The Kollsman Window is a feature incorporated into an aircraft's altimeter, serving as a tool for accurately measuring altitude during flight operations. Positioned at the 3 o'clock position on the altimeter dial, this small barometric setting display enables pilots to calibrate the instrument to current atmospheric conditions. By allowing manual input of local barometric pressure, the window ensures precise altitude awareness and compliance with airspace regulations.
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Jim Goodrich
Jim Goodrich is a pilot, aviation expert and founder of Tsunami Air.
What is the Kollsman window in aviation?
The Kollsman window is a feature found on the altimeter that allows pilots to input local barometric pressure settings from ground stations or weather reports. This window is incorporated into the altimeter, which is a tool for accurately measuring altitude during flight operations.
The Kollsman window is the barometric-pressure setting window located on the face of a pressure altimeter, usually at the three-o'clock position. It is a sub-dial that contains the barometric readings in either inches of mercury (InHg) or millibars, depending on the altimeter model, and provides a means to adjust the altimeter. By turning the knob, the pilot sets the current local station pressure (QNH) into this window. Once QNH is set, the altimeter indicates the height above mean sea level. In this way the Kollsman window eliminates errors that arise from weather-induced pressure changes, allowing the instrument to display the correct altitude only after it is set.
Why is it called the Kollsman window?
The window is called the Kollsman window because it is named after Paul Kollsman. Born in Germany on 22 February 1900 and trained in civil engineering in Stuttgart and Munich, Kollsman emigrated to the United States in 1923. While employed by the Pioneer Instrument Company in Brooklyn, he grew dissatisfied with the low precision of contemporary barometric altimeters and, working from his attic in 1928, developed an altimeter of unprecedented accuracy. In that same year he formed Kollsman Instruments Company with an initial investment of 500 dollars. The instrument reads zero when barometric pressure is 29.92 inches of mercury (760 mm) and converts the sensed pressure into a distance above sea level in feet (meters), enabling pilots to fly during periods of restricted visibility.
The instrument’s reliability was proven in 1929 when Jimmy Doolittle used a Kollsman altimeter during the first demonstrated instrument flight. The Navy subsequently purchased 300 units, launching the commercial success of Kollsman's enterprise. Recognizing this breakthrough, manufacturers incorporated a dedicated setting knob and scale window. Users quickly identified the feature with the inventor, so the dial-set aperture became popularly known as the Kollsman window, and the name endures on every barometric altimeter used today.
How does the Kollsman window work?
The Kollsman window works by letting pilots set the current local barometric pressure. Turning the knob moves the card indication, and this motion is transferred through gears that rotate the entire inside mechanism of the altimeter to change the Kollsman-window reading. Pilots set the altimeter to current local pressure so the instrument indicates zero on ground. This operation determines current atmospheric barometric pressure at the airfield and calibrates the altimeter for accurate altitude measurement. Without an adjustment, the altimeter is subjected to pressure changes from weather as well as altitude, so pilots input local barometric pressure from ATIS to keep the altimeter calibrated while flying from high-pressure weather systems to low-pressure systems.
What is the Kollsman window setting?
The Kollsman setting is the local barometric pressure setting, expressed in inches of mercury or millibars, that eliminates error due to local weather conditions. The Kollsman window is the small aperture on the face of the altimeter where the pilot enters the local barometric pressure obtained from the nearest airport, a weather report, or a ground station. The altimeter setting is entered in the Kollsman window and pilots set the Kollsman window to the setting that they received.
Setting the Kollsman window adjusts the altimeter reading. Without an adjustment, the altimeter is subjected to pressure changes as a result of weather and not just a change in altitude. As you fly from high-pressure weather systems to low-pressure systems you need to adjust your altimeter to get an accurate Mean Sea Level altitude reading. Turning the knob until the needle indicates the airport elevation will give the QNH in the Kollsman window, confirming that the correct local value is displayed.
What are the limits of the Kollsman window?
The Kollsman window range is at least 28.1-30.99 inches of Hg (946-1049 millibars). Altimeters built to this standard have a wider specified range of 27.50-31.50 inches of Hg (931.3 to 1066.7 mb). A sub-dial on most instruments covers 28.1 InHg to 31.0 InHg, so the practical Kollsman window limits coincide with the minimum required scale.
How to read the Kollsman window?
To read the Kollsman window, look first at the small rectangular cut-out beside the main altitude drum: inside it, digits show either InHg or milli-bars. Turn the knob until the displayed number matches the reported local pressure. As you do, the sub-dial rotates and the Kollsman window reads out the new figure in milli-bars in lieu of InHg. Once you have dialled the correct altimeter setting in the Kollsman window, the instrument converts static air pressure into an indicated altitude that equals mean sea level altitude, eliminating error due to local weather conditions. Pilots must update the altimeter setting every 100 NM (185.2 km) below FL180 and cross-check altitude using terrain awareness, visual references, and proper minimums, because misreading the tens-of-thousands hand leads to being off by 10,000 feet (3,048 meters). Temperature affects altimeter accuracy, so resetting is vital for safety.
