Approach Procedures — Page 179, Lesson 260

Let’s pick this up right where the approach procedure design leaves off. We’ve already looked at how the obstacle environment, aircraft equipment, and meteorological reporting all feed into setting approach minima. Now I want to walk you through the two big ideas that actually govern what you fly to: system minima and the calculation of DA/H. First, system minima. The whole point of an instrument approach is that you descend to a certain height, and if you don’t have the runway in sight, you go around. But the authorities don’t just let you pick any height you like. For each method of conducting an instrument approach, there is a specified minimum height above the datum below which the approach is not permitted to continue without the necessary visual reference. That height is called system minima. Think of it as a safety floor — a buffer to allow for anomalies, so the minima stay realistic and give you enough margin above the obstacles, the missed approach areas, and the climb-out areas. Here’s the key relationship: system minima overrides a lower DH or MDH. Let me give you the exact example from the text. For a CAT 1 ILS, the system minimum is 200 ft. Suppose the obstacle clearance height, the OCH, plus the upper margin for a CAT 1 ILS comes out to 170 ft. Well, 170 ft is below the 200 ft floor, so the system minimum prevails — your DH becomes 200 ft. But if your calculated DH is 230 ft, that’s above the system minimum, so the DH stays at 230 ft. The system minimum only kicks in when your calculated value would be lower than the floor. Now, for Air Law, you only need to know the system minima for ILS categories I, II, and III. Let me give you those numbers exactly. CAT I: 60 metres, which is 200 ft. CAT II: 30 metres, which is 100 ft. CAT III: not applicable. And here’s the logic for CAT III — for CAT III, the DH can be zero, so the system minimum for CAT III must also be zero. That’s why it’s listed as not applicable; there’s no floor above zero because you’re allowed to go all the way down to zero decision height. Now the second idea: calculation of DA/H. DA/H stands for decision altitude or decision height — the altitude or height at which you must make the decision to continue or go around. The text tells me there’s a diagrammatic representation of the method of calculating DA/H, and it’s shown in Figure 8.13. That figure lays out the method for determining DA/H for precision approaches. The core idea is that your decision height is built up from the obstacle clearance height, plus margins, and then checked against the system minimum we just talked about. Whichever is higher — the calculated value or the system minimum — that’s what you fly. So the takeaway for you as a pilot: you don’t just memorise a DH. You understand that the published DH is the higher of the calculated value and the system minimum. For a CAT 1 ILS, if the calculation gives you less than 200 ft, you still fly 200 ft. If it gives you more, you fly the higher number. And for CAT III, the floor is zero, so your DH can be zero. That’s the complete picture of system minima and DA/H calculation.