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Alternate air vs Filtered Air Bypass

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  • Alternate air vs Filtered Air Bypass

    I’m in the process of installing the Vans Filtered Air Box on my VFR Bearhawk with a fuel injected engine.
    The Vans FWF kit includes a Filtered Air Bypass, and an Alternate Air intake door. It also includes a Carb heat provision (obviously for a carbureted engine).

    But it got me thinking.......

    I’m building a VFR Bearhawk with a fuel injected engine. Builders commonly install an Alternate Air. But that’s not necessarily the same as a Filtered Air Bypass. One bypasses the filter in case the whole filter becomes blocked, the other may do but doesn’t necessarily. A Carb Heat acts as an Alternate Air Intake, but it doesn’t necessarily bypass the filter.

    My question is: If so many VFR carbureted aircraft have been flying around without a Filtered Air Bypass, should I be concerned about installing one just because I using fuel injection ? Would it make sense to install an Alternate Air intake (that draws warmer air from inside the cowling), but doesn’t bypass the filter ? (There is added complexity to bypass the filter due to the location).

    Also, I would be interested in show of hands to see how many of us have ever used the Alternate Air on a VFR Aircraft. (I’ve only ever used it once, and that on an IFR aircraft in IMC, and I’ve no idea if it by-passed the filter.)

    All thoughts always appreciated.
    Nev Bailey
    Christchurch, NZ

  • #2
    My Alt Air door is unfiltered and sits between the back of the filter and in front of the fuel controller.

    I don't personally know anyone who has ever used an Alt Air door due to a plugged filter. I do, however, know several who have used carb heat to clear an iced up carburetor (including myself)
    Last edited by robcaldwell; 04-19-2021, 06:46 AM.
    Rob Caldwell
    Davidson, North Carolina
    EAA Chapter 309
    BH Model B Quick Build Kit Serial # 11B-24B / 25B
    Build Log:
    YouTube Channel:


    • #3
      I installed the van's airbox on my Patrol last year. Until I started that installation I had never heard of having two bypasses, one filtered and one not. I thought it was overkill and only went with the unfiltered bypass, the big hole on the bottom, in the center of the air filter. I have never needed it, have never needed the alt air on any plane (with the exception of using carb heat which is also alternate air inlet), I get the purpose and the importance of having it but don't see a need to have two.
      Rollie VanDorn
      Findlay, OH
      Patrol Quick Build


      • #4
        I think the most common cause of a blocked filter is freezing rain which shocks into ice when it accellerates across the filter, or debris if working off unusual airfield - think agricultural planes with lots of crop laying around.

        Like Rob, in my years of talking planes and flying them, I have never heard of a VFR flight which encountered a blocked filter and needed the bypass door.

        The bypass door does have unfettered access to the engine intake, if it fails, it poses a risk of dropping parts into the airbox which could be sucked into the engine. So there's a risk to consider.


        • #5
          I've always wondered if flying through heavy snow could pack up the filter. One time went through a pretty good snow shower with my Maule, started seeing the MP drop some, added carb heat which caused more loss of MP, got out of the snow, closed the carb heat and everything returned to normal.


          • #6
            The reading I did on it says more alternate air doors fail, risking damage, than ever "save" the day. But, if I remember right, Transport Canada requires them. I am not sure I wouldn't prefer to use a carb heat setup (I don't have a carb) to clear air filter ice and provide a secondary source of air.


            • #7
              I puzzled over this a lot when building our Patrol. I read of, I think, a couple crashes where parts from the alternated air door on the bottom of the Vans FAB came apart allowing parts to be ingested. I opted to not do the filter bypass for this reason. I did install the alternate air door on the top front of the FAB allowing air to be pulled in from inside the cowling. The thoughts for this included the possibility of snow in the filter and thinking the air in the cowling will be relatively warm and certainly not allow additional accumulation if open. When it is open, it also closes access from the normal front opening. Also, that front opening being relatively small, about 4” by about 2 1/2”, I thought about the possibility of it getting blocked by flying debris or a bird. The alternate would provide air in this case. I also put a course screen at the front of the FAB which the alt air bypasses.


              • #8
                Certified IFR injected aircraft don't have 3 sources of air. Only 2. 1 through the filter and 1, usually completely unfiltered from within the cowl in some way. If the main air inlet is blocked, there's no need to keep filtering the air, just route around, keep flying and fix it when you get on the ground.

                With the exception of turbo setups, and that's a whole different thing, I've never heard of anyone actually needing to use the alternate air on an airplane. I cannot see why you'd need 2.


                • #9
                  I've never used a fire extinguisher but I still carry one ......

                  As for the failure causing damage, on my RV10 I have glassed over all the rivets inside the box holding on the alternate door.


                  • #10
                    Originally posted by PaulSA View Post
                    I've never used a fire extinguisher but I still carry one ......
                    Haha.....and I know you know how to use it ! All those happy memories in the safety training school I bet you’re not missing it
                    Nev Bailey
                    Christchurch, NZ


                    • svyolo
                      svyolo commented
                      Editing a comment
                      I think I would prefer a parachute to a fire extinguisher. And I think I will use a filtered "carb heat" setup.

                  • #11
                    I've gone back and forth, right now I'm not installing any alt air on my vans FAB and disc type filter. Engines consuming parts being the main reason, vibe on any component on an engine requires good design and robust (heavy) materials, hinged parts even moreso, though I know the vans solution is more of a sliding contraption.

                    Ice/snow is a concern, though we shouldn't be in freezing air with any moisture as a matter of practice, right? Ram air and positive pressure upstream of the filter means snow, or supercooled water turned into ice, that gets in will be melted (or sublimated maybe), provided you're in >0C air.

                    The disc filter seems like it would provide a bit of a buffer from ice/snow until it clears, maybe until you leave icing conditions, but it seems that just doesn't seem to be an issue as far as I've found. If I had a conical filter that might change my admittedly not-emperical calculation.
                    Dave B.


                    • Battson
                      Battson commented
                      Editing a comment
                      No need to worry about the sublime. The triple point pressure is 612 Pa (0.0006 atmospheres) for ice to vapour.

                    • Archer39J
                      Archer39J commented
                      Editing a comment
                      Not worried, ice in air that is <0C will apparently sublimate with enough mass flow. Not sure if this translates to the inside of a FAB so I said "maybe".