Which is a performance flight instrument?

The altimeter is classified as a performance flight instrument because it directly measures and displays the altitude of the aircraft above a reference point, typically sea level. This instrument plays a crucial role in maintaining altitude, which is essential for performance during various phases of flight, including takeoff, climb, cruise, descent, and landing. The readings provided by the altimeter help pilots make necessary adjustments to maintain the desired flight level, crucial for safe operation.

Glideslope, while important for approach and landing, is specifically used to assist in vertical navigation and is part of the instrument landing system, but it does not measure overall aircraft performance like altitude. Course and bearing instruments are more about navigational aspects rather than performance metrics, as they help pilots determine their path or direction relative to a specific point rather than directly influencing the aircraft’s performance.