FAA Instrument Rating Airplane (IRA)Flight Instruments and SystemsEasy
An unpressurized single-engine airplane experiences pitot-static icing, and the pilot selects the alternate static source, which draws air from inside the cockpit. Cockpit static pressure is typically lower than outside ambient pressure due to airflow effects. What effect will this have on the altimeter indication?
- AThe altimeter will indicate lower than the actual altitude
- BThe altimeter will indicate higher than the actual altitude
- CThe altimeter indication will be unaffected by the static source change
- DThe altimeter will freeze at the last reading before the switch
Show answer & explanationAnswer & explanation
Correct answer: B. The altimeter will indicate higher than the actual altitude
Because cockpit static pressure is lower than the true outside static pressure, the altimeter interprets this lower pressure as a higher altitude, so it will indicate an altitude higher than actual when using the alternate static source in most unpressurized airplanes.
Why the other options are wrong
- A. This is the opposite of the typical effect for unpressurized aircraft with alternate static sources.
- C. The change in static source does affect the reading, as documented in POH correction tables.
- D. The altimeter continues to respond to the new (lower) pressure source, it does not freeze.
Alternate Static Source Error
Using cockpit air as an alternate static source in unpressurized aircraft typically causes lower sensed pressure, resulting in the altimeter reading higher than actual altitude and airspeed reading higher than actual.
- Airspeed also reads slightly high on alternate static source
- POH provides specific correction values
- Effect direction depends on aircraft type and cabin ventilation
Memory trick: Cockpit air is 'thinner' feeling to instruments, so altimeter thinks it's higher.