Many runway incursion incidents result from pilots/drivers acknowledging air traffic control (ATC) hold short instructions and then continuing across the runway holding point line due to various reasons (poor situational awareness due to complex aerodrome layout, expectation bias, workload, etc).
Taxiway Guidance System
At aerodromes where Category II & III operations take place or where ground movement requirements are complex, a taxiway guidance system may be installed in order to regulate traffic. The system operates by selective switching of the taxiway centerline lighting so that individual sections or routes, each terminating at a lighted stop bar, are illuminated in order to show the way ahead. The taxiing guidance system consists of:
- stop bars and
- selectable segments of green taxiway centerline lights.
The system is designed to provide pilots with visual guidance while taxiing during night operations and during periods of low visibility. Stop bar lighting is prescribed in International Civil Aviation Organization (ICAO) Annex 14 for low visibility operations and is in use at many international airports. Stop bars are considered a valuable defence against aircraft and vehicles accidentally entering a runway without ATC clearance and a solid barrier against the pilots’ error related to poor situational awareness.
Automated and Manual Mode of Operation
When a taxiing route is selected, corresponding segments of taxiway centerline lights on the maneuvering area are switched on automatically. When two or more routes are selected, the system gives priority to the first route in sequence and activates red stop bar lights across conflicting routes, as necessary. A segment of the centerline lights of the conflicting routes that cut across the first route will also be suppressed.
The air traffic controller has the option of over-riding the taxiing route priority by manually selecting or deselecting the appropriate stop bar lights. All taxiing guidance lights on taxiways leading to the runways terminate at the runway holding positions where red stop bar lights remain on unless deselected by the controller. Stop bars are normally installed in association with green lead-on lights which form part of the taxiway centerline lighting beyond the stop bar. The lead-on lights are interlinked with the stop bar so that when the stop bar is ‘on’ the green centerline beyond the stop bar is ‘off’ and vice versa.
Figure: Stop Bar and Lead-on Lights, UK CAA CAP637
The installation of a taxiway guidance system allows landings and take-offs to take place in low visibility conditions where otherwise extensive delays would occur. Stop bar operations require no special equipment to be installed in aircraft or vehicles but require appropriate training and strict adherence to the procedures: the pilot/driver must stop and hold at a lit stop bar, and proceed only when ATC gives the appropriate verbal instruction and switches off the stop bar. The runway stop bar shows red in the direction of approach to the stop bar from the taxiway and is co-located with the runway holding point markings.
In case the stop bar lights have not been switched off after receipt of an instruction to proceed forward, the pilot or vehicle driver must seek clarification from ATC. When following the directional guidance provided by the green taxiway centerline lights and red stop bar lights, pilots are advised to also navigate their taxi route with reference to information and mandatory signs/markings provided at the airport so as to maintain situational awareness of their whereabouts at all times.
- ICAO Doc 9870, Manual on the Prevention of Runway Incursions
- ICAO Annex 14 – Aerodromes, Volume I, Aerodrome Design and Operations