If a static port is blocked, which instruments are most likely to show erroneous readings?

• A. Airspeed indicator and altimeter
• B. Altimeter and vertical speed indicator
• C. Heading indicator
• D. Turn coordinator

Answer: (B)

Static pressure is the reference used by instruments that measure altitude and rate of change of altitude. When the static port is blocked, that ambient static pressure input is lost for those instruments.

The altimeter relies on static pressure to display altitude, so with a blocked static port it can’t sense true changes in height and may hold its reading or react inaccurately to climbs and descents. The vertical speed indicator determines how quickly altitude is changing by monitoring static pressure changes; without a usable static reference, it will show erroneous or unreliable rates of climb or descent.

Heading indicators and turn coordinators use gyroscopes, not static pressure, so they aren’t directly affected by a blocked static port.

Thus, the instruments most likely to show erroneous readings are the altimeter and the vertical speed indicator.