An aircraft has been instructed to descend from FL330 to FL110/11 000 ft altitude. When should the pilot change to QNH?
QNH. The atmospheric pressure adjusted to mean sea level.
Refer to figure.
ICAO Doc 4444
4.10.4 Provision of altimeter setting information
22.214.171.124 The flight crew shall be provided with the transition level in due time prior to reaching it during descent. This may be accomplished by voice communications, ATIS broadcast or data link.
126.96.36.199 The transition level shall be included in approach clearances when so prescribed by the appropriate authority or requested by the pilot.
188.8.131.52 A QNH altimeter setting shall be included in the descent clearance when first cleared to an altitude below the transition level, in approach clearances or clearances to enter the traffic circuit, and in taxi clearances for departing aircraft, except when it is known that the aircraft has already received the information.
When an aircraft is flying at or above the transition level, the vertical position of the aircraft shall be expressed as a flight level with reference to QNE (1013.2). When an aircraft is descending below the transition level, the vertical position of the aircraft shall be expressed as an altitude with reference to QNH. In order to change from QNE to QNH during descent, the pilot needs to know the transition level, which will be provided by the ATC.
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