B

Bob Axsom

Well Known Member
I was getting ready to reinstall my lower cowl after changing the oil and run-up leak check. Saw a funny looking fat washer on one of the platenuts at the bottom of the cowl. It was the magnet from the air filter by-pass magnetic latch. Everything else was missing from the the magnet holder part of this mod. Hopefully everything went out the air exit port but the rivet heads could easily get sucked into the engine if the by-pass door opens during operation. Something new to check periodically. I am certainly going to change the mount.

Note: this is the by-pass kit offered free by Vans to provide some safety by letting the engine suck open a magnetic latched door in the bottom of the airbox - Alternate air. I plan to mount the magnet with aluminum positioning strap (1/2" hole) sandwiched between a retainer (cover) strap and the fiberglass bottom of the airbox. I will use three screw washer nut stacks for attachment. Two screws will be round head (#8s) outboard of the flapper door with the heads in the airbox (I will just have to live with the flow turbulence) and the third will be a 256 flathead right through hole in the magnet with the head inside the airbox but under the door. A 3/48 will fit but none of the traditional aircraft suppliers have them. For now I am installing a 3/48 from Ace Hardware and jam nuts. This would be OK mechanically but I would like to use the screw to concentrate the magnetic flux at the door and eliminate the fiberglass (air) gap in the field. Ace only has Stainless Steel - no help. Aircraft Spruce has MS59959-5 screws that would work but there is no mention of the material (may also be stainless). After a Google search turned up nothing I ordered them any way. Anyone know what MS51959-5s are made of? Also are there any 256 lock nuts out there? If not I will use 256 jam nuts and loctite.
Bob Axsom
 
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FAB and the air bypass door.

When I got my FAB, I called tech support at Van's and we discussed this thing at length. The bottom line, this is an idiot-proof solution for pilots who just don't know when to use CARB HEAT! I left my parts in the bag they came in just in case a "future owner" may want to put it in.

If you trust yourself, get another fiberglass box and bottom plate for the airfilter and get rid of it. It will be cheaper than an engine. It could have a higher risk of creating an engine failure than NOT having the idiot's carb heat solution.

Jerry K. Thorne
East Ridge, TN
RV-9A N2PZ
 
Yeah but....

I trust myself but I do not trust the dinky little carb heat strap on that Van uses. It has resulted in one crash that I know of with an experienced aviator at the controls. The bottom line is, if the induction system (basically the air hole and the filter) gets a fast stuff job, being able to bypass both is a smart option to have. The implementation looks OK but at least in my case it does not hold up. I agree with your sentiments but I'm going to try to evolve this thing into something reliable rather than throw it out. I fully realize your choice may be the better one in the long run - no tick-tacky add on parts sucked into the engine, etc.
 
2-56 lock nuts

Bob, try microfasteners.com for the 2-56 lock nuts and stainless, steel, brass,
nylon screws.
For others the 2-56 flat head screws sure were handy, temp. for holding parts and skin when clecos were in the way during assembly of our 4
Ted
 
Source for nuts OK - screws maybe not

Thanks Ted. They do have a good assortment of small hardware at very reasonable prices. They have the lock nuts OK but I have a specific need for screws that can be magnetized. Stainless, aluminum, brass and nylon appear to be all they have in the 256 (2/56) size. I did query them on the availability of what I need.

Bob Axsom
 
Fixed!

I completed the mod yesterday and supplied failure and my fix photos to Doug Reeves and Van's Aircraft this morning. Note if you installed the by-pass kit you should check that the latch is still working by pressing on the door while the cowling is off. Comment about the carb heat system - because air from it also has to pass through the filter it is susceptible to induction ice problems - in other words it's air path is blocked by the clogged filter. The mod supplied by Van's appears to be a very good idea to me, allowing air to by-pass the filter and go directly into the carburator. The magnetic latch assembly was defficient in my case and I hav since modified it as noted earlier. It is not heated so no carb ice protection is afforded by the by-passing air.

