All Weather Operations — Page 70, Lesson 108

I want to walk you through the next part of All Weather Operations. We're moving into the details of Category III operations, and I'll start with a couple of important notes that apply to the table you've just seen. First note: when a table refers to 'autocoupled to below DH' or 'approved HUDLS', it means the automatic flight control system continues to be used down to a height that is no greater than 80% of the applicable decision height. So if your decision height is 100 feet, the autopilot can stay engaged down to 80 feet or lower, but not above 80% of that DH. This matters because airworthiness requirements may set a minimum engagement height for the automatic flight control system, and that minimum can affect what decision height you're allowed to apply. In other words, if the autopilot can only stay coupled down to 90 feet, you can't use a DH of 100 feet if 80% of 100 is 80 feet — you'd need a higher DH to stay within the 80% rule. Second note: for a Category D aeroplane conducting an autoland, you may use 300 metres RVR instead of whatever lower value might otherwise apply. Category D aeroplanes are the largest — think wide-body jets like a Boeing 747 or 777 — and this gives them a bit more leeway on the visual range requirement for an autoland. Now, let's look at the three subdivisions of Category III operations. These are all precision instrument approaches and landings using either ILS — Instrument Landing System — or MLS, which is Microwave Landing System. The key differences are in decision height and runway visual range. Category III A operations have a decision height lower than 100 feet, and a runway visual range not less than 200 metres. So you can go down below 100 feet without seeing the runway, but you need at least 200 metres of RVR. Category III B operations go lower: decision height lower than 50 feet, or even no decision height at all. The RVR requirement is lower than 200 metres but not less than 75 metres. So you can operate in very poor visibility, down to 75 metres RVR, and you can descend below 50 feet or even have no DH at all. Category III C operations are the most demanding: no decision height and no runway visual range requirements. That means you can land in zero visibility, with no minimum RVR and no decision height. In practice, this is rarely used because it requires full autoland capability and the ability to taxi in zero visibility, which most aerodromes don't support. There's an important note here: if your DH and RVR don't fall within the same Category, the RVR determines which Category the operation is considered to be. So if you have a DH of 80 feet — which fits Category III A — but the RVR is 150 metres — which fits Category III B — the operation is treated as Category III B because the RVR is the limiting factor. For operations where a decision height is used, the operator must ensure that the decision height is not lower than three things. First, the minimum decision height specified in the AFM — the Aeroplane Flight Manual — if one is stated. Second, the minimum height to which the precision approach aid can be used without the required visual reference. Third, the decision height to which the flight crew is authorized to operate. So the actual DH you fly must be the highest of those three values. Now, let's talk about no decision height operations. These are operations where you don't have a decision height at all — you continue the approach all the way to touchdown without needing to see the runway at a specific height. But you can only conduct these if three conditions are met. First, the operation with no decision height must be authorized in the AFM. Second, the approach aid and the aerodrome facilities must be able to support operations with no decision height. Third, the operator must have an approval for CAT III operations with no decision height. There's a practical note here: for a CAT III runway, you may assume that operations with no decision height can be supported, unless it's specifically restricted as published in the AIP — Aeronautical Information Publication — or a NOTAM. So unless the published information says otherwise, a CAT III runway is considered capable of supporting no-DH operations. Let me show you the diagram that illustrates the RVR requirements for Category III approaches against decision height and roll-out. That covers the breakdown of Category III operations and the rules for setting decision heights and conducting no-DH approaches.