(4)
TRANSITION ALTITUDE - The altitude in the vicinity of an aerodrome at or below which the vertical position of an aircraft is determined from an altimeter set to sea level pressure (QNH).

(5)
TRANSITION LEVEL - The lowest Flight Level available for use above the Transition Altitude.

(6)
TRANSITION LAYER - The airspace between the Transition Altitude and the Transition Level.

b.
TRANSITION FROM FLIGHT LEVELS TO ALTITUDES AND VICE VERSA - The vertical position of aircraft at or below Transition Altitude shall be expressed in altitude (QNH). Vertical position at or above the Transition Level shall be expressed in terms of Flight Levels. When passing through the Transition Layer, vertical position shall be expressed in terms of Flight Levels when climbing, and in terms of altitude (QNH) when descending.

c.
TAKE-OFF AND CLIMB - A QNH altimeter setting shall be provided to aircraft by Air Traffic Control prior to take-off. The vertical position of an aircraft during climb shall be by reference to altitudes until reaching the Transition Altitude above which vertical position shall be by reference to Flight Levels.

d. ENROUTE

(1)
VERTICAL SEPARATION - Vertical separation and vertical position of an aircraft during enroute flight shall be expressed in terms of Flight Levels. However, on local or short range flights between adjacent airports, flights may be cleared on a QNH altimeter setting, if the entire flight is at or below the Transition Altitude of the airport concerned.

(2)
CRUISING LEVELS - An aircraft shall be flown at Flight Levels specified for the airway, or if off airway at Flight Levels corresponding to the magnetic tracks, as shown on the cruising altitude diagrams in the Enroute Supplement.

(3)
TERRAIN CLEARANCE - The lowest usable Flight Level providing terrain clearance and/or the prescribed minimum IFR cruising level shall be determined by Air Traffic Control and/or the pilot using QNH pressure values. QNH altimeter setting reports will be available from sufficient locations to permit determination of terrain clearance with an acceptable degree of accuracy. For those areas in which adequate QNH altimeter setting reports cannot be provided, the appropriate authority shall make available in the most usable form the information required to determine the lowest Flight Level which will assure adequate terrain clearance.

e.
APPROACH AND LANDING - QNH altimeter setting will be provided to aircraft in the approach clearance. The vertical position of an aircraft during approach shall be controlled by reference to Flight Levels until reaching the Transition Level below which vertical positioning shall be by reference to altitudes.

(1) After the approach clearance has been issued and the descent to land is commenced, the vertical position of an aircraft above the Transition Level may be made 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 engine aircraft for which an uninterrupted descent from a high altitude is desirable and to airports equipped to control such aircraft by reference to altitudes throughout the descent.
f.
MISSED APPROACH - Vertical position of an aircraft during missed approach will be governed by the same procedures specified for take-off and climb, enroute and approach and landing outlined in the above paragraphs. If descent is stopped above the Transition Altitude the altimeter must be reset to 29.92 (QNE) to insure adequate separation between this level and the adjacent levels above and below.

g.
FLIGHT PLANNING - The levels at which a flight is to be conducted shall be specified in a flight plan.

(1)
　
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