Definition
The way a pressure altimeter works: a sealed aneroid wafer inside the instrument expands as outside air pressure decreases with altitude and contracts as pressure increases. This movement is mechanically linked to pointers on the face of the instrument, which display altitude in feet. The pilot sets a local sea-level pressure value in the Kollsman window so the displayed altitude is referenced to that pressure setting.
Plain English
It is how the altimeter turns changes in air pressure into a reading in feet. As you climb, the air gets thinner, a small sealed capsule inside the instrument expands, and that movement drives the needle to show a higher altitude.
Context Anchor
Used when learning how the altimeter works, setting the altimeter before flight, and understanding why the indicated altitude can be wrong if the pressure setting is wrong.
Derivation
From Latin altus (high) and the Greek-rooted metron (measure), so altimeter literally means 'height measurer.' Knowing this reinforces that the instrument is measuring something — in this case, air pressure — and translating it into height.
Why Pilots Care
Correct altimeter operation prevents controlled flight into terrain, ensures compliance with assigned altitudes, and maintains safe vertical separation from other aircraft.
Analogy
An altimeter is a little like a thermometer that has been marked in feet instead of degrees: it senses one physical thing and displays it as something more useful to the pilot.
Grounding Statement
The altimeter does not actually sense height — it senses outside air pressure and converts it to a height reading based on the pressure setting the pilot has dialled in.
Intuition Check
Altimeter operation does not just mean turning the setting knob. It means how the instrument changes sensed air pressure into an altitude reading; the knob only sets the pressure reference it uses.
Example Sentence 1
During preflight, the pilot reviewed altimeter operation with the student so they understood why the needle moved when they adjusted the pressure setting.
Example Sentence 2
In cruise flight the pilot cross-checks altimeter operation against the assigned altitude to confirm the airplane remains level at the proper height.