You’ll hear it constantly in connection with automatic landing systems and… — Page 388, Lesson 481

Let’s start with the phrase that gives this whole topic its name: all weather operations. You’ll hear it constantly in connection with automatic landing systems and low weather minima. And here’s the trap — the term can be taken to mean that no weather condition can ever stop an aircraft from taking off and landing. That is not the case. No automatic system can perform the landing task in wind conditions beyond those for which the aircraft is certified. That certification limit is governed primarily by the controllability characteristics and the strength factors of the aircraft. And no automatic system can land an aircraft on a runway whose surface is not fit for the operation — for example, a runway contaminated by water, slush, or ice. So "all weather" really means "all weather within the certified envelope," not literally all weather. Now, the category of operation. The main categories are defined in Figure 27.2, and I want you to see them as serving two purposes at once. 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 — namely the ILS localizer and glide path, and the approach, runway, and taxiway lighting. So a category is a joint statement: what the aircraft’s automatic system can do, and what the ground equipment can support. Let’s take Category 1. It 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 stated two ways: either a visibility not less than 800 metres, or a runway visual range not less than 550 metres. Note the "or" — the pilot can use either the prevailing visibility or the runway visual range, whichever is applicable, as long as it meets its own minimum. That’s the Category 1 standard. So the key ideas to hold onto: "all weather" is a certified envelope, not an absolute; and each category couples an aircraft capability with a ground equipment standard. Category 1 gives you the 60-metre decision height floor, with the 800-metre visibility or 550-metre RVR alternative.