ATPL · Navigation · Instrumentation Atpl Ground Training Series… lesson — Page 388, Lesson 481

Let’s start with the phrase that gives this whole chapter its name: all weather operations. I want to be very clear about what that term actually means, because it’s easy to misread it. The phrase is used a lot in connection with automatic landing systems and with describing low weather minima. Now, "low weather minima" just means the lowest weather conditions — the worst visibility and cloud base — under which an approach and landing are still permitted. So "all weather operations" sounds like it means there is no weather that could stop you from taking off and landing. That is not the case. No automatic system can, for example, perform the landing task in wind conditions beyond those for which the aircraft has been certified. That limit is governed primarily by the controllability characteristics and the strength factors of the aircraft. In plain terms, the airframe and its flight controls set the ceiling on what wind the autoland can handle, not the electronics. Similarly, no automatic system can land an aircraft on a runway whose surface is not fit for the operation because of contamination by water, slush, or ice. So "all weather" really means "all weather within the certified limits of the aircraft and the fitness of the runway." Keep that in mind — it’s the central caveat. Now we move to the categories of operation. The main categories are shown in Figure 27.2, and I want you to understand what these categories actually do. They serve two purposes. First, they indicate the stages through which automatic approach and automatic landing development progress — so they designate the capabilities of individual automatic flight control systems. Second, they designate the standards of efficiency of the ground guidance equipment available at airports. That ground equipment is the ILS localizer and glide path, plus the approach, runway, and taxiway lighting. So a category isn’t just about the aircraft — it’s a pairing of what the aircraft can do and what the airport can support. Let’s take Category 1 first. A Category 1 operation is a precision instrument approach and landing with a decision height no lower than 60 metres, which is 200 feet. And the visibility requirement is either a visibility not less than 800 metres, or a runway visual range not less than 550 metres. Notice the "or" — you can satisfy it with plain visibility of 800 metres, or with runway visual range of 550 metres. I’ll define decision height and runway visual range properly as we go, but for now, decision height is the altitude at which you must decide whether to continue the approach or go around, and runway visual range is the distance along the runway you can actually see, measured by instruments. So Category 1 is the baseline — 60 metres decision height, 800 metres visibility or 550 metres RVR. The higher categories tighten those numbers, and we’ll build on that next.