Aerodrome Lighting — Page 426, Lesson 575

I want to walk you through the precision approach lighting systems now, starting with the ICAO CAT I system. This is the arrangement you'll see at many aerodromes, and it's important you understand exactly what's on the ground before you. In the ICAO Precision Approach CAT I Lighting System, the centre line is not a continuous line of lights. Instead, it's made up of barrettes. A barrette is a short row of lights, and here they're arranged along the centre line. There's only one crossbar, and that crossbar is positioned at 300 metres from the threshold. The threshold is the beginning of the runway, the point you cross to land. The centre line itself should be 900 metres in length. That's the standard distance the approach lighting extends out from the threshold. Now, the centre line may be augmented with strobe lights. Strobe lights are the very bright, flashing lights. In this system, they 'ripple' towards the threshold from the start of the centre line. So the sequence of flashes travels from the far end of the centre line, down towards the threshold, guiding your eye in. The visibility of the threshold itself may be enhanced by the use of wing strobes. These are rotating strobe lights, and they sit on either side of the threshold to make that point much more conspicuous. So you have the barrette centre line, the single crossbar at 300 metres, the rippling strobes, and the rotating wing strobes at the threshold. Now, let's move to the more demanding systems. At aerodromes where Category II and Category III approaches are conducted, more complex approach lighting systems are installed. The purpose is to enhance the possibility of the pilot achieving the visual criteria at DH, the decision height, to complete the landing. At Cat II and III, you're flying lower in poor visibility, so you need better visual cues to decide whether to continue. The systems used are various, but all are based on either the Calvert 5 bar and centre line system, or the ICAO barrette system. Both systems should be 900 metres long, and both provide some element of attitude information. That's crucial — the lighting isn't just for horizontal guidance, it gives you pitch information so you can judge your descent path. Here's the key augmentation. In both the Calvert and the ICAO systems, the inner segment — that's the 0 to 300 metres from the threshold — is augmented by supplementary approach lighting. This consists of replacing the centre line of the Calvert system with barrettes, and adding red wing barrettes to both systems. The effect is to enhance the visibility and conspicuity of the inner segment. So in that critical last 300 metres before the threshold, you get brighter, more distinctive lighting. There's a very practical reason for that 300-metre figure. The DH for Cat II is not lower than 100 feet, which equates to 300 metres from the threshold, assuming the pilot crosses the threshold at 50 feet. So at the decision height, you're roughly 300 metres out, and that's exactly where the supplementary lighting is concentrated. The lighting is designed to give you the visual reference right where you need it at DH. Finally, let's look at the Precision Approach Path Indicators, or PAPI. This is an additional aid to a precision approach, providing glide path guidance during the visual phase, after DH. The PAPI system consists of 4 light units, each showing either red or white light through precision defined angles. Each light unit is set to a different mid angle. Below that angle, the unit shows red; above that angle, it shows white. The overall effect is to give you a reference to the median angle, which is set to the required glide path, for example 3 degrees. So as you fly the approach, you look at the four lights. If you're on the glide path, you see a specific combination of red and white. If you're too high, you see more white; too low, more red. ICAO defines PAPI as "a wing bar of 4 lights uniformly spaced." That's the official definition — four lights, evenly spaced, forming a wing bar. So to tie it together: the CAT I system gives you lateral guidance with barrettes and a single crossbar. The Cat II/III systems, whether Calvert or ICAO, add the supplementary inner segment with red wing barrettes to boost conspicuity at the critical 300-metre point. And PAPI gives you the vertical glide path guidance after DH. Each system is built around that 900-metre length and that 300-metre inner segment, because that's where your decision to land is made.