An unpressurized aircraft experiences a blocked static port in cruise flight. The pilot selects the alternate static source, which vents to the unpressurized cabin. Assuming the cabin vents/heater are off, what effect will this most likely have on the altimeter reading compared to the actual altitude?
- AThe altimeter will read the same as the actual altitude with no change
- BThe altimeter will freeze and stop responding entirely
- CThe altimeter will read higher than the actual altitude
- DThe altimeter will read lower than the actual altitude
Show answer & explanationAnswer & explanation
Correct answer: C. The altimeter will read higher than the actual altitude
In most unpressurized aircraft, air flowing over the fuselage creates a venturi effect that produces slightly lower-than-ambient pressure inside the cabin. When the alternate static source vents to this lower-pressure cabin air, the altimeter interprets the lower pressure as a higher altitude, so the altimeter reads higher than the aircraft's actual altitude. Airspeed indications are similarly affected, typically reading slightly greater than actual, and the VSI may show a brief false climb indication when the source is switched.
Why the other options are wrong
- A. Some change is expected due to the pressure differential from the venturi effect around the fuselage.
- B. The altimeter continues to function; it does not freeze when using the alternate static source.
- D. A lower altimeter reading would require higher-than-ambient cabin pressure, which is generally not the case.
Alternate Static Source Errors
When the alternate static source vents to the cabin of an unpressurized aircraft, airflow over the fuselage typically creates lower-than-ambient cabin pressure, causing the altimeter and airspeed indicator to read higher than actual values.
- Cabin pressure is usually slightly lower than outside static pressure due to airflow (venturi effect)
- Altimeter and airspeed typically read higher than actual when using alternate static source
- Pilots should consult the POH for specific correction values for their aircraft
Memory trick: Cabin air is thinner from airflow, so gauges think you're higher than you are