Alternate air questions

[bret]

I'm on the other side of the box, my donor engine in a former life swallowed a screw and got rebuilt because of it. I see alt air is a good idea for IFR ops but I decided to not install one because of all the stories I read on here about pieces getting ingested and causing internal engine damage, and the fact that I am just a VFR guy for now. the odds may be equal to an alt air door falling apart or ingesting a bag or bird without one in either case we have a heavy glider to get us down safe to a road or field.

[Rupester]

I built the intake, filter, and alt air exactly as the Sam James 'structions called it out. (vertical intake FI). Yes, it is an alternate intake path, but as pointed out above it does not bypass the filter. But there are advantages, in that the heated alt air coming in from the heat muff will melt frost or packed snow that might be clogging the filter. I perceived that was really the intent of the Sam James design.

[Ron RV8]

After reading about loose bits getting sucked into the engine, cracking etc I encapsulated a .040 aluminum ring on the bottom of the air box. The clevis is R/C model vintage. So far so good @ 370 hrs...

[Ralph Inkster]

My first build suffered FOD damage as described by Brett, so agree with his concerns. But I still see the value of installing this safety device on filter housings. The valve was installed originally per plans with Proseal and rivets attaching the base plate to the fiberglass housing, at about 700hrs the base plate vibrated loose (?) and wore the rivet holes & rivets enough that 3 of them were sucked up thru the engine causing overhauls on 2 cylinders.

My first attempted repair was to re-rivet the base to the fiberglass with large diameter washers inside the housing for the rivets to squeeze against, all again Prosealed. 100hrs later I noticed these rivets loose again! Third round I tried a different repair method.

My method now consists of drilling the base plate for AN509-8 screws, dimpled (not countersunk!) and mounting to fiberglass housing with Proseal as before. Inside the housing I create a .040 donut shaped plate with nutplates for the #8 screws, this plate also Prosealed. This system has the screw fasteners engaged with metal parts on both sides, not allowing any fastener to fiberglass movement that may eventually wear loose.

I have incorporated this mod on probably 6 aircraft so far with no further issues showing up. Although I have only seen this damage on updraft installations, I do the same for horizontal injected air boxes too.


Ron- I have also seen the hinge screws come loose, on one plane the valve door was hanging at the bottom of the cowling. I suggest you replace the hinge screw with something that is drilled & safety wired.

[Ron RV8]

Annual soon, I will have a look at it...

[edneff]

mine is a normally aspirated O360, no alternate air. Since I don't fly in icing conditions the only case where I can see needing it would be if I had a bird strike directly in the intake snorkel. I considered it but decided that the maintenance and the possibility of issues with unfiltered air entering the engine was not something I wanted to deal with. K.I.S.S.

[rightrudder]

Originally Posted by bret

I'm on the other side of the box, my donor engine in a former life swallowed a screw and got rebuilt because of it. I see alt air is a good idea for IFR ops but I decided to not install one because of all the stories I read on here about pieces getting ingested and causing internal engine damage, and the fact that I am just a VFR guy for now. the odds may be equal to an alt air door falling apart or ingesting a bag or bird without one in either case we have a heavy glider to get us down safe to a road or field.

That's my reasoning as well.