What is the effect of a blocked static port on the altitude indication during a long flight?

• A. Airspeed readings may become unreliable.
• B. Vertical speed indicator will fail.
• C. Altitude readings may become unreliable because the static pressure input is obstructed.
• D. Pitot pressure will indicate the wrong value.

Answer: (C)

Altitude indication relies on static pressure that changes with altitude. If the static port is blocked, the altimeter no longer receives ambient static pressure; the pressure inside the static system is trapped and does not reflect outside air pressure changes as you climb or descend. As a result, the altitude reading can freeze at the value at the moment of blockage or drift to an incorrect value, making the altitude indication unreliable during a long flight.