Definition
The small window on the face of a sensitive altimeter that displays the barometric pressure setting the altimeter is referenced to. Rotating the adjustment knob changes the value shown in the window, which in turn shifts the altitude reading on the main dial.
Plain English
It is the little number box on an altimeter where you set the local air pressure. When you turn the knob, the number in that box changes, and the altimeter's altitude reading shifts to match.
Context Anchor
Seen on the face of a pressure altimeter during preflight, after receiving an altimeter setting, and during altimeter maintenance or accuracy checks.
Derivation
Named after Paul Kollsman, the German-American engineer who, in 1928, developed the first accurate barometric altimeter. The display window where the pressure setting appears took his name and has kept it ever since.
Why Pilots Care
Correct use prevents large altitude errors that could lead to terrain conflicts or ATC altitude deviations.
Grounding Statement
When you turn the altimeter setting knob, the number in the Kollsman window changes, and the altitude shown by the instrument moves with it.
Intuition Check
Do not think of this as an aircraft cabin window. Here, “window” means a small display area on the altimeter face.
Example Sentence 1
Before taxi, the pilot set 30.12 in the Kollsman window after listening to the ATIS.
Example Sentence 2
During the approach she verified the Kollsman window matched the current reported pressure.