Description

Some Runway Incursion incidents could have been prevented if controllers had had better means to detect that the runway was occupied at the time of issuing clearance to the next aircraft to use the runway. Analyses published in the Global Action Plan for the Prevention of Runway Incursions (GAPPRI) even found that 41% of all serious runway incursion incidents could have been prevented by ATC potential conflict detection before clearing another traffic to the runway.

This article describes the typical scenarios for runway conflicts not being timely detected as well as the safety barriers that could prevent such situations or mitigate the consequences.

Currently, controller detection of potential runway conflict is one of the ATM top 5 operational safety priorities identified by EUROCONTROL Operational Safety Group (SAFOPS). It has been identified as an initiator in many high severity incidents (see the Tableau dashboard for details).

Operational Context

Aerodrome conflict scenarios are sometimes difficult to generalise due to the wide range of local variables. It might be difficult to separate the common (e.g. human error, safety barrier, etc.) from the specific (e.g. aerodrome layout, local equipment, local weather, etc.). The following factors have the greatest potential to affect the availability and the effectiveness of the safety barriers:

• Availability of surveillance information affects the likelihood of detection of RWY incursions and incorrect movement on the Manoeuvring Area. The level of ATC service may differ depending on equipage:
  • No surveillance information available
  • Basic Surface Movement Radar (SMR)
  • Advanced Surface Movement & Guidance Control System (A-SMGCS) Level 1 (provides identification of aircraft and vehicles)
  • Advanced Surface Movement & Guidance Control System (A-SMGCS) Level 2 (provides identification and alerting functions)
  • Integrated Tower Working Position (ITWP) using input and display of the ATC clearances in addition to surveillance to enable the use of early warnings to highlight any non-conformance to clearance
• Visibility conditions from the Tower. The possibility of recognising potential threats in good time may differ depending upon visual impairment:
  • day/night
  • fog/mist
  • low cloud affecting high control towers
  • sunlight and glare during day
  • precipitation on windows
  • airport floodlighting during night, especially temporary work in progress
• Runway configuration can influence how the operations on each runway can be affected by the operations on the adjacent ones and therefore how the specific barriers can be applied:
  • Multiple Parallel/Intersecting runways
  • Mixed mode/Single mode operations
• Line of sight from the ATC Tower can differ based on the position, height, design, equipment and distance of the tower. These can restrict the direct view of the relevant part of the runways and manoeuvring area
• ATC Safety Net Alerts – their availability could impact the timeliness of conflict resolution:
  • A-SMGCS Level 2
  • Conflicting ATC Clearances (CATC)
  • Conformance Monitoring for Controllers (CMAC)
  • Airport Ground Systems, e.g. Runway Guard Lasers and Loops

Typical Scenarios

The majority of manoeuvring area conflicts fall into one of the following event types:

• Taking off or landing on runway that is already occupied – either by an aircraft/vehicle being on the runway or an aircraft/vehicle having received a clearance to enter the runway;
• Take-off on runway already occupied– either by an aircraft/vehicle being on the runway or an aircraft/vehicle having received a clearance to enter the runway;
• Entry onto runway by aircraft taxying for departure or by vehicle while at the same time another aircraft is taking off or landing or has received a take-off or landing clearance;
• Runway crossing while at the same time another aircraft is taking off or landing or has received a take-off or landing clearance;
• Surface movement conflict on runway – two or more aircraft/vehicles entering/crossing the runway at reciprocal points
• Departing or landing on intersecting runways – these events have four sub-scenarios:
  • two aircraft departing from intersecting runways
  • two aircraft landing on intersecting runways
  • one aircraft departing and one aircraft landing on intersecting runways
  • departing or landing aircraft against surface movement on intersecting runways
• Departing/landing on a taxiway (not a runway) while there is an aircraft or a vehicle on the taxiway
• Incorrect aircraft movement on the manoeuvring area. There are two subtypes of these events:
  • aircraft takes incorrect taxy route
  • aircraft incorrectly pushes back/enters on to manoeuvring area

Contributing Factors

The manoeuvring area conflicts usually include at least one of the following events:

• Loss of Communication
• Non standard R/T phraseology used
• Incorrect ATC Clearance
• Incorrect readback/hearback
• Non-conformance with ATC clearance due to:
  • spatial/positional confusion
  • misinterpretation or mishear of the clearance
  • procedural non compliance
  • poor CRM and lack of Situational Awareness
  • expectation and continuation biases
• Local environment specifics such as taxiways layout, marking and signage (e.g. not fully compliant to the provisions of ICAO Annex 14), no single runway occupancy frequency, etc

A dedicated study identified the four most frequent ATC contributing factor areas that are present in the sequence of events leading to runway incursions:

• Memory - most commonly a failure to check/monitor i.e. not following normal practice
• Perception - most commonly a failure to see something
• Operational environment - commonly distractions, visual impairments and noise.
• Communication errors - incomplete, incorrect or ambiguous RTF.

