Filter by-pass: Is it necessary?

I'm working on my FAB with the new lever-operated bypass and I was wondering what the point of the bypass is. The only possible scenario I can imagine that would make it necessary would be swallowing a huge bird into the cowl scoop. Are clogged filters common enough to necessitate the bypass?

I'm going to install it regardless because it's what's recommended...but just wondering why of all things in the airplane that don't have a backup, why does the air intake get a backup? Just because it's a trivial thing to add?

We don't

Jamie,
We don't have a bypass because the original ones had a magnet. If the intake became plugged, a door would get sucked open. Some of these fell apart with engines ingesting metal, so I decided not to have that possibility and weighed that against a big bird or whatever.

IF a bird were to get into the opening, I figure that since the FAB box gets a lot wider, the bird would not be big enough to block the entire inlet.....we shall see,

I am aware of two RVs that had off-field landings after the air filter was clogged with snow. The anemic Van's carb heat system did not provide enough heat to melt the snow. I am also aware of an aircraft (not an RV) that was downed by hitting something in flight that clogged the air filter.

I believe every RV should have an air filter bypass.

No to alterante air, for me

Kevin Horton said:

I am aware of two RVs that had off-field landings after the air filter was clogged with snow. The anemic Van's carb heat system did not provide enough heat to melt the snow. I am also aware of an aircraft (not an RV) that was downed by hitting something in flight that clogged the air filter. I believe every RV should have an air filter bypass.

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I agree with Kevin but don't agree with his conclusion. I don't fly in snow so my opinion is you don't need it. I only know of one blockage that occurred, regardless it is YOUR OPTION. Keep in mind there is "Impact Blockage" like snow and Carb Ice, two different things.

My rational, any weather condition that could block the filter, like snow, can be counteracted by applying CARB HEAT. The carb heat door blocks the outside air and eliminates any chance of snow blocking or "Impacting" the filter.

It's obvious when you are in snow or any precipitation, so it's not a burden to remember to use carb heat when in rain, snow, sleet or freezing fog/drizzle/rain. However if you are in any of these conditions, especially for a VFR pilot, you may be in deep dodo already, regardless of bypass air or not.

I am leaving it off for now, but I did order it. I can always add it later. VFR flying no, not needed. IFR flying? Yea worthy of consideration, but the above carb heat anti-blockage procedure still applies.

The old magnetic bypass set-up was worthless, however the new version bypass, the cable operated one looks better, but keep in mind any hardware that can get loose could go directly into the engine. The by pass is right there under the throat of the carb or FI body and with no filter. I don't have my installation instructions handy but you need to assure the hardware you install on the FAB bottom for the bypass can not get loose or sucked into the engine. I leaving it off because of weight, worry of ingestion of parts and I don't fly in snow, if I did I have a fall back, carb heat door blocking the snow from blocking the filter.

There was one case of a bird getting sucked into the scoop of a RV, true story and its documented in a back issue of the RVator. Things can happen but the chance is so small it's not worth the effort to install. According to Van's that is their conclusion if you read the service bulletin, it is dependant on the weather you plan on flying in.

Fuel Injection?

gmcjetpilot said:

VFR flying no, not needed. IFR flying? Yea worthy of consideration, but the above carb heat anti-blockage procedure still applies.

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Unless of course your engine if fuel injected in which case you don't have the carb heat option.

FWIW: When I was mulling this over...

I called Vans and they said "None of the guys here put them on thier planes". I did state my mission was strictly VFR. Good luck.

I have the original magnetic one on and it is still working fine after 120 hrs. I check it periodically by reaching in the cowl outlet and pressing in on it. If it should ever appear to be deteriorating, I will just plug up the hole, and not bother with the cable operated one. I also just fly VFR.

JMHO
Roberta

No Alternate Air for Me

gmcjetpilot said:

I am leaving it off for now, but I did order it. I can always add it later. VFR flying no, not needed. IFR flying? Yea worthy of consideration, but the above carb heat anti-blockage procedure still applies.

Click to expand...

