Approach Procedures — Page 175, Lesson 250

Let’s pick this up right where the approach procedure rules leave off, because this is where the real operational teeth come in. I’m talking about the limitations on continuing an instrument approach. Here’s the rule, and I want you to hear it exactly as it’s written, because it’s a hard limit. An instrument approach shall not be continued beyond the outer marker fix — or, if no outer marker is provided, the equivalent position — in the case of a precision approach. And for a non-precision approach, the limit is below 300 metres, which is 1000 feet, above the aerodrome. The condition is this: you may not continue past that point unless the reported visibility, or the controlling RVR — that’s runway visual range — is above the specified minimum. Now here’s the subtle part, and it’s a classic exam trap. If, after passing the outer marker fix, or the equivalent position, in a precision approach — or after descending below 300 metres, 1000 feet, above the aerodrome in a non-precision approach — the reported visibility or controlling RVR falls below the specified minimum, the approach may still be continued, but only down to the DA/H or MDA/H. DA/H is decision altitude or decision height; MDA/H is minimum descent altitude or minimum descent height. So the rule is: you can’t start the descent past that gate unless the minima are met, but if the minima deteriorate after you’ve passed the gate, you’re allowed to continue down to the decision or minimum descent altitude. And then, if the required visual criteria are obtained at DH — at decision height — the aircraft may be landed. That’s the whole logic of the gate: once you’re committed past the outer marker, you get one chance to see the runway at decision height. Now let’s move into the design side, because this is where the procedure itself is built. The fundamental requirement of an instrument approach procedure is that the aeroplane must be flown in safe airspace. To stay in that safe airspace, two things must hold. First, the required track of the aeroplane must be achievable. Second, the altitude limitations that need to be applied must be commensurate with what is trying to be achieved — meaning the altitude constraints have to match the goal of the approach. And here’s the key principle that governs the whole design: as the procedure takes the aeroplane closer to the runway or aerodrome, and closer to the ground, the safety limitations must be increased, not relaxed. That’s a deliberate inversion of what you might guess — the closer you get to the ground, the tighter and more stringent the safety margins become. Now, why does the design have to be so careful? Because until 3-D satellite navigation technology is widely available and proved reliable, the system of guidance in azimuth and elevation will rely on ground-based equipment, and that ground-based equipment has inherent errors. Azimuth is the horizontal direction, left and right; elevation is the vertical angle, up and down. So the guidance system has built-in error. The saving grace is this: providing the error tolerances are known, and the design of the procedure — which details the flight path to be flown — takes those error tolerances into account, the procedure will be usable. But there’s a human element too. The procedure requires the pilot, or the auto-pilot, to be able to fly the aeroplane to the required basic accuracy to keep the aeroplane in the specified airspace. And because the procedure defines tracks to be made good, the pilot must make allowance for the wind. That’s a direct operational instruction — you don’t just follow the needle, you correct for drift. Finally, the structure. An instrument approach procedure has five separate segments, and each one has a specific purpose. Each of the five segments begins and ends at a designated fix. But — and this is important — it is possible for segments to begin at specified points if no fix is available. For instance, the final approach segment of a precision approach may begin at the point of intersection of the intermediate flight altitude and the glide path. So the segment boundary isn’t always a physical fix; it can be a geometric intersection point. That figure shows you the procedure segments laid out, and the next one, Figure 8.4, gives you the characteristics of each segment. So the takeaway here is the gate logic at the outer marker, the principle that safety margins tighten as you get closer to the ground, the reliance on ground-based guidance with known error tolerances, and the five-segment structure with its fix-based boundaries.