If the static ports are blocked in flight, which two instruments are most affected?

• A. Altimeter and VSI
• B. Airspeed indicator and Altimeter
• C. A/SI and VSI
• D. Altimeter only

Answer: (A)

Static ports provide the ambient atmospheric pressure reference used by the air data system. The altimeter translates static pressure into altitude, and the vertical speed indicator measures how quickly that static pressure is changing, i.e., climb or descent rate. When the static ports are blocked, there is no valid outside static pressure signal for these two instruments, so the altimeter can freeze or read incorrectly and the VSI can’t accurately indicate rate of climb or descent. The airspeed indicator uses both pitot (dynamic) pressure and static pressure, so it becomes unreliable too if static pressure is blocked, but the readings for altitude and rate of change rely entirely on static pressure, making them the most affected.