Description

A Runway Crossing Incursion is considered here to occur when an aircraft or vehicle which requires to cross one or more active runways, in order to complete an intended aerodrome ground movement, comes into actual or potential conflict with an aircraft using the same runway for landing or take off. A crossing aircraft may be one which has just landed or is en route to an intended take off on another runway. In this context, a rotary wing aircraft which is hover-taxiing in ground effect should be considered as equivalent to a fixed wing aircraft or vehicle on the ground rather than as an aircraft ‘in flight’.

Discussion

In accordance to ICAO SARPs, a specific ATC Clearance is required for every runway crossed regardless of whether it is active or not. According to Doc 4444, paragraph 7.6.3.1.1.2 “When a taxi clearance contains a taxi limit beyond a runway, it shall contain an explicit clearance to cross or an instruction to hold short of that runway.”

The majority of aerodromes where more than one runway is simultaneously active chose to ensure that active runways are notified on the ATIS with any changes thereafter advised directly. This has the advantage of focusing the attention of all concerned specifically on the active runways. Whilst the EAPPRI states that “when a taxi clearance contains a taxi limit / reporting point beyond a runway, it shall contain a specific clearance to cross that runway, or an instruction to hold short, even if the runway is not in use” this procedure is not universally adopted. This particularly applies at airports where inactive runways revert to active taxiway use, which is a common practice at many busy airports.

It should be noted that a crossing aircraft may be in service or being towed or taxied out of service, noting that in the latter case, there is frequently no requirement for a pilot to be present in the flight deck since the movement is not occurring as a prelude to an intended flight, following a rejected take off or after a landing.

If an aerodrome is designed so that there is no need for aircraft or vehicles to cross an active runway, then the risk of this type of incursion is confined to discretionary use by ATC of such clearances.

Whilst there are many possible scenarios for this type of incursion, the majority appear to occur as a consequence of either:

• The inadvertent issue of conflicting clearances to a crossing aircraft or vehicle and another aircraft using the same runway for take off or landing
• The inadvertent failure of an aircraft or vehicle to stop at the limit of its taxi clearance or otherwise correctly follow it

In the case of runway crossing incursions at aerodromes with more than one active runway in simultaneous use, there appears to be a greater probability of such incursions involving aircraft taxiing in after landing than those taxiing out for take off - although there does not seem to be any conclusive data to support such a proposition. Whilst there is no common practice on the designation of dedicated landing and take off runways in relation to terminal facilities which are located in a position where either aircraft taxing in or those taxiing out must cross an active runway, a preference for giving taxiing-in aircraft a route without a runway crossing is frequently favoured.

Controllers should note that the height of the Visual Control Room (TWR) may mean that just because runway occupancy cannot be visually monitored from it because of poor visibility, this does not mean that vehicle and aircraft movements are taking place in forward visibility which is sufficiently low to demand the enhanced vigilance which can be expected in such conditions. In this context, it should be noted that whilst available data tends to indicate that low visibility alone is not a significant contributory factor in the majority of runway incursions, probably because of the enhanced vigilance thereby invited, controllers should recognise that Low Visibility Procedures (LVP) status is often not equivalent to low surface visibility and should thus not automatically be associated with such enhanced vigilance.

When such circumstances arise and an incursion begins, adequate situational awareness on the part of at least one of the three parties involved (ATC, the aircraft landing/taking off and aircraft or vehicle crossing) will have already been lost. However, in most cases, the relatively slow speed of the crossing aircraft or vehicle should provide a reasonable chance that the seriousness of the resultant incursion can be contained if situational awareness of one or more of the other parties is properly maintained.

Situational awareness is fundamental to both the prevention of runway crossing incursions and the mitigation of their effects once an incursion has begun. It is widely recognised that the use of more than one language for ATC communications, in respect of runway occupancy, is not in line with the best practice for risk management of runway crossing incursions.

It has often been found that a human error made by any party which creates the conditions for a runway crossing incursion involves either distraction or complacency. The former requires that a sterile environment should be maintained in aircraft flight decks and vehicle cabs and invites, inter alia, an assessment of the need for the wearing of headsets by controllers at all times. It is important to recognise that familiarity with one’s task as a controller (and in the case of aircraft or airside vehicles, one’s own location) should not lead to a reduced focus on any aspect of aircraft ground movement where active runway crossing is routinely necessary.

