A pitot-static system's static port becomes blocked by ice, and the pilot switches to the alternate static source, which vents into the unpressurized cabin. In this cabin, airflow past unsealed openings creates a slightly lower pressure than outside ambient static pressure. What effect will this have on the altimeter indication?
- AThe altimeter will indicate a higher altitude than actual
- BThe altimeter indication will be unaffected
- CThe altimeter will freeze at the altitude when the switch occurred
- DThe altimeter will indicate a lower altitude than actual
Show answer & explanationAnswer & explanation
Correct answer: A. The altimeter will indicate a higher altitude than actual
When cabin static pressure is lower than actual outside static pressure (due to airflow effects, per Bernoulli's principle), the altimeter interprets this lower pressure as a higher altitude, causing it to indicate higher than the aircraft's actual altitude. Airspeed indications are also typically affected, usually reading slightly higher than actual as well.
Why the other options are wrong
- B. The alternate static source directly changes the pressure reference, so an effect will occur.
- C. Instruments do not freeze in place; they respond continuously to the new pressure source.
- D. A lower cabin pressure input would cause the altimeter to read higher, not lower.
Alternate Static Source Errors
When switching to an alternate static source venting into an unpressurized cabin, the pitot-static instruments may show erroneous readings due to differences between cabin and true outside static pressure.
- Cabin static pressure is often lower than outside ambient due to airflow (Bernoulli effect)
- Lower static pressure causes altimeter to read higher than actual
- Airspeed indicator typically reads higher than actual with alternate static source
- Correction values are usually published in the aircraft flight manual (AFM)
Memory trick: Lower pressure inside the cabin tricks the altimeter into thinking you're higher — like a balloon expanding at altitude.