Kollsman Window Altimeter Barometric Pressure Setting
A barometric scale window of a sensitive altimeter used to adjust the altitude for the altimeter setting. The altimeter uses static pressure as its source of operation. Air is denser at sea level than aloft—as altitude increases, atmospheric pressure decreases. This difference in pressure at various levels causes the altimeter to indicate changes in altitude. Most altimeters are equipped with an adjustment knob located at the bottom of the instrument to manually compensate for the changes in atmospheric pressure.. This is called the barometric pressure setting window or “Kollsman Window”.
Kollsman Altimeter Adjustments
To adjust the altimeter for variation in atmospheric pressure, the pressure scale in the altimeter setting window, calibrated in inches of mercury (“Hg) and/or millibars (mb), is adjusted to match the given altimeter setting. Altimeter setting is defined as station pressure reduced to sea level, but, an altimeter setting is accurate only in the vicinity of the reporting station. Therefore, the altimeter must be adjusted as the flight progresses from one station to the next. Air traffic control (ATC) will advise when updated altimeter settings are available. If a pilot is not utilizing ATC assistance, local altimeter settings can be obtained by monitoring local automated weather observing system/automated surface observation system (AWOS/ASOS) or automatic terminal information service (ATIS) broadcasts.
It is important for pilots to understand that the altimeter is a barometric device that measures pressure, not altitude. Some pilots may think of the altimeter as a true “altitude indicator,” without error. In fact, the pressure altimeter is a barometer that measures changes in atmospheric pressure, and through a series of mechanisms and/or computer algorithms, converts these changes, and displays an altitude. This conversion process assumes standard atmospheric conditions,
The “Kollsman Window” is named after an American inventor , Paul Kollsman who invented the first accurate Barometric Altimeter. Prior to his invention the altimeters that existed could only determine an airplane’s altitude within a few hundred feet. In perfect weather during the day this was acceptable however, flying at night or in IFR conditions was extremely dangerous if not impossible. Kollsman’s altimeter responded to changes in barometric pressure and determined altitude with accuracy within a couple of feet. Kollsman produced barometers and instruments at the Kollsman Instruments Company that he founded in 1928 with $500.
Jimmy Doolittle flew with Kollsman’s Altimeter
In 1929 James “Jimmy” Doolittle agreed to try Kollsman’s altimeter in combination with a gyroscopic, artificial horizon gauge , a ground-based radio navigation system for communicating the aircraft’s position, and a cockpit navigation display. This was the historic “first blind flight” using only instruments.
The Kollsman Company continues to produce a variety of Aviation & Aerospace Instruments along with highly sensitive military and security products.