Improving the awareness of both flight crew and vehicle drivers of the entrance points to active runways, continues to be a major feature of attempts to reduce runway incursions.
Surface markings of runway holding points and their accompanying signage may be supported by lit stop bars and runway guard lights. Whilst Runway Guard Lights are usually operated in day or night conditions and lit stop bars are now usually controllable from the TWR, the use of lit stop bars is sometimes restricted to use at night and during Low Visibility Procedures (LVP).
Runway Guard Lights (RGLs)
RGLs are the first line of defence against an unintended incursion of an active runway by an aircraft or vehicle. They serve to raise situational awareness but do not provide or preclude authority to pass. The term was formally introduced by ICAO in 1995 when introducing a new Standard for what had previously been a Recommended Practice for installation of ‘taxi-holding position lights’. In their basic form, they consist of a pair of unidirectional yellow lights which flash continuously. They are positioned at each side of a taxiway at the marked and signed Holding Point where the taxiway is about to join a runway; both aircraft and vehicles are required to wait at this point until given clearance by ATC to proceed. They are sometimes colloquially referred to as ‘wig-wags’. The lights are required to have equal intervals lit and unlit and to flash at between 30 and 60 cycles per minute. They must be in operation whenever RVR is less than 1200 metres and must be switched independently of any stop bar lights (see below). They are often elevated, but this is not a requirement.
The current version of ICAO Annex 14 also requires that where a taxiway, which is used in low visibility conditions, has a ‘wide throat’ as it nears a runway intersection and a stop bar is not installed but additional conspicuity is required, an alternative RGL may be used. This consists of a row of unidirectional yellow lights in line at 3 metre intervals across the taxiway at the Holding Point, inset into the pavement. Adjacent lights must be alternately illuminated and alternate lights should illuminate in unison.
Remotely selectable Stop Bar lighting is increasingly being provided on both a 24 hour basis or just during LVP operations. When not used during normal daytime operations, it may be provided at reduced intensity for night operations. A lighted Stop Bar consists of a row of unidirectional lights embedded in the pavement and spaced equally across the taxiway at the associated holding point, normally at right angles to the centerline. It will show red towards an approaching aircraft when lit. Stop Bars are sometimes installed in association with green Lead-on Lights which are a continuation of the taxiway centreline lighting beyond the Stop Bar which guide an aircraft onto the runway centreline. Where provided, such Lead-on Lights are operated in conjunction with their preceding Stop Bar so that when the Stop Bar is red, the green centreline beyond the Stop Bar is unlit and vice versa.