Prevention - Aerodrome Design and Use

The following risk mitigations are considered best practice:

• Whenever possible, the crossing of active runways should be rendered unnecessary by design. This means that where it is going to be routinely necessary to cross the extended centreline of a runway, a perimeter taxiway should be available which is sufficiently distant from the runway threshold for landing and take off to continue whilst crossing is occurring.
• When the above is not possible, the aerodrome layout should be such that all crossings are made at 90° to the active runway centreline. Crossing traffic on an active runway should never by cleared to cross after entry on a reverse-direction RET since this precludes any visual check for potential runway actual or imminent occupation before entering to cross.
• Aerodromes which have experienced serious runway incursion events have often chosen to modify the detail alignment of those taxiways which may be used as runway crossing entry points by introducing a significant change of direction shortly before a runway intersection occurs.
• A runway longitudinal profile which has one or both thresholds significantly below the elevation of the highest part of the runway can prevent effective checking that a runway is clear before crossing commences and should be regarded as a hazard to be addressed in the long term and recognised until it is so addressed.
• Ground markings, signage and lighting which ensure that both vehicles and taxiing aircraft can readily and unambiguously determine their position - by day or by night and in any forward visibility in which they are permitted to operate - are essential. The provision of H24 controllable stop bars at all holding points where crossings of active runways may routinely occur is highly recommended as is the corresponding provision of H24 taxiway and lead-on lighting at all designated runway crossing points. Where aerodrome design requires the frequent crossing of active runways, consideration should be given to the feasibility and potential risk mitigation provided by Runway Status Lights (RWSL) and the associated FAROS system.
• Airside Driving Permits should only be issued following training which emphasises the importance of continuous situational awareness. This should include a sufficient understanding of ATC communications not only to communicate appropriately and act in strict accordance with instructions but to allow passive monitoring of radio traffic to enhance visual situational awareness. Such Permits should be allowed to remain valid only subject to a formally specified programme of recurrent training.

Prevention - Aircraft and Vehicle Operators

The following risk mitigations are considered best practice:

• The maintenance of a sterile environment should be mandated in both aircraft flight decks and vehicle cabs during ground manoeuvring in order to minimise the chances of distraction.
• Sensible maximum ground speeds should be mandated and enforced by whatever means are available, including in the case of aircraft the use of the OFDM process and in the case of vehicles the use of driver identity recording and random checks of vehicle recorders where fitted.
• Current aerodrome charts at a scale which is sufficient to show all permitted taxiways/runways and all intermediate and runway access holding points and their designations must be available to pilots and drivers in a form that facilitates ready reference to them.
• Where vehicles are likely to have to cross active runways whilst airside, consideration should be given to requiring two people to be present in the driving cab where seating is available in order to add an element of monitoring of the driver’s actions. To be effective, this would require the second occupant to have completed a similar course to a permit holding driver without necessarily being able to drive.
• Where aircraft operators permit their out-of-service aircraft to be towed or to be taxied under their own power without at least one pilot being present on the flight deck, they should satisfy themselves not only that adequate training has been given to approved persons, but that a system of recurrent training is in place.

Prevention - Flight Crew/Vehicle Drivers

Three factors are considered paramount in avoiding involvement in a runway crossing incursion:

• the maintenance of visual situational awareness in respect of ground position.
• the careful monitoring of other traffic in the vicinity by active visual monitoring supported, insofar as primary tasks permit, by passive monitoring of ATC communications with relevant traffic.
• the careful receipt of, read back of and adherence to all ATC clearances supported by active monitoring of the pilot or driver in control by other qualified persons present in the flight deck or vehicle cab respectively.

In addition, flight crew should:

• note that when instructed to follow other traffic, this never automatically includes a clearance to enter or cross an active runway - each aircraft requires a specific clearance.
• aim to complete check list activity before crossing an active runway and if this is not possible suspend such activity until a runway crossing is complete.
• avoid stopping on a runway when crossing it unless they are specifically instructed to do so.
• expect communication with ATC shortly after receiving a "Line up and wait" clearance (usually within 90 seconds). If no further clearance is received, informing the controller that the aircraft has lined up should be considered.

Even with the assiduous application of all these good practices, the final defence against a runway collision, when approaching to or crossing an active runway, is an effective visual check in the direction from which any aircraft in the process of landing or taking off would come. For aircraft, the use of the TCAS display to check for traffic on or approaching the runway about to be entered is considered good practice.

If, for any reason, a risk of collision is perceived once in the process of crossing an active runway, it is important that the decision to stop or continue is made on the basis of an assessment of the visible facts. These may not necessarily accord with any instructions from the control tower due to their remoteness from the point of risk.

Any decision that the best course of action for an aircraft accelerating for take off is to continue, on the basis that over flight of a crossing aircraft or vehicle will be possible, will frequently be made in conjunction with an increase to maximum rated take off thrust from the reduced thrust setting usually used.