Bob Axsom
 
Filter Bypass Door

I had the same problem; although I've come to a slightly different conclusion.
Went to change the oil in my RV-8 the other day and found the Filter
Bypass Door magnet retainer was missing. I found the magnet clinging
to the steel bypass door. It appears that I was lucky in that the
aluminum magnet retainer had simply fallen off and exited the cowling
through the outlet. The rivets had pulled through the thin fiberglass
on the bottom of the air box, and stayed with the retainer.

After pulling the plugs and inspecting the pistons and plugs for
possible damage (luckily none was found) I began to think about the
concept of the filter bypass door. I have an alternate air door
installed as per Van's drawings, but this is in the front of the air
box. Just before my first flight Van's came out with the Filter
Bypass Door kit, which I then installed. The intention of the bypass
door was to provide induction air to the engine if the air filter
became obstructed with ice.

Although I intend to fly IFR, I intend to stay well clear of icing
conditions. Should I inadvertantly find myself in those conditions,
selecting the alternate air door to ON will avoid induction icing.
The RV-8 fuel vents are also prone to icing, and this might well
cause fuel flow problems well before induction filter icing becomes a
problem. So, after some careful thought, I decided to remove the
Filter Bypass Door assembly. The air box was removed and the hole I
had cut for the bypass was carefully glassed over.

It's my feeling that the risk of ingesting the magnet, retainer or
rivets outweighs the benefit of the Filter Bypass Door.

Has anyone else had this problem or come to the same conclusion?

Jon Ross
N207RV - 51 hours
 
Bob Axsom said:
I completed the mod yesterday and supplied failure and my fix photos to Doug Reeves and Van's Aircraft this morning. Note if you installed the by-pass kit you should check that the latch is still working by pressing on the door while the cowling is off. Comment about the carb heat system - because air from it also has to pass through the filter it is susceptible to induction ice problems - in other words it's air path is blocked by the clogged filter. The mod supplied by Van's appears to be a very good idea to me, allowing air to by-pass the filter and go directly into the carburator. The magnetic latch assembly was defficient in my case and I hav since modified it as noted earlier. It is not heated so no carb ice protection is afforded by the by-passing air.

Bob Axsom
Click to expand...

I also know of a high time pilot who ended up with an off airport landing apparently due to carb icing. The RV6 suffered damage to the wings due to fence posts lining a narrow road, but has since been re-built. I'm not sure if his airplane had the U-bracket modication that allows additional cowl air into the inlet when the ram air door is closed. At the time, the FAA questioned the volumn of air allowed by the carb-heat by-pass, but the U-bracket which allows additional air in addition to the 2" pipe, apparently solves that problem.

But non the less, whether it's a bird or ice getting crammed into the one way ram inlet, I still like the idea of an optional air source, bypassing the air filter completely. And I do think of birds, since I also know another RV pilot who hit one and had to make an off airport landing into a field, which flipped the aircraft. The bird didn't go through the ram inlet, but then you never know! A bird could easily close off both the ram inlet as well as the carb heat inlet.

I'll also look into modifications for the magnetic latch, but the door is staying.

L.Adamson
 
One of the comments Van's has included in the installation instructions is, to be careful when riveting, not to dimple the metal plates when forming the shop heads on the rivets. It also warns of damaging the fiberglass with the MFG rivet head while riveting parts to the filter box. A squeezer is best used to assemble this bypass. Proper planning, and various yokes, will enable you to use a squeezer to set all the rivets involved.

Roberta
 
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The rivets were squeezed on N710BJ

Roberta, I understand your view but I disagree with the apparent conclusion. I squeezed the rivets and very carefully complied with the instructions in the installation of this kit. There is no question in my mind that the concept is good but the design needs revision to accommodate field experience that shows the operational risks that are introduced directly by this design. If these things arn't popping off like popcorn in the next few months I will be greatly surprised. The obscurity of the installation makes it highly probable that MANY aircraft with this mod have already failed and the owners are totally unaware of the condition. You have to remove the lower cowl and get down under the engine and then push the door to find out if anything but gravity is holding it closed when everything is sitting still. When you are flying along at speed and maneuvering there is no telling what the free flopping door is doing. I think it is a bad Idea to dismiss this with the rationale that it is a good design and all failures are due to builder incompetence.

