Alternate Static Port

Heya all!

I've been lurking here for about a week and a half now going through all this useful information, and I really have to share how impressed I am with the professionality of the users here!!

I am currently in the very early stages of going for my IFR ticket (after not having flown for like 7 years). Hopefully I should have my money together in a few months. Like many that I've read in the Changing Careers forum, I too have plans to leave the IT field, hence part of the decision to use this username. The other part of the reason is that I see a correlation between pilot and sysoperator....they both "operate systems", constantly fine tuning them to achieve a result. So there's that for my intro

To the question:

Both Gleim and Dogan indicate that when using an alternate static source, the air pressure inside the cockpit is usually lower than the airpressure outside the airplane, "due to the Venturi effect of the outside air flowing over the cockpit" (Gleim).

I suppose what's confusing me is: if air is flowing AROUND (and faster as compared to the air inside) a cockpit, shouldn't the air pressure OUTSIDE be less than the relative air pressure inside the cockpit?


Think of what happens when you are driving on the freeway and you roll the window down an inch or so. The air pressure drops a lot inside the car as soon as the window opens. Why? The lower atmospheric pressure immeaditely outside the car sucks out the air (the low pressure is caused by high speed air flowing around the vehicle: remember high speed = low pressure).

The same thing happens in non pressurized airplanes: small openings around the doors and windows allow cabin air to escape into the lower pressure, high velocity air flowing outside.
Ahh! I understand now! The air inside the cabin gets sucked out via the cracks of the doors/other little openings, causing the pressure to drop. That makes complete sense to me now! heh, guess I just needed to bounce that off somebody. Thank you soo much for the quick reply!


Cap, Roci
Staff member
Remember the Bournolli (sp?) theory?

Fast air = lower air pressure?

The air in the cabin is relatively static, whereas the air outside is moving near whatever airspeed you're cruising at.
Yah, that's what caused my confusion,...the books read "lower pressure inside the cabin as compared to the outside air". I was thinking that the faster air outside should be a lower pressure than inside the cabin, but I stand (sit) corrected....the air rushing outside the cabin "pulls" the air from the cabin, causing a lower density/pressure relatively.

btw...I don't mean to sound arrogant, I'm just a stickler for spelling (not typosesses)'s Bernoulli