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Old 06-21-2015, 08:53 AM
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Mel Mel is online now
Join Date: Mar 2005
Location: Dallas area
Posts: 10,307

Actually the alternate air doors on the "snorkels" are quite successful.

Many people have built alt air doors that slide open manually. These are also quite successful.

It's mostly the "passive" style of doors that fail.
Mel Asberry..DAR since last century
A&P/EAA (LifeTime) Tech Counselor/Flight Advisor/Nat'l Test Pilot School
Specializing in Amateur-Built and Light-Sport Aircraft
North Texas (8TA5)
RV-6 Flying since 1993, 172hp O-320, 3-Blade Catto (since 2003)Sold
Legend Cub purchased 12/2017
FRIEND of the RV-1
Eagle's Nest Mentor
Recipient of Wright Brothers "Master Pilot" Award
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Old 06-21-2015, 10:20 AM
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AAflyer AAflyer is offline
Join Date: May 2009
Location: Belle Mead, New Jersey
Posts: 296

Originally Posted by Vlad View Post
Make a poll Bruce would be interesting to see how many RVs don't have it.
I don't.
Of all the people who might actually experience a blockage of their intake, due to birds/branches/balloons/flying fish/small children…YOU ARE IT, my friend!
Bruce Windom
QB RV-8 N148BW
First Flight: Oct '16
AeroSport Power IO-360 180hp w/dual P-Mags
Catto 3-blade, with black carbon spinner.
10" Dynon SkyviewTouch. That's ALL.
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Old 06-21-2015, 11:58 AM
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N941WR N941WR is offline
Join Date: Jan 2005
Location: SC
Posts: 12,137

There are risks with everything we do, or not do.

Why is the option of adding the alternate air door available? It is my understand it was to give a pilot an "option", if the air intake iced up.

If that is the case and you don't fly IFR in icing conditions, leave it off, as I did.

If you are worried about sucking in a plastic bag on takeoff, I doubt you will have time to sort out what is going on in time to open the alternate air door before hitting the trees at the end of the runway. Not to mention, what are the real odds of that happening? (I understand it has happened but again, what are the odds?)

In the early days, the air door was held shut by a small magnet and if the main intake was blocked, the suction of the engine would allow the door to open automatically. (I have always wondered if it would open at high power settings while on the ground, just when you need filtered air the most.) The problem was that some of them came apart and some of those parts were sucked into the engine. That little problem lead to the current manual door and even some of those have come apart.

In short, you roll the dice and take your chances.

Since I was building a VFR ship, I left it off and in 650+ hours of flight, I have never given it a second thought.
Bill R.
RV-9 (Yes, it's a dragon tail)
O-360 w/ dual P-mags
Build the plane you want, not the plane others want you to build!
SC86 - Easley, SC
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Old 06-21-2015, 04:51 PM
sailvi767 sailvi767 is offline
Join Date: Sep 2010
Location: Charlotte NC
Posts: 888

[ Here is what we have. It's clean and simple. I don't see how any failures could introduce FOD. The barrel rotates and opens the slots as a alternate source of air. I believe it is a ECI system.

Last edited by sailvi767 : 06-21-2015 at 04:59 PM.
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Old 06-21-2015, 05:19 PM
Robert Anglin Robert Anglin is offline
Join Date: Mar 2008
Location: houston, texas
Posts: 900
Default Two cents.

I'm with Mel on this, and have learned the hard way in the certified world that ALT. air is a very good thing to have. Everyone knows carb. icing. It is induction icing that can and does trip a few up with an injected engine. It has been a long time now that I have not been around the factory guys. Maybe Mel knows off the top of his head but I think it used to be required for a Part. 23 build program.. We have a snorkel and won't leave home with out checking to be sure our ALT-air door works. We made it out of the Van's kit, but made it so it will reset in flight and not fall apart..
My two cents. Yours. R.E.A. III # 80888
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Old 06-21-2015, 05:41 PM
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N941WR N941WR is offline
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Originally Posted by Robert Anglin View Post
...Everyone knows carb. icing. It is induction icing that can and does trip a few up with an injected engine. ...
Only one thing, what we are talking about won't help with carb icing as the alt air still goes through the carb or fuel servo for injected engines.

All this does is bypass the air filter in the event the air filter gets plugged.
Bill R.
RV-9 (Yes, it's a dragon tail)
O-360 w/ dual P-mags
Build the plane you want, not the plane others want you to build!
SC86 - Easley, SC
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Old 06-21-2015, 06:43 PM
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plehrke plehrke is offline
Join Date: Sep 2006
Location: Defiance, MO
Posts: 1,461

I think this thread has a mix of answers and opinions on two slightly different subjects; alternate air (blocks off main air intake and opens to pull air from inside cowl but still goes through filter) and filter by pass (opens hole in bottom of FAB in the middle of filter to draw air from cowl into induction without going through the filter.)

I have taken off and covered over the filter by pass door since there has been plenty of trouble with the various designs and the risk of parts going into engine too great for any benefit of getting air to engine if filter clogged. I still have my alternate air door even though I have had to replace the hinge and fix it several times because I feel it has benefits. It has little risk of parts going through the engine if it fails because the filter will stop it.
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Last edited by plehrke : 06-21-2015 at 06:55 PM.
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Old 06-21-2015, 07:21 PM
BobTurner BobTurner is offline
Join Date: Dec 2011
Location: Livermore, CA
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While your definitions are perhaps 'cleaner' or clearer, they re not the customary ones. Virtually every normally certificated aircraft that I've seen labels the knob 'alternate air' when in fact it is a filter bypass.
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Old 06-21-2015, 07:59 PM
brad walton brad walton is offline
Join Date: Jul 2007
Location: Cypress, TX
Posts: 505

Filter bypass on the bottom of the FAB. No problems and it works well and seals tight. I made a ring so it is riveted thru metal on both sides of the fiberglass. What is the failure mode?
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Old 06-21-2015, 10:15 PM
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RV10inOz RV10inOz is offline
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Location: Brisbane Qld. Aust.
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Originally Posted by Ironflight View Post
It is a risk trade - the risk of the intake becoming blocked versus the risk of the door coming apart and FODing the engine. The anecdotal evidence suggest that there are many more cases of the various versions coming apart than there have been blockages (although they HAVE occurred -apparently rarely). I have personally seen so many air boxes that have shaken and vibrated themselves apart that, for me, the blockage is a lower probability than the mechanism failing - so I dont have alternate air doors an any of our current RV's.

Everyone should make their own choice.
Agreed x 1000

I wish we did not have one, but we do, although having the FOD problem, we made a fabricated aluminium (aluminium for you guys ) air box and it has been great.

And we avoid icing conditions in a non FIKI aircraft.

David Brown

DYNON Authorised Dealer and Installer

The two best investments you can make, by any financial test, an EMS and APS!
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