Bob Axsom
 
Problem with Filter By-Pass

Bob:

I agree with you about the installation. Many RV owners may have the problem and not be aware of it.

It may be possible to reach in the cowling air outlet and "feel" the magnet reatiner. If any looseness is detected, then it will be cause to remove the cowling and investigate.

It is poor design practice to retain the magnet with the two small rivets as the Van's drawing depicts. This is a high vibration area, and when you inspect the steel door you will likely see evidence of fretting against the fiberglass. The two rivets that secure the magnet retainer are located in this area. Oversqueezing the rivets during installation may be a factor, but if the rivets are too underdriven, then the rivet shanks would not have stayed with the lost magnet retainer (as in my case), possibly then being ingested into the intake. Not good.

Your solution improves this, but to me the thin fiberglass air box floor will always be suspect.

It's my feeling that if I am inadvertantly flying in icing conditions I will open the cable operated alternate air door. This will prevent filter icing, as induction air will then be routed from the heated lower cowling area.

Best Regards,
Jon Ross
RV-8 80094
N207RV
 
Bob, I apologize if my post offended you. I did see apparent dimpling around your riveting and remembered reading the comment in Van's instructions cautioning about this. When I assembled mine, last year (March), I was careful not to produce said dimples in my assembly of this mod. Mine is still solidly intact after 100 hrs and 14months of flying.

It is very easy to check this mod through the opening in the bottom of the cowl. My 7A provides access along the exhaust pipes and also by removing the nose gear leg fairing. An inspection mirror and a flashlight will further enhance inspection of this area.

Roberta
 
No offense - good Inspection method

No offense, you made good points and your inspection method appears to be a very good approach.
 
Thanks, Bob. Your mod to the airbox sounds like something I will adapt next time the cowl is off. Happy flying.

Roberta
 
another FAB alt air failure

Bob,

Thanks for bringing up this topic. I did as Roberta suggested, looked up through the hole near the nose gearleg at the FAB alternate air trapdoor, and found the failure. The magnet holder was absent and there were two nice little holes where the rivets had been. The magnet was stuck to the door about half way back.

In spite of living up north, I'm planning on removing the alternate air door and glassing it over. The previous worry about having snow block the air filter doesn't seem relevant now. I've flown through light/medium snow near Omaha and through fine ice crystals over the Sawtooths and had no problem with the carb heat on. Both times I changed my flight path to minimize exposure to frozen precipitation, which is probably the most prudent course of action.

Mark
210 hrs on Hobbs... wondering how long I've been sucking up dirty air with the $20,000 vacuum cleaner.
 
Re: Another FAB Air Box failure

Mark:

This is exactly how I found mine; time in service was 51 hours. I suspect that the time in service before failure may vary due to differing vibration levels.

As I said in an earlier post, it would be prudent for builders to inspect this area through the cowling outlet on preflight. Easy to do and the consequences could be dire.

thanks for your input.

Regards,
Jon Ross
N207R


redbeardmark said:
Bob,

Thanks for bringing up this topic. I did as Roberta suggested, looked up through the hole near the nose gearleg at the FAB alternate air trapdoor, and found the failure. The magnet holder was absent and there were two nice little holes where the rivets had been. The magnet was stuck to the door about half way back.

In spite of living up north, I'm planning on removing the alternate air door and glassing it over. The previous worry about having snow block the air filter doesn't seem relevant now. I've flown through light/medium snow near Omaha and through fine ice crystals over the Sawtooths and had no problem with the carb heat on. Both times I changed my flight path to minimize exposure to frozen precipitation, which is probably the most prudent course of action.

Mark
210 hrs on Hobbs... wondering how long I've been sucking up dirty air with the $20,000 vacuum cleaner.
Click to expand...
 
Air Box idea

Guys (and girls) I agree with the potential failure mode of the alternate air door, especially since it seems to have occurred multiple times. I also think it is still a good idea. Prior to this thread I had assembled mine, but I also glued it on with some proseal to absorb the vibration and add some holding capacity to the 2 rivets. Since the proseal is fuel proof, I'm expecting this to work. I'll try to remember to report back as I get some time on it, which is still a couple of months off. I still have align the gear leg fairings, and fair the windshield, then do all of the paint prep work. Hopefully fly in time to make OSH.

