In the PA46 it is SOP to go to alternate air in any visible moisture at or below 0. You lose no power and can fly like that all the time – and many do. Only risk is debris on the ground.
Ok. So I take away that flying on alternate air in IMC at -15C or colder for hours is not an issue in itself.
There might be other issues related to icing in other areas or for other reasons.
In my case (SR22 turbo-normalized) the alternate air is automatic. Based on suction pressure the magnets used in the mechanism will not continue to hold the door shut and it will open. That door is well inside the cowling close to the engine and other pieces that will be quite warm. After all the exhaust pipes from the engine and back to the turbo are close by and they are really hot. When the door opens I get a yellow warning on the MFD.
Just for your peace of mind Stephan – other than ambient heat, the airbox in a SR22 is protected against water ingress by the same physical principle that makes inertial separators work in a turboprop engine.
So here is a case where the question above comes in. This is a GRAMET for tomorrow morning. I will not fly myself as my bird is sick due to a failed software update (different story) but there is something to be learnt for future decision making.
The magenta line represents FL180 which is around were I would be flying. I would be in that stuff for about 2 hours. Trying to get on top it is probably not going to work and lower might have more surprises in stock around the 300 NM marker. I feel it’s a better tactic to simply climb high enough to where it is cold enough and let the alternate air intake work to my benefit.
If you look at the MSLP chart
there might be nasties in that wx, so flying in IMC might get pretty exciting.
Re inertial separators, they work only for big things, not for tiny water droplets.
Re the alt air door in the Cirrus, isn’t it just spring loaded, opening automatically when the pressure across it reaches some value (as in when the air filter gets blocked)?
No springs at all. There are magnets.
For the nasties one has onboard sat WX :-)
How do the magnets work?
They just hold the door in place until the force to open it is greater than what they can hold. I was shown it a little while ago. Seems simple.
Peter are you saying that water droplets are immune to physics?