Prevention – ANSPs

A number of complimentary measures could help reduce runway crossing incursions:

• The issue of a clearance to cross an active runway should always be given directly by the runway controller on the single frequency used for all clearances to access a particular active runway for any purpose.
• Only one language should be in use for all communications on the frequency designated for use by a runway controller.
• Where OJT is in progress, procedures should require that all active runway crossing clearances are specifically monitored by the qualified training controller.
• Unambiguous procedures to cover any malfunction of stop bar lighting at runway holding points should be in place.
• Consideration should be given to the most appropriate way(s) to ensure that the prevailing use of controllable stop bar lighting in support of taxi clearances is effectively communicated to aircraft using an aerodrome.
• If runway crossing is permitted when alternative but less efficient alternatives exist, the circumstances when it is permitted and the process to be followed should be established.
• Arrangements for the display to runway controllers of active runway occupancy by aircraft or vehicles should include runway crossing clearances.
• All holding points or positions at which runway crossing is permitted should have an unrestricted line of sight from the runway control position or equivalent CCTV coverage.
• Exceptional vigilance is required if vehicles or aircraft on one runway are instructed to hold clear of an intersecting runway to await crossing clearance if, as is likely, there are no holding point markings or associated lighting to indicate a specific holding position.
• Systems which alert controllers to potential or actual runway incursions should incorporate an audio alert as well as a visual representation of the location and opposing traffic.

Prevention - Controllers

Good practices for runway controllers which could reduce the risk of runway crossing incursions include:

• Any active runway crossing clearance should be regarded as of equivalent significance to a take off or landing clearance
• Where controllable lit stop bars are not installed at every designated holding point, aircraft taxi clearance limits prior to the expected crossing of an active runway should always correspond to positions where this facility exists
• If a frequency change is not required to cross an active runway, the clearance to do so should not be given any earlier than it would be if such a change was required
• The issue of crossing clearances which are likely to result in late landing clearances should be avoided
• Where circumstances permit, runway crossing clearances should be given as near to either end of a runway as possible and, where this is not possible, full length take offs should be preferred
• In the event that it is realised by a pilot or driver that an incursion has occurred and that a collision has become a possibility, it is essential that any instruction to either party to stop or to continue is made promptly and appropriately
• All active runway crossing clearances should be issued with a requirement to report clear of the runway after crossing and this confirmation should be obtained before any use of the same runway for take off or landing is authorised
• A handover briefing to facilitate a change of runway controller should seek to avoid the interval between the issue of any runway crossing clearance and the subsequent “runway vacated” call
• Whenever possible, runway crossing clearances should minimise the time which an aircraft or vehicle is on the active runway
• Aircraft under tow are best positioned prior to crossing as close as possible to a suitable exit taxiway
• The use of standard taxi routes for runway crossing is strongly advised.
• When the time is critical, controllers should consider informing the crossing aircraft/vehicle of traffic which will subsequently land or take off on the same runway
• Since pilots require a general overview of their expected taxi routing, it may be appropriate in the case of complex detail to provide such an overview first and then divide the detail of the message into segments, placing the clearances and instructions in sequential order. This should help reduce the chances of pilot misunderstanding, while still providing the complete picture.

Mitigating the consequences of a crossing incursion

The two principal ways in which the consequences of an incursion can be mitigated are the recognition of an imminent risk of conflict by at least one of the pilots or drivers involved or by the designated runway controller. In the case of pilots and drivers, it is important that they are aware of the direction of use of any active runway they are about to cross so that their pre-entry check for actual or potential conflict is made in the appropriate direction.

In either case, this recognition may be supported or replaced by activation of a ground or airborne safety net which may communicate situational awareness directly to a driver or pilot or provide it through controller alerting which then may prompt the issue of risk reduction instructions. Early ground safety nets were mainly based on A-SMGCS level 2 but this data is now increasingly being supplemented by fixed position sensors and by automatic vehicle/aircraft position transmission. In the USA, and now for the first time in Europe at Paris CDG (inner northern runway, direct pilot alerting through the RWSL system is improving the mitigation of both any hazardous consequences of an incursion and the chances of preventing one altogether. In the USA, the latest RWSL installations are being supplemented by the FAROS system which annunciates an occupied runway to aircraft on approach to land.

Some Serious Incident Examples

Related Articles

• Runway Incursion
• Taxi-in Runway Incursions
• Runway Incursion and Airport Design
• Runway Identification
• European Action Plan for the Prevention of Runway Incursions (EAPPRI)
• Flight Operations Risk Assessment Checklist - Active Runway Crossing
• Land and Hold Short Operations
• Runway Status Lights (RWSL)

Further Reading

• Analysis of reported runway incursion serious incidents - European data sample of 2013 – 2015, EUROCONTROL, Feb 2017
• Analysis of investigated runway incursion accidents and serious incidents - worldwide data sample of 2006 – 2016, EUROCONTROL, Apr 2017
• SAFO 17012: High Collision Risk During Runway Crossing, FAA, Nov 2017
• Final report: Problematic Taxiway Geometry Study Overview, FAA, January 2018