canopy than up that little air intake.

As you might have guessed, I did not install the alternate air.

Click to expand...

Well, at least two people I personally know, have got the little birdies in the canopy/wind screen.

One did an off field landing and flipped (RV6); and the other, which was a 172, would hardly fly due to intense drag.

I kind of like the idea of an "alternate". Same as having two spark plugs for each cylinder....

L.Adamson

avpro56 said:

The door was constructed to plans, and was in the closed position when it failed. The reason for the failure is that the door is supported by a single screw on one side. The mass of the door held in check by the single screw in a cantilivered manner caused the cracking. This is poor engineering design and practice, especially when you consider that it is attached to an extremity of a reciprocating engine.

Click to expand...

That's not correct. The door is also supported by a tight fitting metal catch, on the oppsite side from the screw; in which the door plate slides in to.

L.Adamson

I have to agree with Adamson, there should be no pressure on that attach point because the top of the door sits in a little cradle. I have check mine and found no signs of any stress, maybe your cable is fastened down to tight.

avpro56 said:

The mass of the door held in check by the single screw in a cantilivered manner caused the cracking.

Click to expand...

Dear Jon, thank you very much for sharing your information with us.

As others have pointed out the door is not cantilevering off the screw when it is in the closed position....it is supported on the opposite side to the screw by a detente lip. However the builder has to form that lip themselves by folding back the supplied collar in that area.

Is it possible that your collar did not have the lip and that your door was in fact truly cantilevering off the one screw.

I have attached a photo of the drawing of the alternate air vent, and I have labelled the pivotting screw and the detente lip (to be folded by builder).

This will give some insight into the mechanism so that we can all be sure we're talking about the same thing.

Left click the photo for larger size and better resolution.

Captain Avgas said:

As others have pointed out the door is not cantilevering off the screw when it is in the closed position....it is supported on the opposite side to the screw by a detente lip. However the builder has to form that lip themselves by folding back the supplied collar in that area.

Is it possible that your collar did not have the lip and that your door was in fact truly cantilevering off the one screw.

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Click to expand...

It could also be posible to have the tab not fitting tightly against the door. The door wouldn't have to move very much under vibration to cause a fatigue failure at the screw location

Captain Avgas said:

As others have pointed out the door is not cantilevering off the screw when it is in the closed position....it is supported on the opposite side to the screw by a detente lip. However the builder has to form that lip themselves by folding back the supplied collar in that area.

Click to expand...

Mine is different. However it was one of the first replacement kits. It has a seperate pre-bent door catch, that's rivited onto the main plate, opposite of the holding screw.

L.Adamson

rvbuilder2002 said:

It could also be posible to have the tab not fitting tightly against the door. The door wouldn't have to move very much under vibration to cause a fatigue failure at the screw location

Click to expand...

Welcome aboard Scott. Your comments on design matters are generally astute (except of course on the nosewheel issue where you were still grimly defending the good ship "Status Quo" even as Captain Van was similtaneously hatching his plan to scuttle the boat with his Service Bulletin....but never mind, everyone is entitled to one mistake).

I think you're right about the danger of not having the tab pressing FIRMLY on the door. It might result in movement that could induce cracking...it might also allow the door to come open more easily when it is not supposed to. If the door moves inadvertently beyond its 1/2" lip there is CERTAIN to be stress cracking over time, either of the door or of the piddling #8 pivot screw (the nut could then ingest into the engine).

Personally I want an alternate air device because I fly a fair bit of IFR. But I have nagging concerns about this particular mechanism. I think its problem is that it may lack sufficient detente area at the tab (1/2" may not be enough). With time and vibration that tab may lose its grip.

There must be a lot of these vents flying now. Is every one a happy camper out there. Chime in if you've had any problem with this device coming open when it's not supposed to. Conversely, I invite those flying to chime in if they have a few hours up and the vent is operating fine.

L.Adamson said:

Mine is different. However it was one of the first replacement kits. It has a seperate pre-bent door catch, that's rivited onto the main plate, opposite of the holding screw.

L.Adamson

Click to expand...

Why was the pre-bent catch ditched. Were there previous problems.

Scott, do you know why.

Alternate Air

L.Adamson said:

That's not correct. The door is also supported by a tight fitting metal catch, on the oppsite side from the screw; in which the door plate slides in to.

