Alternate Air Light comes on when it rains on 252

[turbotrk]

Hi, I have noticed during a flight that the Altrrnate air light came on for a while and then went off. I thought it could have been due to the bracket of the induction intake being not tight. After a couple of months same problem as I was fliying trough rain and I realized that also the time before I was in the same condition. Has anybody had any similar issue?

[kortopates]

Several things could cause this, but I can't imagine how rain would be one them:

1) the induction boot between the cowl and air filter could be pinching off blocking air flow as you climb to altitude - as the difference between ambient air pressure and the upper deck pressure that your turbo is trying to maintain increases. This could be from the boot not being re-installed properly - not being pulled up tight enough allowing it to be pinch in from suction, or a combination of that and a worn flimsy old boot that should be replaced. Also the Air filter attached to the turbo inlet has to oriented with just the right angle to allow the best possible fit for the boot to line up with the fitting on the lower cowl so that it goes on nice and tight.

2) there is also a magnetic on the alternate air source door that could need adjustment if the door is opening with too little required suction (and the induction boot is not pinching in from suction). The service manual calls for it to open at so many inches of vacuum - look there for specific's. 

But most likely is #1

[M20F]

It sounds like you are saying in both instances you were flying through rain. If that is the case the air filter can easily get saturated in heavy rain and block airflow causing the alternate air to engage.

[M20F]

Also to add could be an indication to change your filter if the rain wasn't particularly heavy.