III. Area Control Service
an IFR flight shall be provided with alternative instructions to be complied with in the event that flight in VMC cannot be maintained for the term of the clearance;
c)  the pilot of an IFR flight, on observing that conditions are deteriorating and considering that operation in VMC will become impossible, shall inform ATC before entering IMC and shall proceed in accordance with the alternative instructions given.
Note.— See also 14.2.
14. Essential traffic information

14.1 Essential traffic is that controlled traffic to which the provision of separation by ATC is applicable, but which, in relation to a particular controlled flight, is not separated therefrom by the minima set forth in Sections 2 to 9 inclusive of Part III, Section 3 of Part IV and 6.6 of Part VI.
Note.— Pursuant to Section 1 of Part III, but subject to certain exceptions stated therein, ATC is required to provide separation between IFR flights in airspace Classes A to E, and between IFR and VFR flights in Classes B and C. ATC is not required to provide separation between VFR flights, except within airspace Class B. Therefore, IFR or VFR flights may constitute essential traffic to IFR traffic, and IFR flights may constitute essential traffic to VFR traffic. However, a VFR flight would not constitute essential traffic to other VFR flights except within Class B airspace.
14.2 Essential traffic information shall be given to controlled flights concerned whenever they constitute essential traffic to each other.
Note.— This information will inevitably relate to controlled flights cleared subject to maintaining own separation and remaining in visual meteorological conditions.
14.3 Essential traffic information shall include:
a)  direction of flight of aircraft concerned;
b)  type of aircraft concerned;
c)  cruising level of aircraft concerned and estimated time over the reporting point nearest to where the level will be crossed.
Note.— Nothing in Section 14 is intended to prevent ATC from imparting to aircraft under its control any other information at its disposal with a view to enhancing air safety in accordance with the objectives of ATS as defined in Chapter 2 of Annex 11.
15. Clearance of a requested change in flight plan
15.1 When issuing a clearance covering a requested change in flight plan (route or cruising level) the exact nature of the change shall be included in the clearance.
15.2 If a level change is involved and more than one level is contained in the flight plan, all such levels shall be included in the clearance.
15.3 When traffic conditions will not permit clearance of a requested change in a flight plan, the words “unable to clear” shall be used. When warranted by circumstances, an alternative flight plan should be offered.
15.4 When the alternative flight plan mentioned in 15.3 is offered, the complete clearance, as amended, or that part of the clearance containing the alternative shall be included.
EMERGENCY AND
COMMUNICATION FAILURE

16. Emergency procedures
16.1 General
16.1.1 The various circumstances surrounding each emergency situation preclude the establishment of exact detailed procedures to be followed. The procedures outlined herein are intended as a general guide to air traffic services personnel. Air traffic control units shall maintain full and complete co-ordination, and personnel shall use their best judgement in handling emergency situations.
Note.— To indicate that it is in a state of emergency, an aircraft equipped with an SSR transponder might operate the equipment as follows:
a)  on Mode A, Code 7700; or
b)  on Mode A, Code 7500, to indicate specifically that it is being subjected to unlawful interference.
　
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