Altitude matters for terrain clearance, traffic separation, and fuel planning. The altimeter measures it by sensing static air pressure, which drops as an aircraft climbs. The instrument converts that pressure into a displayed height.

Barometric altimeters need a reference setting. Pilots dial in the local pressure from air traffic control or set a standard value above a transition altitude. Get the setting wrong and every aircraft in the area could be off by hundreds of feet.

Altimeter settings

• QNH, pressure at sea level, used for low-altitude flight
• QFE, pressure at the airfield, gives height above the field
• Standard pressure, 29.92 inHg or 1013.25 hPa, used at high altitude

Radar altimeters work differently, bouncing a signal off the ground. They provide precise height above terrain and feed warnings like ground proximity alerts.