QNH – is the altimeter setting with pressure reduced to Mean Sea Level in International Standard Atmosphere. On setting QNH in the altimeter the a/c will show the altitude above sea level, in Flight or at the Ground.
QFE – is the altimeter setting existing over a particular aerodrome. On setting QFE in the altimeter the a/c will show the height above aerodrome in the Flight and will show 0 feet if aircraft is on ground at the airfield.
Transition Altitude – is an altitude with reference to Mean Sea Level with the existing QNH setting around an Aerodrome area.
All the a/c flying below the Transition Altitude set their altimeter to QNH and report their Altitude with reference to Mean Sea Level.
Transition Level – is set with reference to Standard Pressure Altitude setting of 1013.2.
All the a/c flying at or above Transition Level set their altimeter to Pressure Altimeter setting of 1013.2 and report their altitude as Flight Level.
A climbing a/c changes its altimeter setting from QNH to QFE on crossing Transition Altitude.
A descending a/c changes its altimeter setting from QFE to QNH on crossing Transition Level.
Transition Altitude is usually given in the Jepessen charts.
Transition Level is usually given by the ATC, but in case of unavailability, the Transition Level can also be calculated manually as follows-
| If QNH pressure is above 997 Mb the Transition layer is increased to 1500 feet to get the Transition Level.
Example- Transition Altitude 5000 feet Existing QNH 1012.5Mb Since Existing QNH is above 997 Mb, increase Transition Altitude to 1500 feet. On Adding 5000 +1500 = 6500 feet We get Transition Level as 6500 feet. |
| If QNH pressure is below 997 Mb the Transition layer is decreased to 1000 feet to get the Transition Level.
Example- Transition Altitude 5000 feet Existing QNH 994 Mb Since Existing QNH is below 997 Mb, increase Transition Altitude to 1000 feet. On Adding 5000 +1000 = 6000 feet We get Transition Level as 6000 feet. |
The lowest transition level in India is 4000 feet.
The lowest flight level in India that can be used is FL50.
The highest flight level that can be used in India is FL450/460.
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