L.Adamson

Click to expand...

Yes, I am well aware of the metal capture latch on the opposite side. The latch is small, and the door coming out the latch not is so hard to comprehend. Mine was built carefully with the latch supporting the side opposite the pivot screw.The cover still broke where the photo shows. But I suppose there will always be those that think it was installed incorrectly and therefore the reason for the failure.

All in all, Van's pivoting door solution is clever and low tech but not a durable way to supply alternate air. Nor was the magnetic door; that failed on me as well. After both alternate air designs failing I am not so impressed with the solution for alternate air as offered by Van's. But they do so much else so very right that I find it hard to be otherwise critical.

I do think it is a good practice to have an alternate air source; but the risk of something breaking off and being ingested has to be taken into account during the design of any alternate air device. Many have chosen to fly without an alternate air source rather than install the current kit as offered by Van's. Ask a few of Van's employees if they chose to install this kit on their personal aircraft; the answer will surprise you.

The best way to have alternate air is to have a device that can be opened and closed from the cockpit; it must be simple, and be constructed so that the chance of ingesting a screw or other part is remote. A sliding door as described in my earlier post does just that. Sorry, I have no photos as it's not on my airplane as I've gone to horizontal induction. I'll try to get hold of some photos.

Roll your own; that's why they call them experimental. Of course, your own experience may vary.

avpro56 said:

Ask a few of Van's employees if they chose to install this kit on their personal aircraft; the answer will surprise you.

Click to expand...

Just lay it on the line.....

I can't stand these little secrets..

L.Adamson

stainless??

I just wonder if making the hatch out of stainless instead of aluminum would improve the durability of the part. That approach has worked well with the wheel pant brackets....

2 cents worth....Chris

carguy614 said:

I just wonder if making the hatch out of stainless instead of aluminum would improve the durability of the part. That approach has worked well with the wheel pant brackets....

2 cents worth....Chris

Click to expand...

Or a partial doubler plate could be rivetted to the door to stiffen it up in the hinge area. But I'd be concerned that either of these solutions might just transfer long term bending forces back into that little #8 screw causing it to fatigue and fail (and allow the free nut to ingest into the engine).

The answer is to prevent the door from moving out of that little tab detente. But it's an awfully small tab and there's a lot of engine shaking going on to dislodge it, particularly at shutdown.

In my opinion Jon Ross's experience has indicated that the current alternate vent is not a failsafe design. And any device attached to the induction system that could shed a component into the engine if it malfunctions needs to be failsafe.

We should be indebted to Jon for bringing his vent failure to our attention so that we may individually address the situation as we see fit.

Jon Ross has emailed me the attached pix of a custom built alternate air vent on his friend's RV7. It is a sliding vent using UHMW grooved blocks and an aluminium door.

The builder was not happy with Van's design and wanted something he could feel more comfortable about.

It is relevant that this design can be opened AND closed from the cockpit (unlike Van's design which cannot be closed from the cockpit once it has been opened).

In my opinion this design could be improved slightly in a couple of areas but it is definitely heading in the right direction. I like the sliding arrangement.

Left click on pix for enlargement.

I've got a couple of observations on this approach. First off, the blocks and hardware are going to be heavier than the standard version. This could be important in light of another current thread regarding cracks in the airbox mounting plate. Also, are the attaching bolts anchored inside the airbox to prevent ingestion in the event of a nut coming loose?

szicree said:

I've got a couple of observations on this approach. First off, the blocks and hardware are going to be heavier than the standard version. This could be important in light of another current thread regarding cracks in the airbox mounting plate. Also, are the attaching bolts anchored inside the airbox to prevent ingestion in the event of a nut coming loose?

Click to expand...

Hi Steve,
I'd be confident that the sliding principle could be executed with a design that was SIGNIFICANTLY less bulky. I'd be tempted to consider fabricating a lightweight perimeter frame from 2 stepped aluminium sheets. But even in UHMW it could be much less bulky.

I don't think those metal locknuts are ever coming undone. They're the same type used on engine mount bolts. But you might be more comfortable with castle nuts and pins.

The fastener that I'd be worried about ingesting would be that little #8 screw that Vans uses.

