Authorization for an aircraft to proceed under conditions specified by an air traffic control unit.
Note 1.— For convenience, the term “air traffic control clearance” is frequently abbreviated to “clearance” when used in appropriate contexts.
Note 2.— The abbreviated term “clearance” may be prefixed by the words “taxi”, “take-off”, “departure”, “en-route”, “approach” or “landing” to indicate the particular portion of flight to which the air traffic control clearance relates.
Source: ICAO Doc 4444 PANS-ATM
Clearances are issued solely for expediting and separating air traffic and are based on known traffic conditions which affect safety in aircraft operation. The traffic conditions include:
- aircraft in the air;
- aircraft on the manoeuvring area;
- vehicles on the manoeuvring area;
- obstructions not permanently installed on the manoeuvring area.
ATC clearances do not constitute authority to violate any applicable regulations for promoting the safety of flight operations or for any other purpose; neither do clearances relieve a pilot-in-command of any responsibility whatsoever in connection with a possible violation of applicable rules and regulations.
If an air traffic control clearance is not considered suitable by the pilot-in-command of an aircraft, the flight crew may request and, if practicable, obtain an amended clearance.
ATC clearances are issued for controlled flights only, i.e.:
- When a flight plan specifies that the initial portion of a flight will be uncontrolled, the pilot needs to obtain a clearance from the relevant ATC unit before entering controlled airspace.
- When a flight plan specifies that the first portion of a flight will be subject to ATC, and that the subsequent portion will be uncontrolled, the aircraft is normally cleared to the point at which the controlled flight terminates.
- For flights through intermediate stops, the initial clearance limit is be the first destination aerodrome and new clearances are to be issued for each subsequent portion of flight. By prior arrangement between ATC units and the operators however it is possible to clear the flight through intermediate stops.
A flight is subject to ATC clearance depending on the flight rules and airspace class, i.e.:
- IFR flights are subject to ATC clearance in airspace classes A, B, C, D and E.
- VFR flights are subject to ATC clearance in airspace classes B, C and D. They are not permitted in class A airspace and are not controlled in class E.
ATC clearances (except departure and arrival clearances) contain the following items in the order listed:
- aircraft identification;
- clearance limit;
- route of flight;
- level(s) of flight for the entire route or part thereof and changes of levels if required;
- any necessary instructions or information on other matters (e.g. SSR transponder operation, approach or departure manoeuvres, the time of expiry of the clearance, etc.).
The clearance limit is described by specifying the name of the appropriate significant point, or aerodrome, or controlled airspace boundary. When prior coordination has been effected with units under whose control the aircraft will subsequently come, or if there is reasonable assurance that it can be effected a reasonable time prior to their assumption of control, the clearance limit shall be the destination aerodrome or, if not practicable, an appropriate intermediate point, and coordination shall be expedited so that a clearance to the destination aerodrome may be issued as soon as possible.
The route of flight is to be detailed in each clearance when deemed necessary. Subject to airspace constraints, ATC workload and traffic density, and provided coordination can be effected in a timely manner, an aircraft should whenever possible be offered the most direct routing.
Instructions included in clearances relating to levels shall consist of:
- cruising level(s) or, for cruise climb, a range of levels, and, if necessary, the point to which the clearance is valid with regard to the level(s);
- levels at which specified significant points are to be crossed, when necessary;
- the place or time for starting climb or descent, when necessary;
- the rate of climb or descent, when necessary;
- detailed instructions concerning departure or approach levels, when necessary.
Standard clearances for departing and arriving aircraft are described in a dedicated article.
The flight crew shall read back to the air traffic controller safety-related parts of ATC clearances and instructions which are transmitted by voice. The following items shall always be read back:
- ATC route clearances;
- clearances and instructions to enter, land on, take off from, hold short of, cross, taxi and backtrack on any runway;
- runway-in-use, altimeter settings, SSR codes, level instructions, heading and speed instructions and transition levels.
Other clearances or instructions, including conditional clearances, shall be read back or acknowledged in a manner to clearly indicate that they have been understood and will be complied with.
The controller shall listen to the readback to ascertain that the clearance or instruction has been correctly acknowledged by the flight crew and shall take immediate action to correct any discrepancies revealed by the readback.
Unless specified by the appropriate ATS authority, voice readback of CPDLC messages shall not be required.
Amendment of Clearance
When issuing a clearance covering a requested change in route or level, the exact nature of the change shall be included in the clearance.
When traffic conditions will not permit clearance of a requested change, the word “UNABLE” shall be used. When warranted by circumstances, an alternative route or level should be offered.
When an alternative route is offered and accepted by the flight crew, the amended clearance issued shall describe the route to the point where it joins the previously cleared route, or to the destination (if the aircraft will not re-join the previous route).