SESKStuff Everyone Should Know
SESK: Stuff Everyone Should Know · sesk.info/pilot-alternate-static-source

When a pilot switches to the cabin alternate static source, the altimeter usually reads what?

Slightly higher than the actual altitude.

  1. Slightly lower than actual
  2. Exactly correct
  3. Slightly higher than actual ✔

Why: Air flowing around the fuselage makes cabin pressure a bit lower than outside. Lower pressure looks like higher altitude, so the altimeter reads high. The airspeed indicator reads fast too.

Huh, didn't know that: The vertical speed indicator shows a momentary climb when the alternate static source is first opened.

The dad joke
Why is the altimeter so confident? It always thinks it's above it all.
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Sources

  1. FAA Pilot's Handbook of Aeronautical Knowledge (FAA-H-8083-25C), Chapter 8 https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/faa-h-8083-25c.pdf

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