On the altimeter, the Kollsman window is used to set the current sea-level pressure. Which option best completes this statement?

• A. Sea-level pressure
• B. Outside air temperature
• C. Dynamic pressure
• D. Mach number

Answer: (A)

The altimeter converts ambient air pressure into a reading of height, and it needs a reference pressure to do that conversion. The Kollsman window provides that reference by setting the current sea-level pressure (QNH). With the local sea-level pressure entered, the instrument displays altitude above mean sea level, because it uses the updated reference pressure to translate the observed pressure into height. If you were to use a standard setting instead, you’d read pressure altitude instead of height above sea level. The other terms—outside air temperature, dynamic pressure, and Mach number—aren’t what the window adjusts; they relate to temperature effects on altitude readings, airspeed measurements, and speed through the air, respectively.