Problem with Air Filter By-pass

[Bob Axsom]

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

[n2prise]

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

[Bob Axsom]

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.

[Ted Farmin]

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

[Bob Axsom]

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

[Bob Axsom]

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

[DeltaRomeo]

Bob (and all), the link to the pictures is:


http://www.vansairforce.net/builderm.../airbypass.htm

[avpro56]

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

[L.Adamson]

Originally Posted by Bob Axsom

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

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

[robertahegy]

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