Definition
A sealed, flexible metal capsule (or stack of capsules) with most of the air evacuated, used as the pressure-sensing element in instruments such as altimeters, vertical speed indicators, and barometers. As outside air pressure changes, the capsule expands or contracts, and that mechanical movement is converted into an instrument reading.
Plain English
A small sealed metal can with the air mostly removed. When outside air pressure changes, the can flexes, and that flexing is what drives the needle on the instrument.
Context Anchor
Seen in aircraft instrument discussions, especially altimeters, pressure instruments, and airframe maintenance descriptions.
Derivation
From Greek 'a-' meaning 'without' and 'neros' meaning 'liquid' — literally 'without liquid.' Older barometers used a column of mercury (a liquid) to measure pressure. An aneroid does the same job without any liquid, using a flexing sealed capsule instead.
Why Pilots Care
Forms the core of the altimeter, allowing pilots to determine altitude based on pressure changes.
Analogy
Think of a sealed, empty metal can that slightly crushes or expands as outside air pressure changes. An aneroid element uses that same basic movement, but in a precise way.
Intuition Check
Aneroid does not mean electronic or automatic. It means pressure-sensing without liquid, using a sealed flexible chamber.
Example Sentence 1
The altimeter uses an aneroid capsule that expands as the aircraft climbs and outside pressure drops.
Example Sentence 2
Maintenance technicians check the aneroid for leaks during instrument inspections.