How does an altimeter determine altitude?

• A. It uses GPS
• B. It measures static pressure and uses it to drive a calibrated mechanism so that altitude is shown
• C. It measures dynamic pressure
• D. It measures stagnation pressure

Answer: (B)

Altitude is determined from the ambient static pressure. As you climb, the air pressure around the aircraft drops, and the altimeter’s sealed aneroid capsules respond to that static pressure change. The capsules move a gear train that drives the clock-like display, so the hands show a number corresponding to the height above a reference pressure surface.

The reference pressure is set with the Kollsman window. By adjusting it to the local sea-level pressure (QNH), the readout becomes the altitude above mean sea level. Without that setting, the instrument shows altitude relative to a standard pressure surface, not necessarily true sea-level height.

This instrument relies on static pressure alone; it does not use GPS for altitude and it isn’t driven by dynamic or stagnation pressure, which are associated with airspeed rather than altitude.