Where is alternate air for an M20J?

[RobertE]

I've been flying this aircraft for a couple of years without knowing the answer to the question so, to avoid embarassment, I should probably post anonymously.  Anyway, where is that thing?  Is it just a flap that isn't actuated by the pilot?

[jetdriven]

Its a spring loaded round door on the backside of the airbox, below the air filter.  You push it in with a 3-4 lb force to test.

 

[Alan Fox]

The Ram air is also a source of alt air....

[mulro767]

.

[jetdriven]

If you need alternate air because the air filter ices over, pulling the ram air results in a pretty quick engine failure when the sense tubes of the fuel injector servo freeze over.

[jlunseth]

I have had induction ice two or three times in the high teens/flight levels.  Usually, it is caused by flying through clouds of ice crystals.  They are pretty, but not compatible with your engine drawing breath.  You will see MP start to drop.  Generally it is a slow, inexorable drop.  Then the alt air door operates automatically, or you operate it manually, and the engine is happy again.  Yes, I would not think that going to ram air, which directs unfiltered air into the engine, would work very well in the circumstances where alt air is needed.

 

There is supposed to be a loss of power when the door operates, because warm air is used.  I generally don't see that in my aircraft.  My supposition is that since it has a differential pressure controller for the turbo wastegate, which senses the difference between ambient and induction pressure and works to maintain that difference, the wastegate closes a little to hold pressure and the power output remains the same as before the door operated.  That would not be the case in a NA aircraft I don't believe.  There would be some loss of power.

[RobertE]

Is there, in fact, a way to operate it manually on a J model?

[tomn]

Here's a picture of the alternate air door on my E. It is located on the back of the air filter box just below the fuel servo/induction boot. As has been mentioned, it opens automatically when the filter gets blocked to the point that air will not pass through. The vacuum from the intake overcomes the spring and the flap opens allowing air from inside the engine compartment to be drawn in - keeps the engine running. Its a good idea to check this occasionally to be sure it opens. Press on the spring and push in towards the filter box and make sure it opens freely.

Tom

[tomn]

ok, here's the picture...