Dear Bob and Richard,

One detail which appears to be missing from discussions about the use of alternate static vents that are located in the cockpit is exactly how the static pressure inside the aircraft differs from outside. My ATC reference also appears to leave this out.

It would appear that due to the venturi effect of the flow of air over the cockpit, this alternate static pressure is usually lower than the pressure provided by the normal static source.

So when the alternate static source is used, the following differences in the instrument indications should occur: the altimeter will indicate higher than the actual altitude, the airspeed will indicate greater than the actual airspeed.

What happens to the VSI, and does the use of a 'storm window' change any of this?

Cheers, rubber

I've never understood how the venturi effect over the cockpit affects the alternate static port.

But i have read that if you don't have an alternate static source available, you can break the glass on one of the instruments and use the cockpit air as the alternate source. I can't remember which instrument it was though.

I believe it's the VSI glass you break. It is then rendered unserviceable.
rubber

Here's something you might enjoy reading rubber. It explains the situation very eloquently and brings a lot to the discussion

www.cfibastian.com/wp/2010/11/the-tale-breaking-vsi-glass/

Well that changes everything.
So it's the ASI we should break instead?

But that guy is a chopper pilot. Is his ASI a little less valuable than that of a fixed wing pilot?

Thanks Rich for pointing out the article.

rubber

Going back to the original post - how does the alternate static source affect the pressure instrument readings? And what effect does opening/closing the storm window have?