"Automatic" Aux Inlet

Chris Heitman designed and installed an aux inlet door for his RV-9A. His design is actuated by the negative pressure resulting from a blocked intake. The well thought out design is presented with pictures and testing he did using a shop vacuum to simulate a blocked inlet. Take a look at:

http://my.execpc.com/~cjh/altair.html

Superior design!

rbregs,

All we need now is a little kit with the parts, it will be cheaper than the Vans contraption (hey, the push-pull cable alone costs $ 25,- !) and it will be lighter also (no push-pull cable!). Additionally it will work better because it will work immediately and without the need for the pilot to control it! Between the engine quitting, the pilot trying to find out what is wrong, coming up with the idea of opening the alternate-air-door and hopefully restarting the engine, there might be several valuable seconds, if not maybe half a minute! Now where are these birds, plastic bags, etc. most frequently to apear?: at take-off and landing altitudes??, right! In that case half a minute or even a few seconds may be to late! So...... this automatic-alternate-air-door could save your butt!
I wish I knew what these springs were!

Regards, Tonny.

Reviving old thread: Insidious Failure Mode Hypothesis

I'm reviving this old thread for 2 reasons:

1. To propose a new hypothesis for the failure mode of Jon Ross' alternate air mechanism.
2. To determine whether there have been additional failures of the alternate air mechanism in the 20 months since this was last discussed.

Jon Ross had a fatigue failure of the alternate air door which several astute readers have noted would be unlikely to happen if the door were properly engaged in the metal "capture latch" or "catch" opposite the screw.

Jon notes that he was careful to build this properly per plans, so that the door fit tightly into the catch:

"The latch is small, and the door coming out the latch (is not) so hard to comprehend. Mine was built carefully with the latch supporting the side opposite the pivot screw. The cover still broke where the photo shows. But I suppose there will always be those that think it was installed incorrectly and therefore the reason for the failure".

So here is a hypothesis of a failure mode that I propose would fit the data - that is, that Jon built the door carefully, so that it fit tightly into the catch, but that the door still experienced a fatigue failure through no fault of Jon's.

Kevin Horton notes on his website a couple of months ago that his prop control was noted to have a different coefficient of thermal expansion than its sheath, so that there was relative motion of the prop control lever on the engine after the engine warms up WITHOUT MOTION OF THE PROP CONTROL IN THE COCKPIT. The result was that when the engine was cold, he was able to achieve full rpms. When the engine warmed up, his maximum rpms decreased by 100. After careful (and painstaking) sleuthing by Kevin, he concluded that the differential expansion of his control cable resulted in a fairly significant rpm loss when his engine reached operating temperature. See:
http://www.kilohotel.com/rv8/article.php?story=20090918205911519

I think everybody agrees that fatigue failure of the alternate air door could occur if the door were not tightly seated against the catch (or latch), since relative motion from vibration could occur if the cable were backed off even slightly. So what I have been wondering is whether the control cable for the alternate air door, when it heats up, could have a similar effect to what it had on Kevin's prop control: namely, that the cable is effectively "pulled" a little bit, even though the cockpit knob is untouched, allowing the door to disengage slightly from its catch, thereby inviting vibration-induced relative motion, fatigue, and ultimately failure. This would be a very insidious failure mechanism since things would always look normal (door tight against catch) during inspection with the cowl off when things have cooled down. Yet every flight, the alt-air door could have loosened up, and been shaking for the 50 operational hours it took to fail.

So here's a further thought: perhaps a test could be performed to determine the relative motion of the Vans cable at operating temperature, and then determine whether this motion is sufficient, given the mechanism geometry, to cause the door to become unseated at the latch.

Have there been any alternate air failures since Jon's failure that anybody is aware of?

It is not possible to reclose it securely without dropping the cowl and doing it by hand. Once opened, the cover drops slightly and won't go back into the catch.

By the way, this revival comes at a great time for me because I was just about to go looking for this or a similar thread. I am doing the first annual of my -6A and this component has generated the only major squawk. I discovered that the metal ring has been resting on the lower cowl for an indeterminate period. I glued and riveted it per the plans but the ring simply pulled away from the fiberglass bowl. For awhile it was apparently supported by the actuator cable and clamp, until that, too, separated by having the nutplate pull through the bowl. No damage to the cover plate during all this, so I am going to give it another try. I have made a backing ring to reinstall the inlet; it sandwiches the bowl material between itself and the inlet ring. I'm glassing a patch for the cable nutplate and will return it to service and then inspect it pretty regularly to see what happens.

