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12.1.1.1 After approach clearance has been issued and the descent to land is commenced, the vertical position of an aircraft above the transition level may be expressed by reference to altitudes (QNH) provided that level flight above the transition altitude is not indicated or anticipated.
Note.— This is intended to apply primarily to turbine-engined aircraft for which an uninterrupted descent from a high level is desirable and to aerodromes equipped to control such aircraft by reference to altitudes throughout the descent.
12.1.1.2 When an aircraft which has been given a clearance as number one to land is completing its approach using QFE, the vertical position of the aircraft shall be expressed in terms of height above aerodrome elevation during that portion of its flight for which QFE may be used, except that it shall be expressed in terms of height above runway threshold elevation:
II. General Provisions
a)  for instrument runways, if the threshold is 2 metres (7 feet) or more below the aerodrome elevation, and
b)  for precision approach runways.

12.1.2 For flights en route the vertical position of aircraft shall be expressed in terms of:
a)  flight levels at or above the lowest usable flight level;
b)  altitudes below the lowest usable flight level;

except where, on the basis of regional air navigation agreements, a transition altitude has been established for a specified area, in which case the provisions of 12.1.1 shall apply.
12.2 Determination of the transition level

12.2.1 Approach control offices or aerodrome control towers shall establish the transition level to be used in the vicinity of the relevant aerodrome(s) for the appropriate period of time on the basis of QNH reports and forecast mean sea level pressure if required.
12.2.2 The transition level shall be the lowest flight level available for use above the transition altitude established for the aerodrome(s) concerned. Where a common transition altitude has been established for two or more aerodromes which are so closely located as to require co-ordinated procedures, the appropriate air traffic services units shall establish a common transition level to be used at any given time in the vicinity of the aerodrome concerned.
Note.— See Part III, 4.2, regarding the determination of the lowest usable flight level(s) for control areas.
12.3 Provision of information

12.3.1 Appropriate air traffic services units shall at all times have available for transmission to aircraft in flight, on request, the information required to determine the lowest flight level which will ensure adequate terrain clearance on routes or segments of routes for which this information is required.
Note.— If so prescribed on the basis of regional air navigation agreements, this information may consist of climatological data.
12.3.2 Flight information centres and area control centres shall have available for transmission to aircraft on request an appropriate number of QNH reports or forecast pressures for the flight information regions and control areas for which they are responsible.
12.3.3 The transition level shall be included in approach clearances when so prescribed by the appropriate authority or requested by the pilot.
12.3.4 A QNH altimeter setting shall be included 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 already have received the information.
12.3.5 A QFE altimeter setting shall be provided to aircraft on request or on a regular basis in accordance with local arrangements; it shall be the QFE for the aerodrome elevation except for:
a)  non-precision approach runways, if the threshold is 2 metres (7 feet) or more below the aerodrome elevation, and
b)  precision approach runways,

in which cases the QFE for the relevant runway threshold shall be provided.
　
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