Getting to know your Kollsman Window and Barber Pole
Reading the Kollsman Window ( Barometric Reading)
The Kollsman window is located at the 3 o’clock position on the altimeter dial. This window allows access to read a sub-dial, which contains the barometric readings. The arrowhead indice located precisely at the 3 o’clock position on the altimeter’s main dial is used as the reference point for reading the barometric sub-dial. Most altimeters will have a sub-dial, which covers the readings from 28.1 InHg (inches of mercury) to 31.0 InHg. On the sub-dial each major indice is read as 0.1 InHg, each minor indice is read as 0.02 InHg.
The Altimeter in Figure 1 reads 29.92 InHg
As previously, stated, weather conditions will greatly affect the pressure of the atmosphere (the barometric reading). Altimeters report altitude as a function of atmospheric pressure. Typically pilots will obtain a local barometric reading from the nearest airport. They will then set the Kollsman window to the setting that they received. This action will adjust the altimeter reading, eliminating error due to local weather conditions.
Some altimeters will have a Kollsman Window, which reads out in milli-bars in lieu of InHg. These are usually altimeters designated for use in Europe. However use of milli-bars has become more common in the U.S. during the last few years. Milli-Bars is just another unit of measurement, 1013.2 milli-bars = 29.92 InHg.
The Barber Pole
The “Barber Pole” on the face of the altimeter is visible only when the altitude is above sea level. When the altitude is below sea level the barber pole is no longer visible. This is provided to avoid the error of reading –1,000 Ft. as being +10,000 Ft.