I agree with your thoughts on the subject. I don't fly my RV in those conditions either. I had the magentic door, and was one of the first to discover it's malfunction. After I removed it I posted the details of the malfunction to this list. Following Van's instructions, I installed the new type of pivoting door, hoping for the best. It was evident to me that the new alternate air door design was flawed; I thought it might crack but I installed it anyway, with the intent of keeping my eye on it.

Aftaer about 50 hours, this is what I found:



It's obvious that the single screw holding/pivoting the door is not a good design. What is really needed is a good sliding type of door. I encourage other builders to carefully monitor their alternate air door if they have the new type installed. Going without it installed may be less likely to cause problems. If the crack occurs in such a way as to let a small piece become injested it could well cause an engine failure.

For the naysayer, engine/prop vibration on this engine was at .02 IPS after careful balancing, way before the new type of door was installed.

Magneted door not option for Airflow systems

I installed that door and found out months later that it will not work with Airflow Perfromance fuel injection systems. As I remember, with AFP installed it will actually have enough draw to pull the thing open.

I was going to take it out anyway. To me (VFR) the odds of hitting a bird and sucking it up are less than putting one through the windshield or hitting a control surface.

Has anyone else had problems with the latest pivoting alternate air door? I was just looking at the package and deciding if I wanted to install it. After seeing the pic that Jon posted, I'm pretty sure I'm not going to mess with it.

I am putting a better carb heat muff on, and I'm strictly a VFR kinda guy, so perhaps I'll just forget alternate air, unless Van's comes up with a more robust version of this.

Cheers,
Rusty

Slider sounds interesting for those designers

13brv3 said:

Has anyone else had problems with the latest pivoting alternate air door? I was just looking at the package and deciding if I wanted to install it. After seeing the pic that Jon posted, I'm pretty sure I'm not going to mess with it.

I am putting a better carb heat muff on, and I'm strictly a VFR kinda guy, so perhaps I'll just forget alternate air, unless Van's comes up with a more robust version of this.

Cheers, Rusty

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avpro56's idea of a sliding door with no nuts and bolts sounds pretty cool. VAN ARE YOU READING? For those who feel they must have this, may be you can come up with a sliding door with bonded on guides, no fasteners to fall out or get sucked up.

One more observation, the way the current metal pivot door works, I doubt you will be able to re-closed it once open. Once you rotate the door out of the slot, holding down one edge, I doubt you can get it under that flange again and close with just a light duty control wire.

gmcjetpilot said:

One more observation, the way the current metal pivot door works, I doubt you will be able to re-closed it once open. Once you rotate the door out of the slot, holding down one edge, I doubt you can get it under that flange again and close with just a light duty control wire.

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Good observation George. Actually, it's stated in the instructions for the mod that you have to manually close it. As hokey as that sounds, for someone like me who "might" use it once in a lifetime, that would be just fine.

Funny thing is that I originally ran two bowden cables, for the carb heat, and alt air. Then I ordered the oil cooler shutter thing. For a while, I was wondering which two items I'd use the cables for, but then I decided not to install the oil shutter. Problem solved... until now. Now that I'm also not using the alt air, I have a spare cable

I'm actually thinking about a sliding door, which would be easy to make. Hmmm.

Rusty

Me too

I also found my magnet clinging to some steel in the airbox during inspection and reported it here and to Van's. I came up with a fix for mine and it is posted under Builder Mods. I still have that in place and it is working as intended. I also riveted a 8-32 platenut to the inside center of the door for racing in the AirVenture Cup. In that situation I install a metal strip across the bottom of the airbox with a screw in the platenut. At other times the screw is in the platenut (for a couple of reasons) but the strip is in a side pocket in the airplane. I fly IFR and I'll live with the by-pass as is. By the way the original design had no filter by-pass - this was something that was developed after pilot experience showed that something was needed. I still have the second iteration fix in the envelope it came in and after seeing the failure photograph in this thread, it's going to stay there without question. Thanks for showing that.

Bob Axsom

Bob Axsom

Magnetic alt air door

My magnetic door fell apart too, after 30 hours. I have an AEIO-360 B1B. Vans told me that another flaw in that design is that it can be sucked open at high power settings with any induction system not just Airflow. I would like to have alt air but I'm not sure I like the new design either.

Neil