Approach Procedures — Page 165, Lesson 243

Let’s start with the big picture. When we design an approach procedure, the whole problem is different from a departure. On departure, the main worry is getting airborne and climbing away. On approach, the primary concern of the procedure designer is terrain clearance — making sure the aircraft stays safely above the ground until the pilot is in visual contact and can actually land. So the initial design of an instrument approach procedure is dictated by the terrain surrounding the aerodrome. That’s the first thing that shapes everything else. But terrain isn’t the only factor. The design is also affected by the type of operations and the types of aeroplane flying the procedure. Those factors, in turn, influence the types and positioning of navigation aids in relation to the runway or aerodrome. And just like we saw with departure procedures, airspace restrictions can also affect the design. So the designer juggles terrain, aircraft types, operations, nav aids, and airspace. Now, a key responsibility here sits with ATC. If a pilot reports that they’re unfamiliar with an instrument approach procedure — or if it’s clear to ATC that the pilot is unfamiliar — then the ATCU, the Air Traffic Control Unit, is responsible for describing the procedure to the pilot. There’s one exception: if the aircraft is cleared for a straight-in approach, that responsibility doesn’t apply. So ATC must brief the pilot unless it’s a straight-in clearance. Next, let’s talk about speed and aircraft categories. Just like with departures, speed is a critical consideration. The critical speed is Vat — that’s the speed at which the aircraft crosses the threshold of the landing runway. Vat matters because it affects the spacing of aircraft on the approach. But there’s a separate set of speeds — the approach speeds — and those determine the dimensions of the areas within which manoeuvring may be carried out, and hence the OCH calculations. OCH is the Obstacle Clearance Height. So Vat drives spacing; approach speeds drive the manoeuvring areas and the OCH. Let me define Vat precisely. Vat is the speed at threshold based on 1.3 × VS0, or 1.23 × stall speed VS1g, in landing configuration at maximum certificated landing mass. VS0 is the stalling speed in the landing configuration, and VS1g is the one-g stall speed. So Vat is a calculated threshold speed, not just a number you read off. Now, the table — Figure 8.1 — relates speed to aircraft category, and the categories run from A through E. Let me walk you through each one. Category A: Vat is less than 91 knots. The initial approach speed range is 90 to 150 knots, with a note of 110 knots for track reversal or racetrack procedures — that’s the asterisk. The final approach speed range is 70 to 100 knots. And the maximum speed for visual circling is 100 knots. Category B: Vat is 91 to 120 knots. Initial approach speed range is 120 to 180 knots, with 140 knots for track reversal or racetrack. Final approach speed range is 85 to 130 knots. Max speed for visual circling is 135 knots. Category C: Vat is 121 to 140 knots. Initial approach speed range is 160 to 240 knots. Final approach speed range is 115 to 160 knots. Max speed for visual circling is 180 knots. Category D: Vat is 141 to 165 knots. Initial approach speed range is 185 to 250 knots. Final approach speed range is 130 to 185 knots. Max speed for visual circling is 205 knots. Category E: Vat is 166 to 210 knots. Initial approach speed range is 185 to 250 knots. Final approach speed range is 155 to 230 knots. Max speed for visual circling is 240 knots. Notice the pattern — as the category letter increases, the aircraft gets faster, and all the speed ranges climb accordingly. The asterisk on the initial approach speed range for categories A and B — the 110 and 140 — that’s the maximum speed for track reversal or racetrack procedures. For categories C, D, and E, there’s no asterisk shown, so that note applies only where it’s marked. Now, the last piece — Minimum Sector Altitudes, or MSA, also called Terminal Arrival Altitudes, TAA. For each aerodrome, MSA/TAA are established to provide at least 300 metres — that’s 984 feet — of obstacle clearance within 25 nautical miles of the navigation aid, the IAF, or the IF associated with the approach procedures for that aerodrome. IAF is the Initial Approach Fix, and IF is the Intermediate Fix. So within 25 NM of those fixes, you get at least 300 metres of clearance. MSA/TAA is shown on all instrument plates, and it will be the lowest altitude permitted at the appropriate fix — normally the altitude at which the procedure begins. So an arriving aircraft is not allowed to descend below MSA, except in three specific situations. One: the aerodrome and underlying terrain are visible and will remain so. Two: the aircraft is under radar control and being radar vectored. Three: the aircraft is flying a published approach procedure. Those are the only ways you can legally go below MSA. So to tie it together — the approach procedure is terrain-driven, ATC has a briefing duty for unfamiliar pilots unless it’s a straight-in, aircraft are categorised A through E by Vat with defined speed ranges, and MSA/TAA gives you a hard floor of 300 metres clearance within 25 NM of the relevant fixes, with three specific exceptions for descending below it.