ATC Clearances

An air traffic control clearance is an authorisation for an aircraft to proceed under conditions specified by an air traffic control unit. It is the controller's main tool for keeping controlled flights apart: every clearance is built on the traffic the unit knows about and tells the pilot where the aircraft may go, at what level and under what conditions. The term is usually shortened to clearance and prefixed by the phase it covers: taxi, take-off, departure, en route, approach or landing.

A clearance is permission, not a duty to fly into danger. Under ICAO and EASA rules it does not relieve the pilot-in-command of responsibility for the aircraft, its terrain clearance and compliance with the rules of the air, and the FAA makes the pilot-in-command the final authority for the safe operation of the aircraft. Knowing what a clearance contains, what it does not authorise and when it expires is basic instrument flying, and a favourite subject of IR, ATPL and FAA instrument examinations.

What a clearance is

Clearances are issued solely to expedite and separate air traffic. A clearance must be obtained before any flight, or part of a flight, is operated as a controlled flight. IFR flights need a clearance in Classes A to E; VFR flights need one in Classes B, C and D, but not in Class E (see airspace classification). Special VFR, flight within a control zone below VMC minima, is also a clearance, requested by the pilot.

The word CLEARED means authorised to proceed under the conditions specified; APPROVED means only that permission for a proposed action is granted. Each clearance covers its own part of the flight: a route clearance is not a clearance to enter a runway or to take off, which require separate clearances from the aerodrome controller (see aerodrome control phraseology).

A pilot who cannot accept a clearance says UNABLE, preferably with a reason and an alternative, and continues under the clearance previously issued until an amended one arrives. A clearance that would take the aircraft below the applicable minimum altitude outside radar vectoring, or into conditions it cannot fly, is queried, never simply read back. Under 14 CFR 91.123, US pilots may deviate from a clearance only in an emergency, in response to a TCAS resolution advisory, or after obtaining an amended clearance.

Elements of an IFR clearance (CRAFT)

ICAO lists the contents of a clearance as:

1. aircraft identification;
2. clearance limit;
3. route of flight;
4. level or levels for all or part of the route, and changes of level;
5. any other instructions or information, such as departure or approach manoeuvres, communications and the time of expiry of the clearance.

A departure clearance in Europe might be "G-CD, cleared to Lambourne via flight planned route, climb FL90, squawk 5023"; where a SID is to be flown, the clearance names it: "cleared via (SID designator) departure". The FAA teaches the same content as CRAFT: Clearance limit, Route, Altitude, Frequency for departure control and Transponder code. The US altitude item often includes an expect altitude, such as "climb and maintain 4,000, expect 8,000 ten minutes after departure".

The FAA also uses the abbreviated clearance, "cleared to (destination) airport as filed", when the filed route can be approved with little or no change. It always names the destination and any departure procedure to be flown, but it does not include the filed en route altitude, which is assigned separately or given as an expect altitude. At airports with a pre-taxi clearance programme, pilots may call clearance delivery or ground control not more than ten minutes before the proposed taxi time to receive the clearance before taxiing, or receive it by data link: a pre-departure clearance (PDC) needs no readback, whereas a CPDLC departure clearance requires a response from the crew.

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The pilot reads back the safety-related parts of every clearance. SERA.8015 makes the list fixed: ATC route clearances; clearances and instructions to enter, land on, take off from, hold short of, cross, taxi on and backtrack on any runway; the runway in use, altimeter settings, SSR codes and newly assigned communication channels; level, heading and speed instructions; and transition levels, even when they come from the ATIS. The readback ends with the call sign. If it is wrong, the controller replies NEGATIVE followed by the correct version. The FAA's AIM asks for readback of altitude assignments and restrictions, vectors, runway assignments, hold-short instructions and, in general, any part of a clearance containing numbers.

Clearance limits

The clearance limit is the point to which an aircraft is granted a clearance: an aerodrome, a navigation aid, a reporting point or a controlled airspace boundary. It may lie short of the destination, for example at a holding fix or a point on the route. An aircraft reaching its limit without a further clearance holds there and waits (see holding patterns).

Controllers give the pilot something to plan with while the next clearance is pending. EXPECT CLEARANCE AT (time) is used when a clearance cannot be issued immediately. An expect further clearance time, given with a holding clearance, is when the pilot can expect an onward clearance; it is not permission to leave the fix. An expected approach time, by contrast, is when ATC expects a delayed arrival to leave the holding fix to begin its approach. Both matter after a radio communication failure: ICAO uses the expected approach time to time the descent, and the FAA the expect further clearance time to leave the clearance limit.

Amended and conditional clearances

A clearance can be changed at any time. RECLEARED means that a change has been made to the last clearance and that the new clearance supersedes it or part of it; CANCEL annuls a previously transmitted clearance. Some clearances hand the timing to the pilot: "when ready descend FL80" allows the pilot to leave the present level at a time of their own choosing, and the FAA's "at pilot's discretion" does the same, with the rule that a vacated altitude may not be re-occupied. Others fix it: "cross OKTET at or above FL120" imposes a level at a significant point.

