Which instrument is least affected by using an alternate static pressure?

• A. Attitude indicator
• B. Altimeter
• C. Vertical speed indicator
• D. Airspeed indicator

Answer: (D)

When the normal static source is blocked, an alternate static pressure source can distort readings on instruments that depend on static pressure. The altimeter relies directly on static pressure to show altitude, so changing the static reference makes its indication noticeably off. The vertical speed indicator tracks the rate of change of static pressure, so it also shows significant error or momentary spurious readings when the static source is changed. The attitude indicator’s gyro is largely independent of ambient static pressure, so it remains relatively unaffected, though some minor indications can occur if the instrument’s case is vented to cabin air. The airspeed indicator, however, measures dynamic pressure as the difference between pitot (ram) pressure and static pressure. Since its primary signal is this differential, changing the static reference introduces only a smaller error compared with the other instruments, making it the least affected.