Vic
 
alternative alternate air door?

The alternate air door, with its little rubber hinge, also struck me as a bit puny. Has anyone installed a manually operated door on a good, solid hinge and might have some photos to post?
Mark
 
I would stick with the reinforced rubber

I would avoid putting any metal hinges in this area because of additional metal ingestion potential. I think the designer thought about that and that is why we see the "unusual" hinge configuration.

Bob Axsom
 
glass magnet retainer

I decided to keep the alternate air door. Here is how I permanently(?)attached the magnet.
altairdoor13cp.jpg


Mark
 
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Door should not be on fiberglass?

I installed the bypass door 18 months ago on my RV-6A - so might not be remembering properly, but I'm almost sure that my door is installed on the aluminium plate on the bottom of the filter - thus no danger of the failures that have been described. It was a pain to sand the foam to provide a good entry profile, but it did not look too bad when I finished. Maybe I mis-read the instructions, but it appears that mounting the door on a metal plate is far preferable to mounting on a thin piece of fiberglass? I'll try to get some pictures over the next couple of days.

Pete
 
roberta thanks for the tip on easy access thru the nose gear strut opening. i checked mine and it appears ok at 115 hours. i like mark's fiberglassing over the magnet retaining plate and will probably do that at next oil change. fred
 
FAB bypass door

I emailed Van's about the problem so that they wouldn't get broad-sided by a bunch of crazed RV'ers at OSH... gotta give them a chance to check into it before getting all wrapped around the axle. Tom Green wrote back that Van had installed a micro-switch on the prototype bypass door also. It is highly unlikely that Van would offer the bypass door if it had been popping open during testing. So, does the door pop open only on certain engines?

The guys at Van's are looking over the info(apparently several different types of solutions have been emailed to Van's) and hopefully will comment on it.

Haven't glassed over the hole yet, but thinking about it. I always put carb heat on when flying in snow/ice crystals/freezing fog. And the odds of sucking up a bird... yes, it could happen, but is probably rare as hen's teeth.
 
Thanks, Fred. So far mine has been OK. I've been making it a semi preflight item. First sign of trouble, it goes. I wasn't real happy about it in the first place, but I figured it should be OK since it was a Van's design.

Roberta
 
O-320 or O-360?

May be this is a dumb question, but, I am running an O-320 where the filter does not reach to the bottom of the fiberglass FAB enclosure. Are you guys who are having problems all running O-360s? Is an O-360 filter deeper, or the FAB thing thinner? I have an aluminum plate on the bottom of my filter, the door is in that plate, no chance of it breaking away. The plate is about 1/2" off the bottom of the fiberglass FAB thing. So, is this only an O-360 problem?

Pete
 
FAB issues

It seems the issues are:
1) rivets popping out (from the magnet retainer)--easily fixed, see photos in this thread
2) whether or not the door opens during normal operations--if not, no problem, if so, big problem. Sure would be neat if someone could mount a mini-camera to actually observe the door during normal operations.

Penguin, if your air filter doesn't extend down to the FAB, how does it seal? When building my FAB, it required squeezing the top plate down onto the filter while it was in the FAB and then drilling the FAB/top plate. I'm running an O-360. Maybe you've got a different set up? Any photos?
 
FAB on an O-320

My FAB has an oval plate that bolts to the bottom of the carb, the filter then is bolted up to the plate (plate is larger than the filter) using long through bolts bearing on another plate - that I have built the bypass door into - that fits into the lower side of the filter (care is required when tightening the bolts as its possible to start to crush the filter). The FAB itself then bolts up to the top plate. I have had to mould an intake lip into the bottom of the FAB to mate up to the bottom plate to feed the bypass door if required.

I'm guessing that in order to reduce the height/ugliness of the O-360 cowl bulge Van's have tried to make the O-360 FAB as low profile as possible. I'll try to get some pictures this evening.

Pete
 
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