What is the primary purpose of the Kollsman window on the altimeter?

• A. Indicates temperature
• B. Corrects for atmospheric pressure changes
• C. Measures airspeed
• D. Displays altitude above sea level

Answer: (B)

The key idea is that altitude readings must be matched to the current atmospheric pressure. The Kollsman window provides an adjustable reference pressure for the altimeter. By inputting the local barometric pressure (the current QNH), the instrument converts the sensed pressure into an altitude that reflects true height above mean sea level. In other words, it corrects for changes in atmospheric pressure so your altitude reading is accurate under the present weather conditions.

This isn’t about temperature, which is not read from the Kollsman window; it doesn’t measure airspeed; and while the altimeter indicates altitude, that indication only becomes correct when the Kollsman window is set to the proper local pressure.