On the TB20, the main advice I recall is to use it in icing conditions.
I can well understand that but isn’t there a problem: if you fly with alt air on, you are flying on the backup system. Should one not use the standard air path and change to the alternate only if there is an issue? I think that with regard to fuel servo icing this is different i.e. you should use alt air as a fuel servo protective measure, but what about on planes with a carburettor?
Also alternate air robs the engine of a fair bit of power, so really kills high altitude climb performance.
I thought it was only to be used when you are suspicious that the main air intake is gettting blocked by ice (power reduction …).
I think @Aviathor has a story about alternate air in icing conditions 
You follow the POH/AFM.
As Jason says.
If you get in clouds even at positive temps, your filter will get soaked and freeze once you climb above the 0deg isotherm, even if you’re not in clouds anymore.
So (unless your POH says otherwise) go to alt air before entering moisture, and stay there until you’ve exited the runway; potentially you may consider going back to the filtered intake once you’ve spent a fair bit of time in positive temps in the descent, but you could still have some chunks of ice stuck behind the filter that could come into the engine so it’s only recommended if you suspect contamination on the runway.
your filter will get soaked and freeze once you climb above the 0deg isotherm
That isn’t however the case in reality, for the foam filter in common use e.g. Brackett BA7112. It may get water-soaked but I have never ever seen it freeze up. Actually I don’t think it gets water-soaked at all – all the time you are not using alternate air, because a lot of air is passing through it which keeps it from getting saturated.
You follow the POH/AFM.
A more educational discussion would be around the reasons.
What does cause the blockage in your view if it’s not the water soaking the filter freezing?
It could get saturated by dust. In the Bonanza the alternate air door is spring loaded. I don’t remember ever using it manually. On the turbonormalized Bonanza it is closed by magnet and an LED will come on when open. This may happen in icing conditions (TKS equipped).
What does cause the blockage in your view if it’s not the water soaking the filter freezing?
I have never seen it blocked, in rain or in icing conditions.
What I have seen is ice accretion in the air duct behind the filter (when prop TKS was not in use) and one way you can tell is that when you try to open the alternate air door, it won’t open unless you pull pretty hard. There are reports of it not opening at all. That is obviously a nasty engine failure mechanism for systems where the door is automatic – this is one suspected case (the plane was sold very fast afterwards and no data came out).
Peter wrote:
A more educational discussion would be around the reasons.
Yes but we aren’t discussing an aileron. The reasons and use are entirely installation independent. You mention losing power, well that barely applies in a turbocharged engine – other than a tiny ram air effect lost. So I don’t think there is such a thing as a generic answer on when it should be used other than the one I gave.
But if your question is when should I use the alternate air door in a TB20, I get it. Is there a problem in that airframe with following the POH?
The reasons and use are entirely installation independent.
A good discussion topic then
