I fly an Arrow II, and by the Alternate Static Source lever, the placard reads:
“Alternate Static Source
All cabin vents and storm windows must be closed,
Heater and defroster must be on”
I get the part about cabin vents and the storm windows, but why should the heater and defroster be on?
The POH doesn’t even mention anything about the alternate static.
I assume to try to get as close to ambient static pressure as possible. The vents and storm windows could distort with dynamic pressure and or suction. the defrost and heater may bring more ambient air into cabin.
Yeah – that could be it. Bringing ambient/static pressure into the cockpit without adding too much dynamic pressure. Good point. Thanks. I just thought such an old (1974) bird would have enough ambient pressure already without having to open anything … :-)
Just pull the alternate static lever and observe the altimeters in all configurations with windows/vents/defroster open and closed. Would be interesting to see if there are differences.
Some (most?) light aircraft have a cabin air exhaust which helps pull air through the cabin when the vents are open. I would suspect with the windows and ventilation system closed it could create a slight suction!?
I tried a few configurations in the Beech F33A. Cabin pressure is always a little below ambient pressure. But the strom window makes the difference. If you open that the cabin pressure will fall quite a lot. You can feel the air being sucked out. The same for other parts of the cabin. This must also be the reason why closing the door in flight is so hard…
Yeah – good idea. Thats in the books for the next “testflight”. Will post here what I see.
“Some (most?) light aircraft have a cabin air exhaust which helps pull air through the cabin when the vents are open.”
Are you sure that the cabin ehausts are not permanently open? My POH is not clear on this point, but I read it as they were just open and not connected to the air intake controls.
In any case they could be responsible for some of the suction, when everything else is closed.
I can imagine another reason for the heater and defroster being fully open. The POH gives correction figures for the instruments when you use the alternate static source. The correction will depend on whether heater and defroster are open or closed.
It would be impractical to provide correction figures for all configurations so one is chosen which would never be wrong. If the static source blockage is because you are flying in icing conditions, you would most likely need full heater and defroster anyway. If the reason is something else, then opening the heater and defroster won’t hurt.
