Aerodrome Lighting — Page 434, Lesson 584

I want to walk you through the aerodrome lighting systems that govern taxiway movement in low visibility. We're starting with taxiway centre line lights, then moving to stop bars, intermediate holding position lights, and road-holding position lights. Let's begin with taxiway centre line lights. These are provided on an exit taxiway, a taxiway, and an apron when the aerodrome is intended for use in runway visual range conditions less than 350 metres. The purpose is to give continuous guidance from the runway centre line all the way to the point on the apron where aircraft commence manoeuvring for parking. So the pilot follows these lights from the runway, along the exit, down the taxiway, and onto the apron until reaching the parking area. There's an important exception: these lights need not be provided where there is a low volume of traffic and taxiway edge lights plus centre line markings already give adequate guidance. So if traffic is light and the edge lights and markings are sufficient, you can omit the centre line lights. Now, taxiway centre line lights must also be provided on a runway that forms part of a standard taxiway route and is intended for taxiing in runway visual range conditions less than 350 metres. The lights themselves are fixed, variable intensity, and show green. They're designed so the light is visible only from aeroplanes on or in the vicinity of the taxiway — that's a deliberate restriction to avoid confusing pilots elsewhere. Here's a key detail: within the ILS/MLS critical/sensitive area, the centre line lights become alternating green and yellow. ILS is the Instrument Landing System, MLS is the Microwave Landing System. The critical and sensitive areas are protected zones around those systems where aircraft and vehicles must not interfere with the signals. The alternating green and yellow pattern is the same for a high-speed exit. But where aircraft may follow the same centre line in both directions, all the centre line lights shall show green to aircraft approaching the runway. So direction of travel determines the colour coding. Next, stop bars. A stop bar is a row of red lights showing in the direction of taxiing aircraft. When illuminated, they require the aircraft to stop and not proceed until cleared by ATC — Air Traffic Control. One or more stop bars, as appropriate, should be provided at a taxiway intersection or taxi-holding position when you want to supplement markings with lights and provide traffic control by visual means. A stop bar shall be provided at every taxi-holding position serving a runway when that runway is intended for use in runway visual range conditions less than 350 metres. So in low visibility, every holding point for a runway gets a stop bar. There's a practical consideration: where the normal stop bar lights might be obscured from the pilot's view — for example, by snow or rain, or where the pilot may be required to stop the aircraft in a position so close to the lights that the aircraft's own structure blocks them — then a pair of elevated lights should be added to each end of the stop bar. Those elevated lights sit higher so the pilot can still see them. Now, intermediate holding position lights. These consist of three fixed unidirectional lights showing yellow in the direction of approach, and they're positioned at right angles to the taxiway. So three yellow lights, fixed, pointing toward the approaching aircraft, mounted across the taxiway. Finally, road-holding position lights. These comprise either — and the excerpt cuts off there, but the point is they come in one of two configurations, which we'll pick up when we continue. Let me make sure you have the full picture. The common thread is runway visual range, abbreviated RVR, and the 350-metre threshold. Below that value, you need centre line lights on taxiways and aprons, and stop bars at every taxi-holding position serving a runway. The colours carry meaning: green for proceed along the centre line, alternating green and yellow in the ILS/MLS critical or sensitive area and on high-speed exits, red for stop at a stop bar, and yellow for the intermediate holding position lights. Each light system is a visual control that works with markings and ATC clearances to keep aircraft moving safely in conditions where the pilot can't rely on normal visual references.