Alternate Static Source
What is alternate static source?
An alternate static source is a manually-operated valve or port that supplies static (ambient) air pressure to the pitot-static instruments—the altimeter, airspeed indicator, and vertical speed indicator—when the primary static port becomes clogged, iced over, or otherwise blocked. On many aircraft this draws air from inside the cabin, which is at slightly lower pressure than outside, so expect the altimeter and airspeed to read a bit high and the VSI to momentarily show a climb when you switch to it. Think of it as your backup plumbing for the static system when the normal pickup quits.
Also known as: alternate static source valve, alternate static port
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