Alternate air?required?

[chrishalfman]

I prosealed the thing to the bottom of my fab. I looked at the rivets and decided they might hold it in place for the proseal to set up, but wasn't sure of much beyond that. I did have a failure when I clamped down the opening cable to the door. Testing out the function, and the the vibration with the end sprung pretty hard not being able to rotate, snapped the end off in the first 10 hours of flight. So, I bought new cable innards and I learned the wire needed to rotate on the bolt. Kinda basic after it was pointed out to me. Anyway, mine has been doing ok, since.

[tgmillso]

In reading this thread there appear to be a number of cases of alternate air systems failing and getting gobbled up by the engine. The question I have is for the horizontal induction air box setup, where the filter face is essentially parallel with the direction of flight, has anyone in snow or ice conditions had to actually activate their alternate air door? On the Cessna 172S as an example, the filter sits on the front of the cowl directly facing the airflow, so in theory the higher inertia ice and snow particles are going to slap themselves straight into the filter. I wouldn't dream of flying an aircraft like this in IFR conditions unless I had an alternate air door. On the RV-7 and -8 IO-360 setup the heavier snow and ice particles would in theory continue into the engine fins, because the air drawn into the filter and induction system has to pull a tight right angled bend.
Theory aside, has anyone actually had to open one due to icing, or is the issue as good as solved with this right angle bend?
Cheers.
Tom.

[mr.sun]

I guess I have the same question about ice/snow blockage in the snorkel filter on the io-360 horiz induction engines. I have (inadvertantly) had ice on the wings/canopy multiple times with no engine power loss. Am considering a filter bypass coming off the #3 baffle that might increase MAP in cruise. Any thoughts out there?

[Weasel]

I have picked up Ice on a -4 with Carb, -7 with fuel injection, -8 with fuel injection, -10 with fuel injection on various occasions and never required carb heat or alt air. however each of these airplanes had it available. if I were building today....don't know what I would do pretty sure if taking off from an unfamiliar short grass field with obstacles or hostile terrain on the departure end and a plastic bag covered my air intake at 50 ft AGL, I seriously doubt I would remember to open the alt air door in the heat of the moment. Something more automatic like DanH has constructed would be more beneficial for that argument.

Also.....don't fly in icing conditions. No No

[tgmillso]

So I'm coming to the conclusion that these sliding alternate air doors are more trouble than they are worth. Thus I'm going to skip it for now and install a spring loaded one with an open/closed indicator light in the future along the lines of the one Dan H has fabricated, and it common place in many GA aircraft. In the meantime I'll keep clear of anything that resembles ice or snow.
Tom.

[SgtZim]

I can't find any instances so far of a plane crash due to plastic bag or other fod ingestion. One case in England came up in my search, but investigators said the bag was stuffed in the engine prior to washing, and was apparently overlooked before flight. Still odd that the (twin engine) plane got airborne, then the bag (shifted?) and blocked the intake.

I'm also looking for more data on icing incidents on fuel injected engines. I picked up over 1/4" of clear ice in an Archer one time, but that was a carb engine - I don't think I picked up significant ice anywhere in the induction system. I climbed back up quickly when it first showed up, and did not have a problem descending to land other than accumulating a lot of clear ice on the plane.

I'm still not sure if I'll use the bypass on my plane, but I think icing is the much larger concern. Plastic bags in flight seem to be about as threatening as fish dropped by eagles.

[sf3543]

Seems to me that the Carb Heat, as installed on the typical RV, would do the same thing as the alternate air door in the event of the plastic bag scenario.
Although the alternate door would provide unfiltered engine compartment air in the event of a blockage in front of the filter, which could include the carb heat door, in the event of ice.
For me, since my planes are VFR, I don't see the need for it. I have also heard first hand of at least two instances where alternate air door pieces came apart and got sucked in.
For IFR planes, or maybe FI installations with out carb heat, maybe it is a good idea if you feel the need, but it does add another point of failure.

[bkc3921]

As has been stated, many things can clog your filter during takeoff roll or in flight..debris of all kinds, as well as part of the engine itself, i.e. oil....and yes, these occasions are very rare (don't we do most things for that 'one in a million'..?) I have the filter bypass on my snorkel IO360. As with everything 'VANS', it is up to the builder. FWIW, if you DO have this, please be SURE and include "open filter bypass" on your emergency checklist if you have an engine out. You may not have seen what clogged the input, and I believe this simple step might be easy to overlook in an emergency before you attempt a restart.

[johnbright]

Originally Posted by DanH

It is possible to build a good alternate air supply.

Pray thee... do tell.

[rv8ch]

Sorry - could not resist the title.

Some discussion here:

http://www.vansairforce.com/communit...ad.php?t=99604

Unfortunately a lot of photos are no longer avail.

Look for writeups by danh - he's done some amazing work.

Mickey