9.4 An air traffic services unit receiving notification of an aircraft’s intention to change from IFR to VFR flight shall, as soon as practicable thereafter, so inform all other air traffic services units to whom the IFR flight plan was addressed, except those units through whose regions or areas the flight has already passed.
10. Clearances and information
10.1 Scope
10.1.1 The issuance of air traffic control clearances by air traffic control units constitutes authority for an aircraft to proceed only in so far as known air traffic is concerned.
10.1.2 Clearances are based solely on expediting and separating air traffic and do not constitute authority to violate any applicable regulations for promoting the safety of flight operations or for any other purpose.
10.1.3 Clearances are based on known traffic conditions which affect safety in aircraft operation. Such traffic conditions include not only aircraft in the air and on the manoeuvring area over which control is being exercised, but also any vehicular traffic or other obstructions not permanently installed on the manoeuvring area in use.
10.1.4 If an air traffic control clearance is not suitable to the pilot-in-command of an aircraft, he may request and, if practicable, obtain an amended clearance.
10.1.5 Clearances issued by controllers relate to traffic and aerodrome conditions only and do not relieve a pilot of any responsibility whatsoever in connexion with a possible violation of applicable rules and regulations.
10.2 Issuance
10.2.1 Air traffic control units shall issue such air traffic control clearances as are necessary to meet the objectives of collision prevention and the expedition and maintenance of an orderly flow of air traffic.
10.2.2 Aircraft flying through a terminal control area shall, where possible, be cleared by the most direct route from the entry to the exit point of the terminal control area. Similarly, aircraft arriving and/or departing within a terminal control area shall, where possible, be cleared by the most direct route from the point of entry to the aerodrome of landing or from the aerodrome of departure to the point of exit.
10.2.3 Aircraft intending supersonic flight shall, whenever practicable, be cleared for the transonic acceleration phase prior to departure.
11. Control of air traffic flow
11.1 When it becomes apparent to an air traffic control unit that traffic additional to that already accepted cannot be accommodated within a given period of time at a particular location or in a particular area, or can only be accommodated at a given rate, that unit shall so advise other air traffic control units known or believed to be concerned. Pilots-in-command of aircraft destined to the location or area in question and operators known or believed to be concerned shall also be advised of the delays expected or the restrictions that will be applied.
Note.— Attention is drawn to the guidance material contained in the Air Traffic Services Planning Manual (Doc 9426) regarding flow control.
12. Altimeter setting procedures
12.1 Expression of vertical position of aircraft
12.1.1 For flights in the vicinity of aerodromes the vertical position of aircraft shall, except as provided for in
12.1.1.1 and 12.1.1.2, 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 ascending and in terms of altitudes when descending.
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