Take-off without ATC Clearance

Description

Take-off without ATC Clearance is an aerodrome-related safety event that is somewhat similar to Landing without ATC Clearance. Both involve air traffic control, typically happen during critical and high-workload stages of the flight, and can result in similar hazardous outcomes (e.g. runway incursion, loss of separation, collision, etc.).

The main difference between the two events (aside from the flight phase) are the causal factors involved. Landing without ATC clearance can be caused by one or more of a variety of factors - loss of communication, loss of situational awareness (e.g. unintentional use of an incorrect runway, taxiway or even aerodrome), poor communication, high workload, etc. In the takeoff without ATC clearance scenario, poor pilot-controller communication (e.g. improper phraseology) is by far the most common precursor. Normally, if a flight crew finds out that there is a communication failure during the taxi phase, they will return to apron to investigate and will not attempt to take off. Also, while the inadvertent use of a wrong runway (or a taxiway) for take off sometimes happens, it is impossible to imagine that a pilot would try to use a wrong aerodrome.

Note: this article does not examine scenarios in which a valid takeoff clearance is given but is not followed correctly due to the aircraft being in the wrong place (i.e., attempting takeoff from a taxiway or from a wrong runway).

Consequences

Taking off without ATC clearance may lead to:

  • Runway Incursion - The aircraft may have been cleared only to the runway holding point. Also, at relatively complex aerodromes, taking off may mean crossing other runways.
  • Rejected Take Off - The tower controller would likely observe the aircraft starting the take-off roll and may instruct the crew to cancel it.
  • Loss of Separation - If the take-off without clearance is not aborted, either because of lack of communication or due to the pilots non-compliance with the instruction (e.g. for safety reasons), there is a chance that the aircraft that has just taken off will come into close proximity with other traffic in the vicinity of the aerodrome.
  • Aborted landing(s) for aircraft on final. This may happen as a consequence of a runway incursion.
  • Increased workloads for controllers and pilots due to the sudden change of plan. At busier aerodromes, several aircraft may need to be given avoiding-action instructions or other clearance amendments (e.g. orbits/holding patterns).

Contributing Factors

  • Call sign confusion (i.e., the flight crew of one aircraft taking a clearance intended for another one) - This is more likely to happen at complex aerodromes with more than one runway.
  • Incorrect phraseology - The most notorious example of this is the use of the word take-off in a phrase not related to a take-off clearance but instead to a different type of ATC clearance; see (the B742 / B741, Tenerife Canary Islands Spain, 1977 accident). A distorted ATC message containing the word "take off" can easily be misinterpreted as a takeoff clearance.
  • Expectation bias - For example, the instruction "for departure contact tower" issued by the GND controller might be misinterpreted as "after departure contact tower." This was the case in the B738, Eindhoven Netherlands, 2012 occurrence.
  • Cockpit workload - The pre-takeoff part of the flight is a time of high workload for pilots, and there is risk that they will fail to notice the absence of a clearance or "will assume" that their take-off clearance has been issued.

Safety Barriers

  • Strict adherence to SOPs - If in doubt whether a take-off clearance has been received, the flight crew should clarify the situation with the tower controller.
  • Radiotelephony discipline - While standard phraseology and proper communication techniques should be followed at all times, the following aspects must be stressed in relation to the risk of takeoff without ATC clearance:
    • The phrase take off should only be used by ATC when issuing or cancelling a take-off clearance. In all other instances departure or airborne should be used.
    • A blocked frequency often means that two aircraft (usually with similar call signs) have accepted the same clearance. The controller should promptly intervene in such circumstances to clarify the situation. Pilots, if in doubt whether a clearance was meant for them, should clarify rather than assume.
    • Incorrect readback detection. An incorrect readback may mean that the pilot has confirmed what he or she thought they heard or expected to hear (e.g., line up, cleared for take-off) and not the actual clearance (e.g., line up and wait).
    • Unambiguous hold short, line up and take off clearances - If the controller has any doubt that the crew might interpret the clearance incorrectly, they should emphasise (restate) the relevant part of the clearance. If the flight crew has any doubt about whether or not the clearance received contains a take-off part they should seek clarification instead of assuming "the most probable" scenario.
  • Proper handling of conditional clearances - Normally, conditional clearances should be used by ATC for line up clearance, and not for take-off clearance. Controllers should explicitly state that a further (take-off) clearance is to be issued at a later time.
  • Controller vigilance - If the controller observes that an aircraft has just started its take-off roll and there is any safety concern, the controller should instruct the flight crew to stop immediately. If the error is detected early enough, the rejected take off will have no negative impact on flight safety (operational efficiency may suffer, however, as the aircraft may need to lose time to taxi back to the beginning of the runway).

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