Definition
A small, sealed instrument component, usually disc-shaped, made of two thin metal diaphragms joined at their edges to enclose a controlled volume of gas or a partial vacuum. As outside pressure changes, the capsule expands or contracts, and that movement is mechanically linked to a needle or pointer on a flight instrument such as an altimeter, vertical speed indicator, or airspeed indicator.
Plain English
A tiny sealed metal pouch inside a flight instrument that swells or shrinks as air pressure changes, and that movement is what makes the instrument's needle move.
Context Anchor
Seen in discussions of spacecraft, high-altitude aircraft, escape systems, and protected instrument or equipment containers.
Derivation
From the Latin 'capsula', meaning 'small box' or 'little case'. The instrument capsule is literally a small sealed case, which fits the original meaning closely.
Why Pilots Care
If a capsule develops a leak or sticks, the instrument it drives will read incorrectly. Understanding that a sealed capsule is the heart of these instruments helps explain why pitot-static system checks and instrument calibration matter.
Intuition Check
Capsule does not mean a medicine pill here. In aviation, it means a sealed or protected enclosure that holds people or equipment.
Example Sentence 1
When the aircraft climbs, the altimeter capsule expands as outside pressure drops, and the pointer rotates to show the higher altitude.
Example Sentence 2
During overhaul the technician tests the capsule for leaks before reinstalling the instrument.