During cruise flight in an unpressurized single-engine airplane, a pilot's primary static port becomes blocked by ice, and the pilot switches to the alternate static source, which is vented inside the cabin. Compared to readings from the primary source, what effect will this most likely have on the airspeed indicator?
- AIndicated airspeed will read higher than actual airspeed
- BIndicated airspeed will read lower than actual airspeed
- CIndicated airspeed will be unaffected
- DIndicated airspeed will freeze at the last known value
Show answer & explanationAnswer & explanation
Correct answer: A. Indicated airspeed will read higher than actual airspeed
Airflow over the fuselage typically creates a slightly lower pressure inside an unpressurized cabin than the true ambient static pressure outside. Because the airspeed indicator compares this lower static pressure to pitot (ram) pressure, the indicated airspeed reads higher than actual. The altimeter will similarly read slightly high, and the VSI may show a brief false climb during the transition.
Why the other options are wrong
- B. This would occur only if cabin pressure were higher than ambient, which is not the typical case.
- C. There is a measurable, predictable effect due to the pressure differential inside the cabin.
- D. The instrument continues to respond to pressure changes; it does not freeze.
Alternate Static Source Effects
When the alternate static source (vented inside an unpressurized cabin) is used, cabin pressure is usually slightly lower than true ambient pressure, causing pitot-static instruments to read differently than normal.
- Airspeed and altitude typically read slightly higher than actual
- VSI may show a brief false climb during the transition
- Pilots should consult the POH for exact correction values
Memory trick: Cabin air is 'thin' from airflow, so gauges think you're higher and faster than you are.