Approach Procedures — Page 185, Lesson 261

Let's pick this up with the specific data for ILS and MLS approaches. When we're determining the decision altitude or decision height — that's the DA/H — for an ILS or MLS, we can't just use the generic obstacle clearance criteria. We have to account for the physical size of the aircraft itself, because a bigger aircraft needs more clearance. So the criteria specify two things: the wing span, and the vertical distance between the wheels and the glide path aerial. The glide path aerial is the ground-based transmitter that sends the vertical guidance signal up to your aircraft. The vertical distance between your wheels and that aerial is a critical clearance figure. Here's the table. For Category H — that's helicopters — the wing span is 30 metres, and the vertical distance between the wheels and the GP aerial is 3 metres. For Categories A and B, the wing span is 60 metres, and the vertical distance is 6 metres. For Categories C and D, the wing span is 65 metres, and the vertical distance is 7 metres. And there's a special category, DL, which has been included specifically to cater for the A380 — that's the double-deck large aircraft. Its wing span is 80 metres, and the vertical distance between the wheels and the GP aerial is 8 metres. Now, there are other criteria for ILS that you need to know. CAT I approaches are flown with a pressure altimeter. CAT II approaches are flown with a radio altimeter and a flight director. The missed approach climb gradient is 2.5%. And the glide path angle — the angle of the descent path — has a minimum of 2.5° and a maximum of 3.5°. But for CAT II and CAT III, the glide path angle is required to be 3°. Next, let's look at the calculation of MDA/H — that's the minimum descent altitude or minimum descent height. This is for non-precision approaches. The method is shown diagrammatically in Figure 8.15. The key idea is that you start from the obstacle clearance altitude or height, and you build up the minimum descent altitude or height from there, ensuring you have adequate clearance over all obstacles in the final approach segment. Then we have the calculation of MDA/H for VM(C) — that's visual manoeuvring, or circling. This is shown in Figure 8.16. When you're circling to land, you're flying a visual manoeuvre after the instrument approach, and the minimum descent altitude or height is calculated differently because you need clearance for the entire circling pattern, not just the final approach path. Now let's talk about descent gradients. The design of procedures must allow adequate space for descent from the published height crossing the facility — that's the point where you cross the navigation aid — down to the runway threshold. This is achieved by establishing a maximum allowable descent gradient for each segment of the procedure. The most critical segment is the final segment, where the threshold speed — or your ability to decelerate to it — will be a function of the gradient. The optimum descent gradient in the final approach should not exceed 5.2%. That's 50 metres per kilometre, or approximately 300 feet per nautical mile, which is equivalent to a 3° glide path. Where a steeper gradient is necessary, the maximum permissible is 6.5% — that's 65 metres per kilometre, or 400 feet per nautical mile, equivalent to a 3.8° glide path. For a precision approach, the operationally preferred glide path angle is 3°, and this is mandatory for CAT II and CAT III. An ILS glide path in excess of 3° is used only where an alternative means of satisfying obstacle clearance requirements is impractical. Finally, let's cover high rate descents. Gradients over 6.5% may result in descent rates exceeding the recommended maximum rate of descent for some aircraft. Pilots flying such approaches should be aware of this before starting the approach. High rate descents are not permitted as a means of avoiding noise abatement procedures. Where glide paths greater than 6.5% are established — for example, 9.5% or 5.5° at London City — several conditions must be met. The authority of the state in which the aerodrome is situated must give specific approval. The operator must be approved to carry out high rate descents. Specially approved aircraft must be used. And pilots must be specially trained. So to summarise the key numbers: optimum final descent gradient is 5.2%, maximum is 6.5%, and anything above that is a high rate descent requiring special approval. The glide path angle for CAT II/III is mandatory at 3°. And the aircraft category determines your clearance figures for ILS/MLS — with DL being the special category for the A380.