What does the altimeter measure in relation to the aircraft?

The altimeter is a critical instrument in aviation that measures the altitude of the aircraft relative to sea level. It operates based on the principles of atmospheric pressure, using the standard atmospheric model to interpret changes in pressure as changes in altitude. When the aircraft climbs or descends, the pressure changes; thus, the altimeter indicates how high or low the aircraft is in relation to a predetermined reference point, usually sea level.

This capability is crucial for maintaining safe flight operations, especially during takeoff, landing, and in controlled airspace. By correlating pressure readings to altitude, pilots can ensure they maintain the correct flight level, which is essential for avoiding terrain and other aircraft. The altimeter is calibrated to provide an accurate reading based on the current atmospheric conditions, allowing for reliable altitude data that is vital for navigation and safety.