What is the primary purpose of an altimeter?

The primary purpose of an altimeter is to measure the height of the aircraft above a specific reference point, typically sea level. Altimeters accomplish this by measuring atmospheric pressure, which decreases with altitude. As the aircraft ascends, the pressure outside the aircraft drops, and the altimeter responds to this change, displaying the aircraft's altitude on a dial or digital readout. This information is crucial for maintaining a safe flying altitude, avoiding terrain and other aircraft, and ensuring compliance with air traffic control regulations.

The other choices relate to other instruments and aspects of flying. For example, measuring forward velocity pertains to a speed indicator or airspeed indicator, not an altimeter. Indications of rate of climb are provided by a variometer, which indicates how quickly an aircraft is gaining or losing altitude. Lastly, turned angles would be associated with instruments like a heading indicator or a GPS system, rather than an altimeter. Understanding the distinct roles of each instrument is essential for effective operation and navigation in aviation.