Aerodrome Lighting — Page 434, Lesson 579

I want to walk you through the aerodrome lighting chapter now, and we're going to start with the visual approach slope indicators, the VASI. This is the system that helps you fly the correct glide path on approach, purely by looking at light colours. The VASI stands for Standard Visual Approach Slope Indicator. Picture two sets of three lights, positioned as what we call wing bars. Each set is designed so the lights appear either white or red, depending on the angle at which you view them. So the colour you see is not a property of the bulb itself — it's a function of your viewing angle relative to that bar of lights. Here's how you read it. When you're approaching on the glide slope, the first set of lights appears white and the second set appears red. That's your "on glidepath" indication. If both sets appear white, you're flying too high. If both sets appear red, the approach is too low. So the rule of thumb is: white over white, you're high; red over red, you're low; white over red, you're on the slope. I want you to hold that sequence firmly in mind, because it's the entire basis of the VASI. Now let's move to runway lighting, starting with the runway edge lights. These are provided for a runway intended for use at night, or for a precision approach runway intended for use by day or night. They should also be provided on a runway intended for take-off when the operating minimum is below an RVR of the order of 800 m by day. RVR is runway visual range — the distance over which a pilot can see the runway surface markings or lights from the cockpit. So if you're taking off with a visibility minimum below roughly 800 metres, even in daylight, you need edge lights. Physically, the edge lights are placed along the full length of the runway in two parallel rows, equidistant from the centre line. They must be uniformly spaced. There's one exception: at intersections of runways, the lights may be spaced irregularly or omitted, provided adequate guidance remains available to you as the pilot. Now the characteristics. Runway edge lights are fixed, variable intensity white, showing in the direction from which approaches are made. But there are two important colour changes. First, in the case of a displaced threshold, the lights between the beginning of the runway and the displaced threshold show red in the approach direction. Second, a caution zone may be established over the last 600 metres, or one-third of the runway, whichever is least — and in that zone the lights are yellow. So you see white along the main length, yellow in the caution zone near the far end, and red before a displaced threshold. And one more point: when the edge lights are intended to provide circling guidance, they shall show all round, which means omni-directional — visible from every direction, not just along the approach path. Next, the runway threshold and wing bar lights. These are provided for a runway equipped with runway edge lights — with one exception: they're not provided on a non-instrument or non-precision approach runway where the threshold is displaced and wing bar lights are provided instead. So let's separate the two cases. When the threshold is at the extremity of the runway, the threshold lights are placed in a row at right angles to the runway axis, as near to the extremity of the runway as possible. When the threshold is displaced, the lights are in the form of a barrette — that's a wing bar — on either side of the displaced threshold. Both the threshold lights and the wing bar lights are fixed, unidirectional lights showing green in the direction of approach to the runway. Finally, we have the runway threshold identification lights, abbreviated RTIL. These are used at the threshold of a non-precision runway when additional threshold conspicuity is necessary, or where the threshold is permanently displaced. They are flashing white lights, unidirectional in the direction of the approach to the runway. So the key contrast here: threshold lights are steady green, but threshold identification lights are flashing white, and they exist purely to draw your eye to a threshold that might otherwise be hard to pick out. Let me tie the colours together, because that's the part that will stick with you. VASI gives you glide slope information in white and red. Edge lights are white, turning yellow in the caution zone and red before a displaced threshold. Threshold and wing bar lights are green. Threshold identification lights are flashing white. Each colour and each flashing characteristic has a specific job, and on a night approach you'll be reading all of them at once.