What instrument is primarily affected by the pitot tube blockage?

The primary instrument affected by pitot tube blockage is the airspeed indicator. The pitot tube measures dynamic air pressure, which is essential for calculating airspeed. When the pitot tube becomes blocked, the instrument is unable to receive the necessary data, leading to erroneous airspeed readings. If the blockage occurs while the aircraft is in level flight, the airspeed indicator may indicate a lower airspeed than actual or may read zero, depending on altitude and flight conditions.

The other instruments listed do not rely on the pitot tube for their operation. The altimeter, for example, measures static pressure rather than dynamic pressure, allowing it to function independently of the pitot tube. The attitude indicator shows the aircraft's orientation relative to the horizon and does not depend on air pressure, while the heading indicator is based on magnetic or gyro inputs to show the aircraft's heading. Each of these instruments uses different sources of data, making them unaffected by issues related to the pitot tube.