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Alternate air?required?

AAflyer

Well Known Member
What's the VAF opinion on installing/omitting the alternate air door? I have an IO-360 with Superior sump in an -8.
Pros & cons?
 
I have had two versions, original magnet and slider. Seemed like the issues were worst then the advantages so permanently bonded a metal plate over the whole thing. Now have none.
I think you should consider having one if you ever fly in snowing conditions (of coarse none of us should fly in known icing conditions)
 
The convincing argument, for me anyway, is the proverbial plastic bag blowing across the runway at takeoff. That has happened to others and would be a potential disaster for sure. As much of a PITA as it is, I think it is absolutely necessary.
 
I have the identical set up: RV-8, Superior IO-360. After reading several horror stories about the doors coming apart, blowing open, etc. I decided to skip it. It seems to me that it adds as much risk as it alleviates. Take my advice for what it's worth! I'm sure many will disagree!

Mark
 
If there were a leak free, can't fall apart in flight version...I would likely install it. But there isn't. So I haven't. Rv8, superior sump, intake on the ramp.
 
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.
 
I've removed quite a few failed/failing vertical induction alt air doors, a number of them have come in with missing pieces (wonder where that went?). I personally don't have one.
 
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.
 
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! ;)
 
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.
 
image.jpg1_zps34mpuslk.jpg
[ 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.
George
 
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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
 
...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.
 
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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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.
 
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?
 
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 :D ) air box and it has been great.

And we avoid icing conditions in a non FIKI aircraft.
 
image.jpg1_zps34mpuslk.jpg
[ 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.
George

Ok, what is this piece, how does it work, who makes it etc etc etc.
 
image.jpg1_zps34mpuslk.jpg
[ 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.
George

Ok, what is this piece, how does it work, who makes it etc etc etc.

What is this piece - from above ="alternate source of air"
how does it work, from above ="The barrel rotates and opens the slots"
who makes it - from above = "I believe it is a ECI system"

etc: who is buried in Grant's tomb? - = Grant, it is answered within the question

Bad day?
 
I contacted Robbie Attaway about the alternate air system. It is not a ECI system. He designed the system and holds the copyright. Robbie believes it is the best system out there and I have to agree. It's very elegant and well made. He made several others but left the RV world to work overseas. He is back now and would make more or consider selling the patent but they are expensive. They only work with forward facing injected systems.
George
 
Easy there Bill...

I made a pretty good search of the ECI site looking for this, and it's not to be found - so I'm asking the same questions....

EDIT - George beat me to it. Bill can take the day off.
 
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Easy there Bill...

I made a pretty good search of the ECI site looking for this, and it's not to be found - so I'm asking the same questions....

EDIT - George beat me to it. Bill can take the day off.

Sorry, I could not help my self. :eek: Guess I'm not as funny as I think.

Seriously, it is a slick looking design, very clever. It would appear to provide a non biased flow when in use. But - it does take some valuable space in the flow direction. DanH's unit is packaged in a very short length, and with a healthy flow area for the air filter. My installation will be challenged with a set back cowl. The Will James FAB appears that it will fit in that length and includes a spring operated alternate air door. Not as well documented as Dan's, but saves some development time.

I was thinking that maybe the alt air (filter bypass) was not really needed except in winter, but g-zero just caught a plastic bag reported here less than a month ago. His takeoff was executed on alt air and he returned to find the debris.
 
Durability aside, the question is pressure drop, both for the chosen filter, and when running on alternate or bypass air. The rotating sleeve approach is pretty cool, but should be checked for restriction.

Being located right at the throttle body inlet, it should also be checked to make sure it doesn't upset metering when used with a conventional Bendix-style fuel control (Bendix RSA, AFP FM150 or FM200, Precision Silver Hawk EX5, etc). They all rely on ram and venturi pressure to meter. The unit in the photo is an ECI, which is more like a Continental constant flow system. It delivers fuel based on RPM and throttle opening, not airflow measurement.
 
I had an oil line come apart on a rented Piper Arrow while crossing the Rockies at 12,000 feet. The oil quickly saturated the air filter, resulting in a loss of power. The Alternate Air lever (filter bypass) helped restore power long enough to find a place to put the airplane down safely.

During my RV-7A build, I made sure a filter bypass/alternate air system was installed.
 
