A planes alitmeter is basically a barometer which measures the ambient air pressure and displays a reading in feet or meters. Higher up the atmosphere, the air pressure is lower and vice versa. This relation between air pressure and altitude is non-linear and the altimeters are calibrated according to an ideal curve representing the "standard atmosphere".
Pilots can choose/set the pressure at which the altimeter reads zero by turning a knob or entering a value (displayed in the "Kollsman window" of the altimeter). When flying up high, all pilots set this "zero pressure" at the same value, namely 1013.25 mbar. This is the mean/standard atmospheric pressure at sea level. This altimeter setting is reffered to as QNE and the corresponding altitudes are referred to as Flight Levels. For example FL300 reads as 30000ft above the 1013.25 mbar line. This way of setting the altimeter avoids collisions since each plane is instructed by flight control to fly at a different Flight Level. Due to local weather and other factors, the actual pressure at sea level can fluctuate a lot. As a result, the altimeter, when set to QNE, does not display the planes actual altitude above sea level. While flying up high, this is not an issue since obstacles (mountains / building / ...) are far below. QNE has the advantage that everyone uses the same 1013.25 mbar reference and it eliminates the need to constantly reset the pressure while flying cross country.
QNE doesn't work when flying low. During take-off and landing, pilots need to know the correct altitude to avoid collision with the surroundings (mountains, ...). In order to accomplish this, they switch to a different altimeter setting which is named QNH. They request the local flight control to send the correct pressure setting (measured locally at the airport) and adjust their altimeter accordingly.
Pilots can choose/set the pressure at which the altimeter reads zero by turning a knob or entering a value (displayed in the "Kollsman window" of the altimeter). When flying up high, all pilots set this "zero pressure" at the same value, namely 1013.25 mbar. This is the mean/standard atmospheric pressure at sea level. This altimeter setting is reffered to as QNE and the corresponding altitudes are referred to as Flight Levels. For example FL300 reads as 30000ft above the 1013.25 mbar line. This way of setting the altimeter avoids collisions since each plane is instructed by flight control to fly at a different Flight Level. Due to local weather and other factors, the actual pressure at sea level can fluctuate a lot. As a result, the altimeter, when set to QNE, does not display the planes actual altitude above sea level. While flying up high, this is not an issue since obstacles (mountains / building / ...) are far below. QNE has the advantage that everyone uses the same 1013.25 mbar reference and it eliminates the need to constantly reset the pressure while flying cross country.
QNE doesn't work when flying low. During take-off and landing, pilots need to know the correct altitude to avoid collision with the surroundings (mountains, ...). In order to accomplish this, they switch to a different altimeter setting which is named QNH. They request the local flight control to send the correct pressure setting (measured locally at the airport) and adjust their altimeter accordingly.
