Which instrument indicates altitude by sensing pressure changes?

• A. A device that measures airspeed.
• B. A navigation instrument showing ground speed.
• C. A vertical speed indicator.
• D. A flight instrument that indicates altitude by sensing pressure changes.

Answer: (D)

Altitude is determined from ambient static pressure: as you climb, the outside pressure drops, and the altimeter converts that pressure change into a readout of altitude. It uses an aneroid cell that expands or contracts with pressure and translates that movement into feet displayed on the instrument. You set the local pressure (the QNH) so the reading reflects altitude above mean sea level; on the standard setting (29.92 inHg or 1013.25 hPa) you get pressure altitude. Temperature can skew readings, which is why pilots cross-check with other data and charts. The other instruments measure different things—airspeed from dynamic pressure, ground speed from navigation data, and vertical speed from the rate of change of pressure—so they do not indicate altitude directly. The altimeter is the instrument that indicates altitude by sensing pressure changes.