When using an alternate static source, which instruments are most likely to show readings different from ambient air data?

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

Answer: (C)

Using an alternate static source means the static pressure feeding the instruments is not the true ambient pressure. The altimeter directly uses static pressure to indicate altitude, so any change in that pressure source will shift the altitude reading away from the actual outside air, often by a noticeable amount. The vertical speed indicator measures the rate of change of static pressure, so a different static input alters the signal it uses to show climb or descent, leading to readings that don’t match the real rate of altitude change.

Attitude indicators and turn coordinators rely on a gyroscopic system rather than static pressure, so they aren’t directly affected by switching the static source. The airspeed indicator uses both pitot (dynamic) pressure and static pressure, so it can be affected as well, but the most direct and consistent effects from an alternate static source show up on the altimeter and VSI.