Other solution.....

I liked the idea of an automatic opening door, but went for the following solution (post#6)
http://www.vansairforce.com/community/showthread.php?t=48852

Regards, Tonny.

flion said:

By the way, this revival comes at a great time for me because I was just about to go looking for this or a similar thread. I am doing the first annual of my -6A and this component has generated the only major squawk. I discovered that the metal ring has been resting on the lower cowl for an indeterminate period. I glued and riveted it per the plans but the ring simply pulled away from the fiberglass bowl. For awhile it was apparently supported by the actuator cable and clamp, until that, too, separated by having the nutplate pull through the bowl. No damage to the cover plate during all this, so I am going to give it another try. I have made a backing ring to reinstall the inlet; it sandwiches the bowl material between itself and the inlet ring. I'm glassing a patch for the cable nutplate and will return it to service and then inspect it pretty regularly to see what happens.

Click to expand...

I just finished my 3rd annual. I have had the magnetic door, which has required work at least every year. This year, the magnet itself had vibrated off (the 2 rivets holding it apparently came apart in spite of the epoxy around them (I found it sticking to the nosewheel leg).
I figured I'd finally install the 'new' retrofit door. Given my experience with the high vibration and resulting disintegration of components down here, I modified the install. I glassed in the metal ring to the airbox on the outside, keeping room for the door to still swing. Then, I riveted the ring on with another metal ring on the inside of the airbox. I considered using small pieces of aluminum to support the rivets on the inside, but figured they could come loose and get sucked into the engine. So, I made an aluminum ring for the inside that supported ALL the rivets. That way, no single rivet could let anything loose, and the bigger piece of metal is too large to get sucked into the engine. I'll be watching it closely for a while to see how (if) it all stays together.
As far as the door itself, I inadvertently used a nutplate for an AN3 bolt, so I drilled the hole in the door bigger and have a larger piece of hardware in this important area. I made sure the catch holds the door securely, too.

Seb Trost
RV-7A 360 hours

So far it looks like I prefer the "slider" design the best and probably will try making one to fit on the side of my RV-8's snorkel.

Just curious, has anyone thought to try using "Reed Valve" material bonded to the inside surface. That way it is similiar to the magnetic door but without all the metal parts count. It should remain closed unless a certain amount of suction was present to open them when the filter became blocked.
Reed valves on 2 stroke motorcycles flex open and closed thousands of times without pre-mature failure, so I would think it would last forever as an alternate air doorway given it will only open when a blockage occured.

Just pulled my plane out for condition inspection, 102 hrs and the Van's sliding door alt air is trashed... glassing up the hole and taking it out.

LifeofReiley said:

Just pulled my plane out for condition inspection, 102 hrs and the Van's sliding door alt air is trashed... glassing up the hole and taking it out.

Click to expand...

I wasn't talking about the round sliding door that van's supplies.
I was talking about a sliding plate that tracks along a grooved channel.

LifeofReiley said:

Just pulled my plane out for condition inspection, 102 hrs and the Van's sliding door alt air is trashed... glassing up the hole and taking it out.

Click to expand...

I put larger "solid" rivets in mine at 80+ hrs, as a few had come loose. At 137 hrs, they're still there. I plan to leave it..........at this point. It's just that redundancy thing, since that one hole is the only source of air with out it.

L.Adamson --- RV6A

Well... I was a couple of hours from digesting three rivets into my engine, and we're not going there again. I might say, it was installed per the plans... bad idea and a **** poor design.

The magnet system

I installed the magnet latch system long ago and it was failing but I saw it before it went all the way. I developed an aluminum mount for the magnetand installed it with SS screws. That worked fine. Then I got the new mod and layed it on the shelf never to be installed. One of the original failues was due the induction ice and it was written up in some detail where the author was called by his buddy using his cell phone to report that he had landed off airport with damage and injuries and needed help. I have kept my modified magnet latched door in the bottom of the FAB but for racing I lock it down with an aluminum angle and a #8 screw and platenut. I have to admit I have it locked down most of the time.

Bob Axsom