FAA Ground School Practice Test

Which instrument becomes inoperative if the pitot tube becomes clogged?

1. Airspeed

2. Altimeter

3. Vertical speed indicator

4. Attitude indicator

The correct answer is: Airspeed

When the pitot tube becomes clogged, it obstructs the flow of air that is essential for measuring dynamic pressure. The airspeed indicator relies on this dynamic pressure to determine the aircraft's airspeed. If the pitot tube is blocked, the instrument cannot accurately measure the difference between the static air pressure and the ram air pressure, which results in the airspeed indicator becoming inoperative. In contrast, the altimeter and vertical speed indicator use static pressure and are not affected by the pitot tube's blockage. Therefore, these instruments will continue to function normally as long as they have an unobstructed static source. The attitude indicator is completely independent of both the pitot tube and static pressure, as it primarily relies on gyroscopic principles for determining the aircraft's orientation. This makes the airspeed indicator the only instrument that becomes inoperative due to a clogged pitot tube.