What instrument is used for altitude indication?

The altimeter is the instrument specifically designed for altitude indication in aircraft. It measures the altitude by comparing the atmospheric pressure outside the aircraft to a standard reference pressure. As an aircraft ascends or descends, the changes in atmospheric pressure are translated into altitude readings.

When the aircraft climbs, the external air pressure decreases, and the altimeter will show a higher altitude. Conversely, when the aircraft descends, the pressure increases, and the altimeter indicates a lower altitude. Accurate altitude readings are essential for maintaining safe vertical separation from other aircraft and for compliance with air traffic control instructions.

The other instruments listed have different functions. The airspeed indicator measures how fast the aircraft is moving through the air, while the vertical speed indicator indicates the rate of climb or descent. The heading indicator provides the aircraft's orientation or heading relative to magnetic north. Each plays a critical role in flying, but the altimeter is the only one that specifically indicates altitude.