John: I would disagree with the lack of need for an alternate air door--it
saved my bacon when returning from OSH in 1998 in my IV-P. Climbing through
FL200 in wet soup the MP started dropping rather quickly over 5 minutes. No
ice on the airframe that I could tell. Declared an emergency and nosed over.
Still losing MP until around FL180, when the Alt Air Door warning light on
my panel illuminated (indicating that the door had opened). MP immediately
came up to near normal and I leveled off at 16,000 (it was 0/0 on the
ground) and headed for the edge of the front. Popped into sunshine and after
about 30min in the clear the light went out.

I surmised that the icing potential high in the clouds plus the high angle
of attack in the climb let ice pile up in the air intake under the cowl and
starve the engine-before any buildups were apparent on the wing leading
edges. Over time in clear air, the ice sublimated and all returned to
normal. Nothing notable on the ground before or after the event.

Doug

Douglas W. Johnson, MD PA