Using an alternate static source typically causes which instruments to show readings different from ambient air data?

• A. Airspeed indicator
• B. Attitude indicator
• C. Altimeter
• D. Altimeter and VSI

Answer: (D)

Using an alternate static source changes the pressure that the static system feeds to the instruments. The altimeter gets its information from static pressure alone, so when that pressure isn’t ambient, the altitude it shows will not match the true outside air data. The vertical speed indicator relies on how static pressure changes over time, so a different static pressure causes it to indicate an incorrect rate of climb or descent. The airspeed indicator uses both pitot (total pressure) and static pressure, so its indication can be affected too, but the most consistent off readings come from the altimeter and VSI.