Prevention Barriers

The following barriers, when deployed and employed correctly, are capable of alerting ATC in time to prevent runway incursions and ground conflicts. With regard to events on the Manoeuvring Area, these barriers are capable of alerting ATC before standard levels of safety are compromised:

• ATCO memory aids for issued (not issued) clearances. Used to enhance the controller’s situational awareness and remind of issued clearances. Examples of these are:
  • Buttons or switches used by the controller when a clearance affecting the manoeuvring area is issued.
  • Flight strip arrangement rules (e.g. if a take-off/landing clearance is issued to an aircraft, its flight strip is put into a red holder).
• ATCO direct visual detection. One of the main responsibilities of a tower controller is to constantly observe the situation on the manoeuvring area and verify that all clearances are properly complied with.
• ATCO visual detection using remote camera displays. These are used when parts of the manoeuvring area cannot be observed from the control tower (e.g. due to obstacles, aerodrome layout, etc.). They supplement the direct visual detection.
• ATCO resolution following pilot/driver report. This barrier relies on voluntary information from another person. While usually this cannot be legally described and written down as a procedure, experience shows that this barrier turns out to be one of the most effective tools.
• ATCO detection of occupied runway by use of basic Surface Movement Radar (SMR). This tool provides surveillance information in poor visibility conditions when direct visual detection and remote cameras are not effective. It also helps controllers to identify smaller objects (vehicles or aircraft) which are at relatively long distance from the control tower. The disadvantage is that normally there are no visual or audial warnings and therefore the controllers needs to divert their attention from visual observation in order to use the SMR.
• Advanced Surface Movement & Guidance Control System (A-SMGCS) level 1. This tool is an enhanced version of the SMR. It provides identification of both aircraft and transponder-equipped vehicles. The tool relies on the proper use of transponders. If procedures are strictly followed, A-SMGCS provides reliable information on the traffic situation both on the manoeuvring area and the immediate aerodrome vicinity.
• Advanced Surface Movement & Guidance Control System (A-SMGCS) level 2. This tool is an enhancement of the A-SMGCS Level 1. It provides conflict alerts in addition to aircraft/vehicle identification. The logic is similar to STCA used in area control. While this feature is a great enhancement compared to A-SMGCS Level 1 it only warns the controller of immediate risks and does not take into account the issued clearances (which might be conflicting).
• Integrated Tower Working Position (ITWP). This is an ATC system that displays both traffic situation and controller input (ATC clearances) which enables the use of “early warning” features that:
  • highlight any non-conformance to clearance;
  • highlight the potential consequences of any incorrect clearance.
• Use of discrete names for holding positions and pilot reports (e.g. reporting point REKRA instead of P2). This barrier helps reduce the chance of mishearing or misunderstanding a clearance.
• A procedure stating that all vehicles entering a runway should display high visibility flashing/strobing lights on in all visibility conditions. Following such a procedure enhances the situational awareness of all participants (controllers, pilots, vehicle drivers and other people on the manoeuvring area) as it gives them information about the flight crew intentions. This also improves the effectiveness of the “ATCO resolution following pilot/driver report” barrier.

Barrier Effectiveness

A dedicated study has concluded that:

• ATCO detection after alert from the use of input and display of the ATC clearances and surveillance data (ITWP) is likely to be the most effective ATC tool in the prevention and mitigation of runway incursions and ground safety events. This functionality is not yet widely available; however, this study does lend strong support.
• Proactive alerts from pilots and drivers that lead to ATC detection and resolution are likely to be very important barriers, especially in reducing the risk of collision in runway incursions.
• ATC direct visual detection and the use of A-SMGCS level 2 are both strong barriers in the prevention and mitigation of runway events.
• ATC detection of incorrect runway presence, using remote camera displays is a strong mitigation barrier as it does not necessarily depend on good visibility and line of sight.
• ATC memory aids are likely to be strong barriers that aid ATC perception and memory. It is these areas of ATC action however that fail most often in actual events.
• The barrier that halted actual safety events most often was acting upon alerts received from pilots and drivers. The study therefore supports initiatives such as Local Runway Safety Teams in promoting the “one team ethos”.

The barriers that were breached most often were:

• ATCO Direct Visual Detection;
• ATC Memory Aids;
• belated (last-minute) ATCO Direct Visual Detection;
• ATCO detection with A-SMGCS level 1.

The barriers that most often successfully stopped the event were:

• ATCO resolution after pilot/driver alert;
• belated (last-minute) ATCO Visual Detection;
• belated (last-minute) pilot detection.

Further Reading

• Operational Safety Study: Controller Detection of Potential Runway and Manoeuvring Area Conflicts, by EUROCONTROL SISG
• 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
• EUROCONTROL NM Top 5 Operational Safety Review: Controller Detection of Potential Runway Conflict, by EUROCONTROL, 2022