Which instrument indicates the aircraft's altitude?

The altimeter is the instrument responsible for indicating an aircraft's altitude by measuring the atmospheric pressure surrounding the aircraft. As the aircraft ascends or descends, the air pressure changes, and the altimeter reflects these changes in altitude. It is calibrated to recognize standard sea-level pressure, allowing pilots to determine their flight altitude above mean sea level (MSL) accurately.

Understanding the functionality of the altimeter is crucial for safe flying, as precise altitude readings are essential for navigation, compliance with air traffic control, and avoiding terrain and obstacles. On the other hand, the airspeed indicator measures how fast the aircraft is moving through the air, the vertical speed indicator gauges the rate of ascent or descent, and the heading indicator shows the current direction the aircraft is facing but does not provide altitude information.