A conditional clearance takes effect only when a stated condition is met. The controller gives it as call sign, condition, clearance, then a brief repeat of the condition: "G-CD, behind the landing Airbus A320, line up runway 24 behind." The pilot reads back the condition as well as the clearance, ending with the call sign. When a conditional clearance involves an active runway, the aircraft or vehicle on which it depends must be seen by both the controller and the pilot, and the clearance may refer to one movement only.

Release and void times

The FAA has a set of departure restrictions for IFR flights, especially from airports without an operating control tower, where a departure must receive either a departure release or a hold for release:

• Hold for release: the pilot may copy the clearance but may not depart under it until ATC issues a release, and delay information is given with it. It does not prevent a VFR departure, but the pilot should first cancel the IFR flight plan and use a VFR transponder code.
• Release time: the earliest time at which the aircraft may depart.
• Clearance void time: the time after which the clearance may not be used if the aircraft is not airborne. The pilot must tell ATC of their intentions as soon as possible; ATC normally states a time for doing so, which cannot exceed 30 minutes after the void time, after which the aircraft is considered overdue and search and rescue procedures begin. Other IFR traffic for the airport is held off meanwhile. An expired void time does not cancel the flight plan but withdraws the authority to depart IFR until a new release is issued.
• Expect departure clearance time (EDCT): a runway release time in a traffic management programme; the aircraft is expected to depart no earlier than five minutes before and no later than five minutes after it.

In ICAO terms, a time of expiry is simply one of the other instructions a clearance may contain.

Cruise clearances and expected altitudes

A cruise clearance is a US procedure that assigns a block of airspace from the minimum IFR altitude up to and including the altitude specified. The pilot may level at any altitude within the block and climb or descend within it at their discretion, but once a descent has begun and the pilot has reported leaving an altitude, that altitude may not be re-occupied without a further clearance. A cruise clearance is also an approach clearance: it authorises the pilot to proceed to the destination airport and make an approach there, following the published procedure.

An expect altitude authorises nothing by itself. It lets the pilot plan and, after a radio failure, it becomes one of the altitudes the pilot must consider under 14 CFR 91.185, from the time or point stated. A pilot on an IFR flight plan may also request VFR-on-top, an IFR clearance in which the pilot chooses a VFR cruising altitude, keeps VFR visibility and cloud clearances, sees and avoids other traffic and still follows the IFR rules on minimum altitudes, position reports and the route. It is not available in Class A airspace.

Approach clearances

An approach clearance authorises the pilot to fly the instrument approach procedure named in it. It says nothing about the runway: a separate landing clearance must be received before touchdown, and without one the pilot flies the missed approach (see instrument approach procedures). Under PANS-ATM, an IFR flight is not cleared for an initial approach below the appropriate minimum altitude unless the pilot has reported passing a suitable point defined by a navigation aid, reports that the aerodrome is and can be kept in sight, is conducting a visual approach, or its position has been determined by radar. An expected approach time is passed whenever a hold of 30 minutes or more is expected, and a revised one whenever it changes by 5 minutes or more.

The FAA's rules add detail. A pilot being vectored is not expected to turn onto the final approach course until cleared for the approach, and the clearance normally comes with the final vector and an altitude to maintain until established on a published segment: "maintain two thousand until established on the localizer, cleared ILS runway three six approach". "Cleared approach" without a named procedure allows any authorised instrument approach at the airport, but not a visual or contact approach. An aircraft cleared to a holding fix and then cleared for the approach has not received new routing: it proceeds via the holding fix and any published feeder route. ATC withdraws the authority with "cancel approach clearance".

A practice instrument approach is an instrument approach flown for training. In the United States, a VFR pilot requests it stating the approach and that the flight is VFR; ATC normally replies "maintain VFR" and provides IFR separation only where facility procedures allow. At the end of the approach the tower issues the clearance the pilot needs, such as "cleared low approach", "cleared to land" or, in the pattern, "cleared for the option".

Cancelling IFR

Under ICAO and SERA, only the pilot-in-command can change a flight from IFR to VFR, by transmitting "cancelling my IFR flight" and any changes to the current flight plan; the controller acknowledges with "IFR flight cancelled at (time)". ATC does not invite cancellation, and the pilot should cancel only when the flight is expected to continue in uninterrupted visual conditions. A flight planned to begin IFR and continue VFR is filed with flight rules Y, with the change point in the route field. Where the arrival aerodrome has no ATS unit, the flight plan is closed with an arrival report as soon as possible after landing; without one, the alerting service begins.

In the United States, a tower closes an IFR flight plan on landing. At an airport without an operating tower, ATC cannot know that the aircraft has landed, and until the pilot cancels by radio, through a remote outlet to Flight Service or by telephone, no other IFR arrival or departure can use the airport. Cancelling IFR is impossible in Class A airspace, where VFR flight is not permitted.

Library articles are written for study and exam preparation. They do not replace your aircraft's approved documentation, your operator's procedures or the regulations themselves.