FAA Sport Pilot Airplane (SPA)Aircraft Performance and SystemsMedium
During flight, a pilot suspects the static port is blocked and opens the alternate static source, which is vented inside the unpressurized cabin. Cabin pressure in flight is normally slightly lower than the outside static pressure. As a result, compared to the actual values, the altimeter will now indicate an altitude that is
- Aunaffected, since only airspeed changes
- Bhigher than actual altitude
- Cidentical to actual altitude
- Dlower than actual altitude
Show answer & explanationAnswer & explanation
Correct answer: B. higher than actual altitude
Cabin air pressure is typically lower than outside static pressure due to airflow effects, so feeding this lower pressure to the altimeter causes it to interpret the lower pressure as a higher altitude. The airplane will therefore fly slightly lower than the altimeter indicates.
Why the other options are wrong
- A. The airspeed indicator and VSI are also affected by the alternate static source.
- C. There is a known error introduced by using the alternate static source in this way.
- D. This is the opposite effect; lower pressure input causes higher, not lower, indicated altitude.
Alternate Static Source
A backup static pressure source, often vented inside the cabin, used when the primary static port is blocked.
- Cabin pressure is usually slightly lower than outside static pressure
- Causes altimeter to read higher than actual altitude
- Causes airspeed indicator to read higher than actual airspeed
- Pilot should consult POH for specific error corrections
Memory trick: Lower cabin pressure fools the altimeter into thinking you're higher