Approach Procedures — Page 175, Lesson 247

Let's pick this up right where the approach procedure definitions leave off. We've already covered the general structure of an instrument approach, so now we're diving into the two big categories that define how you fly them: precision and non-precision. First, let's talk about the precision approach. This is the gold standard. The defining feature is that the ground equipment provides both track guidance and vertical (height) guidance all the way down to the runway. The classic example is the ILS, the Instrument Landing System. It gives you lateral guidance via the localizer and vertical guidance via the glideslope. As technology has advanced, these systems have become incredibly accurate in both track and height. And with modern FMS, the Flight Management System, the on-board computers interpret that received data and can even control the aeroplane, reducing the visual element to the absolute minimum. Now, here's a critical point for your Air Law exam. ILS systems are categorized by their accuracy of operation. These categories are defined by System Minima — that's the minimum weather conditions required for the approach. But beware: there are anomalies between ICAO and JAR OPS requirements. For your Air Law exam, only the ICAO requirements are examinable. Let's go through them carefully. Category I — System Minima is 60 metres, which is 200 feet. The DH, or Decision Height, must be greater than or equal to 60 metres (200 feet). And the RVR, Runway Visual Range, must not be less than 550 metres. Alternatively, if RVR isn't available, ground visibility must not be less than 800 metres. Category II — System Minima drops to 30 metres, or 100 feet. The DH must be less than 60 metres (200 feet) but greater than or equal to 30 metres (100 feet). And RVR must not be less than 350 metres. Category IIIA — Now we enter the no-system-minima territory. There's no system minima defined. The DH is less than 30 metres (100 feet), or there's no DH at all. RVR must not be less than 200 metres. Category IIIB — Again, no system minima. The DH is less than 15 metres (50 feet), or no DH. And RVR is less than 200 metres but more than 50 metres. Category IIIC — The ultimate. No system minima, no DH, and no RVR requirements whatsoever. That's a full autoland capability in zero visibility. Now, let's contrast that with the non-precision approach. This is where there is no ground equipment that can provide height, or elevation, data to the aircraft. The procedure is defined as non-precision, even though the track guidance accuracy might be just as good as that required for precision. So you have lateral guidance, but no vertical guidance from the ground. Non-precision procedures can be established where track guidance is provided by a VOR, a VHF Omni-directional Range, or an NDB, a Non-Directional Beacon. They can also use the track guidance elements of precision systems — for example, the ILS localizer only, or PAR in azimuth only, which is Precision Approach Radar. Another type of non-precision system is SRA, Surveillance Radar on a reduced range scale. The key difference here is that because there's no reference to touchdown for non-precision procedures, the approach always... well, that's where the text cuts off, but you can already see the fundamental distinction: precision gives you both lateral and vertical guidance with defined categories and minima; non-precision gives you lateral guidance only, and you have to manage your descent differently. Take a look at the figures on your screen — Figure 8.2 shows the ILS categories, and Figure 8.3 and 8.4 illustrate the procedure segments and their characteristics. These will help you visualize how the approach segments fit together.