For flights in the vicinity of aerodromes and within terminal control areas, the vertical position of aircraft shall be expressed in terms of…
Refer to figure.
ICAO Doc 4444
4.10 ALTIMETER SETTING PROCEDURES
4.10.1 Expression of vertical position of aircraft
4.10.1.1 For flights in the vicinity of aerodromes and within terminal control areas the vertical position of aircraft shall, except as provided for in 4.10.1.2, be expressed in terms of altitudes at or below the transition altitude and in terms of flight levels at or above the transition level. While passing through the transition layer, vertical position shall be expressed in terms of flight levels when climbing and in terms of altitudes when descending.
4.10.1.3 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; and
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 4.10.1.1 shall apply.
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