Definition
A flight instrument that displays the airplane's altitude by measuring the surrounding atmospheric pressure and converting it into a height reading, typically referenced to mean sea level when set to the local altimeter setting.
Plain English
The cockpit gauge that tells the pilot how high the airplane is flying. It works by sensing how much air pressure is pressing on it — the higher you go, the lower the pressure — and turning that into a height in feet.
Context Anchor
Seen on the instrument panel during preflight, taxi, takeoff, climb, cruise, descent, and landing. During a visual preflight assessment, the pilot checks that the altimeter appears normal and shows a reasonable value for the airport elevation when set correctly.
Derivation
From Latin altus meaning 'high,' combined with the Greek-derived suffix -meter meaning 'a device that measures.' So altimeter literally means 'a device that measures height.'
Why Pilots Care
Accurate altimeter readings are required for terrain clearance, airspace compliance, and safe approaches.
Intuition Check
An altimeter does not directly measure the airplane’s height above the ground. It measures air pressure and displays altitude based on the pressure setting selected by the pilot.
Example Sentence 1
Before taxiing, the pilot set the altimeter to the current local setting given by the tower.
Example Sentence 2
While en route the altimeter read 6500 feet as the pilot crossed the ridge line.