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.

Not always the case on certified planes - sometimes the "Carb Heat" function is also a filter bypass, like on the Grumman Cheetah, and it's labeled as "Carb Heat".

However, the Carb Heat air goes through the air filter on my Grumman Tiger and there is no filter bypass available. This might be allowed by the use of a coarse screen on the start of the Air Intake to Filter Box hose above #3 cylinder...


If this is the case, perhaps the RV risk could be reduced by adding a coarse screen in the cowling intake and letting the carb heat function everyone has act as the safety measure for the dead bird/paper bag ingestion problem?


The Grumman knob for "Alternate Air" is for the static ports bypass only...
 
Mine is MIA

Pulled cowls yesterday for oil change, door and all mounting hardware has left the mother ship. About to rebuild without alt air. New parts on the way from Van's already, but, seriously wondering how to have the function without risk of more fastener failures. The fiberglass filter holder is just too flimsy to support pop rivets (at least when they fail they should go down and out the cowl instead of up and in the throttle body, thats what i'm telling myself).

RV8%20Intake_zpsttwynx9i.jpeg
 
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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. :D 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. :cool: Anyway, mine has been doing ok, since.
 
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.
 
Alternate air

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?
 
Just a data point

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
 
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.
 
Cite an accident please?

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. :)
 
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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.
 
Filter by-pass..Put in on your checklist

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.
 
Use aftermarket conical filter on inlet ,very little chance of being totally covered, +
they can be cleaned and reused ! Tom
 
Is there an upgrade available?

Now I have to make the call as I'm doing the firewall forward stuff. I was leaning to going ahead and installing the Van's bypass (IO with snorkel), BUT:

A. The door and the seat fit poorly. I can sand the door and get it flatter, but the seat/flange is warped and twisted in the stamping process. I'm not sure I can grind it flat enough to seal well, especially after dimpling the holes on the flange. I worked hard to get a good fit and seal on the filter, don't want to add a permanent leak in the system.

B. Speaking of the flange - The instructions have you machine countersink the snorkel in 5 places to mate with the flange dimples and you pop rivet into those - makes a very marginal attachment as you have machined away most of the fiberglass that the pop rivets are gripping (crushing and pushing away as they set).

C. Still some potential for hardware to go down the pipe.

Question for the experienced mechanics and industry wonks - how much leakage past the filter is ok? The snorkel install says I should drill a drain hole at the low point of the snorkel - also I'm not doing that. I have a sniffle valve on the bottom of the manifold to drain moisture and stuff.

So, I'm back to not installing this system at this time. Has anyone made a better mousetrap?
 
Better Mousetrap

HERE is a link to a thread I posted several years ago. Opens AND closes from the cockpit, nothing to ingest into the engine. 4 years and 365 hours of trouble free operation - well . . . . . I haven't had to ACTUALLY use it thank goodness, but it still functions properly and hasn't failed ;)
 
Thanks Dan, that looks really nice. I'm filing your link for future reference - one of those nice add-ons I can do once the plane is flying.



HERE is a link to a thread I posted several years ago. Opens AND closes from the cockpit, nothing to ingest into the engine. 4 years and 365 hours of trouble free operation - well . . . . . I haven't had to ACTUALLY use it thank goodness, but it still functions properly and hasn't failed ;)
 
Alternate air door

I had an alternate air door come off a certified plane and get lodged ahead of fuel servo. It was the spring loaded type connected to a push/pull cable. If i were designing one, I'd probably make it from a push/pull cable that if failed, couldn't be injected into motor. I don't prefer the magnet or spring loaded type.
 
There is an old racing adage about winners and speed hardware - "Its not what you put in, it's HOW you put it in".

I think it could apply to the alternate air/filter bypass hardware.

I have the horizontal injected snorkel rotating round door type.

I used the Vans hardware but I did not follow their directions much.
Happy at 750hrs and no indications of change anytime soon.
And dispite the Vans note that the door cannot be reclosed/reset via the cable. Mine does easily.

Here's the first of 3 pages on the subject.
http://websites.expercraft.com/rzbill/index.php?q=log_entry&log_id=63249

I did not talk about it in the blog but I did spend time massaging the parts to make sure they were both FLAT. Not kinda flat, FLAT.
 
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