Conditional Clearance Runway Incursions

Conditional Clearance Runway Incursions


This article identifies possible reasons and factors that explain how a conditional clearance can contribute to a runway incursion. It also includes advice on potential risk mitigation actions and measures.

Conditional clearance benefits

If properly used, conditional clearances can help speed up the flow of air traffic. This is especially true when the frequency occupancy time is at peak e.g. the controller has to issue several clearances at the “same” time. Normally this would cause delays to all aircraft except for the first. The situation can be avoided by using a conditional clearance, i.e.:

  • issuing the clearance in advance (when time permits, i.e. there are some seconds of silence on the RTF frequency);
  • the controller specifying when the action is supposed to start; and
  • the flight crew performing the action at the right time.

Thus, the use of conditional clearances can help the controller maintain an expeditious traffic flow.

As always, the controller is supposed to monitor the situation and to intervene in case the instructions are not complied with (e.g. in case of misunderstanding or delay of action by the flight crew) but this is achievable.

Conditional clearance misunderstanding

While the use of conditional clearances can help speed up the flow of traffic there are also some risks associated with misunderstanding that need to be taken into account:

  • The flight crew might misinterpret the conditional clearance as a line-up or take-off clearance (e.g. due to mishearing or due to expectation bias);
  • The flight crew might misidentify one or more elements of the conditional clearance (e.g. if instructed to follow specific aircraft on final);
  • The controller might issue an ambiguous or unclear instruction or use non-standard phraseology.

Contributory factors

There are a number of factors that need to be considered when issuing a conditional clearance. While not safety hazards on their own, these factors can easily contribute to a runway incursion if neglected:

  • Aerodrome layout – e.g. taxiways crossing the runway at acute angles might prevent the flight crew from seeing the conflicting traffic;
  • Premature acceptance – when the flight crew acknowledges a conditional clearance before identifying the conflicting aircraft;
  • Aircraft painting mismatching the aircraft callsign – e.g. if an operator is doing a flight on behalf of another operator;
  • Low sun angles may prevent the departing aircraft from correctly identifying (or even spotting) the conflicting aircraft;
  • More than one aircraft on final (e.g. one aircraft is about to land and there is another aircraft 5NM from touchdown).

Mitigation actions and measures

The following summary of best practices is based on experience and safety occurrences investigations. The suggested actions and measures are not intended to replace or take precedence over local regulations and procedures but could be used in conjunction to prevent runway incursions associated with the use of conditional clearances.

  • The controller should issue a conditional clearance only when there is a reasonable amount of certainty that the flight crew will correctly identify the “conditional” aircraft before accepting the clearance. If the controller has any doubt about this, they should clarify the situation.
  • The controller should ensure the unique identification of the conflicting traffic. Only one aircraft in the vicinity should match the description. The controller should have visual contact with the “conditional” aircraft to avoid the risk of giving wrong information (e.g. in case a particular flight is being performed by a different operator or for some reason the aircraft livery does not correspond with the operator designator which is part of the conditional clearance).
  • The controller should exercise extra care when the visual conditions are not optimal (e.g. at night, sunrise, sunset, mist, etc.) as in these cases the correct identification of aircraft may be very difficult.
  • The conditional clearance should refer to the first aircraft that is supposed to pass in front of the aircraft being cleared. In case of multiple aircraft (e.g. two airplanes on short final) the controller should either wait for an unambiguous situation or make sure the flight crew has correctly understood the full picture to avoid misunderstanding.
  • Adherence to standard ICAO phraseology (e.g. repetition of the condition after the clearance) would reduce the risk of misinterpretation of the conditional clearance (e.g. considering it as a line-up clearance).
  • If necessary, amendment to the Manual of operations regarding the use of conditional clearances that takes into account the local aerodrome specifics should be considered.
  • The flight crew should identify the conflicting aircraft before accepting the conditional clearance. If there is any doubt clarification should be requested before clearance acknowledgment.

Conditional clearance-related incidents

On 3 August 2017, a Boeing 737-900ER landing at Medan was in wing-to-wing collision as it touched down with an ATR 72-500 which had entered the same runway to depart at an intermediate point. Substantial damage was caused but both aircraft could be taxied clear. The Investigation concluded that the ATR 72 had entered the runway at an opposite direction without clearance after its incomplete readback had gone unchallenged by ATC. Controllers appeared not to have realised that a collision had occurred despite warnings of runway debris and the runway was not closed until other aircraft also reported debris.

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