Description

This article provides general information on conditional clearances and additionally 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. The safety relevant effects of conditional clearances used during enroute phases, e.g. time or position-based climb/descent clearances are explained in the article CPDLC-General Safety Considerations.

The conditional clearance

A conditional clearance is a clearance issued by an air traffic controller which does not become effective until a specified condition has been satisfied.

Example of conditions that may be included in a conditional clearance include:

• AFTER THE DEPARTING AIRCRAFT;
• BEHIND THE LANDING AIRCRAFT;
• BEHIND THE <airline name> A340.

ICAO Format ICAO specifies the precise format of a conditional clearance: In all cases a conditional clearance shall be given in the following order and consist of: a) identification; b) the condition; c) the clearance; and d) brief reiteration of the condition, for example: “SAS 941, BEHIND DC9 ON SHORT FINAL, LINE UP BEHIND”. Note.— This implies the need for the aircraft receiving the conditional clearance to identify the aircraft or vehicle causing the conditional clearance. (ICAO Doc 4444 - PANS-ATM: Chapter 12, para 12.2.7) Remark: The procedure makes currently no provision for vehicles to be included in the process of receiving conditional clearance. They may only be the subject of a conditional clearance.

Risk Factors

While the use of conditional clearances can help speed up the flow of traffic thus increasing the operational efficiency of the airspace their use bears significant safety risks which, if not eliminated or properly mitigated, can lead to safety relevant events such as runway incursions. The Global Action Plan for the Prevention of Runway Incursions (GAPPRI) highlights that especially the misapplication and misinterpretation of conditional clearances can be contributing factors to such events. 

Conditional clearance misunderstanding 

The following possible threats and errors associated with the use of conditional clearances have to be considered by the controller: 

• The pilot/crew might misinterpret the conditional clearance as aan actual line-up or take-off clearance (e.g. due to mishearing or due to expectation bias);
• The pilot/crew might misidentify one or more elements of the conditional clearance (e.g. if instructed to follow specific aircraft on final);
• The aircraft or vehicles concerned may not be observed by the appropriate controller or pilot.
• The aircraft or vehicle causing the condition is not the first aircraft/vehicle to pass in front of the other aircraft concerned.
• The controller might issue an incorrect, ambiguous or unclear instruction or use non-standard phraseology or might not detect an incorrect read-back.
• A pilot/crew might take the clearance intended for other aircraft (e.g., due to call sign confusion).

A danger of misunderstanding, ambiguity or other confusion could exist in a conditional clearance, for example:

• If more than one similar aircraft (e.g. same aircraft type and same operator) were passing in front of the subject aircraft; or,
• If an aircraft was in an unfamiliar livery; or,
• If the subject aircraft was not where the controller thought (e.g. at a different runway holding point);

So that the pilot/crew might follow the wrong aircraft. To reduce the risk of such confusion ICAO additionally specifies that:

Conditional phrases, such as “behind landing aircraft” or “after departing aircraft”, shall not be used for movements affecting the active runway(s), except when the aircraft or vehicles concerned are seen by the appropriate controller and pilot. The aircraft or vehicle causing the condition in the clearance issued shall be the first aircraft/vehicle to pass in front of the other aircraft concerned. (ICAO Doc 4444 - PANS-ATM: Chapter 12, para 12.2.7) However, especially the requirement that the aircraft or vehicle concerned are seen both by the controller and by the pilot/crew might, in practice, not be fulfillable as expected due to further factors influencing frontline operator’s vision or line of sight.

Contributory factors

There are a number offurther factors that need to be considered when issuing a conditional clearance. While not safety hazards on their own, these factors can in combination with the issue of a conditional clearance easily contribute to a runway incursion if neglected: Aerodrome layout – e.g. taxiways crossing the runway at acute angles might prevent the pilot/flight crew from seeing the conflicting traffic (aircraft or vehicle); Too early issuing of a conditional clearance by the controller leading to Premature acceptance – when the flight pilot/crew feels obliged to acknowledges a conditional clearance before identifying being able to identify 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, reduced visibility (e.g. due to rain or haze) or just too far distance of the aircraft/vehicle causing the condition may prevent the pilot/crew of the departing aircraft or the controller from correctly identifying (or even spotting) the conflicting aircraft or vehicle; 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 GAPPRI recommendations and 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 by ANSPs and Aircraft Operators to develop and implement policy and robust procedures to enable controllers and pilots/crew to prevent runway incursions associated with the use of conditional clearances. ANSPs should regularly assess conditional clearance operational procedures and practices; consider if the operational use of conditional clearances can be removed or reduced and monitor controllers use of conditional clearance as part of routine operational supervision and ongoing competency assessment. ANSPs should synchronize the stop bars and conditional clearance operational procedures including decisions on when stop bars should be turned off after a conditional clearance and a correct read-back. ANSPs should consider appropriateness of using conditional clearances at taxiways that are not perpendicular to the runway; during lower than CAT I conditions; at CAT II or III RWY holding positions that are not combined with CAT I holding positions or where there is a significant distance from the runway holding position to the runway. The cControllers should issue a conditional clearance only when there is a reasonable amount of certainty that the flight pilot/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 cControllers 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 cControllers 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 pilot/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; at some aerodromes, the use of conditional clearances was removed for specific operations (specific holding positions or specific operators) associated with higher risk and/or previous incidents. Aircraft Operators should highlight to pilots/crews the risks associated with conditional clearances and include the subject in recurrent ground school and simulator training; ensure pilots/crews incorporate these risks in their TEM briefing; highlight the importance of correct ATC readback and visual contact with the “conditional” aircraft; include land and hold short operations guidance in documentation and training. The Pilots/flight crews should identify the conflicting aircraft before accepting the conditional clearance. If this is not possible, they should inform ATC or reject the clearance, e.g. by using the wording “unable”. If there is any doubt over the clearance,there is any doubt clarification with ATC should be requested before clearance acknowledgment. Any crewmember’s doubts should be treated as though the entire crew has doubts.

Accidents and Incidents

The following events include the use of conditional clearance as a contributory factor: