Escape From Frame

Altimetry:

The conversion of pressure values into altitude values is done in the altimeter, according to the so - called standard atmosphere, as determined by the International Commission for Aviation (ICAN). 

This standard atmosphere has the following characteristics:

The pressure at average sea level amounts to 1,013.25 mbar and the temperature decreases by 6.5°C per 1,000 meters difference in height, until a height of 11 km (standard tropopause);

Altimeter at QNH will:

1- indicate the airport's official level during the take off and landing;

2- indicate the plane's altitude above the sea level's average height, during the flight.

The QNH above a certain area is the lowest QNH observed above this areas and is used by regional control centers, with regard to security in the aviation sector.

Altimeter at QFE will:

1- be zero at the airport itself;

2- indicate an altitude above the airport during the flight.

Since the real difference in pressure, in the air column between the plane and the ground, only seldom corresponds to the one according to the standard atmosphere, the values on the altimeter somewhat deviate from the real values.

QNE:

During the flight the altimeter will indicate the altitude (according to the standard atmosphere of 1,013.2 mbar) above this pressure level, the so-called "Flight Level".

    

See at: A.I.P section two: Taking off & climbing / vertical separation / approach and landing