Autoland — Page 393, Lesson 482

We’re starting the Autoland chapter now, and the very first thing we have to nail down is the category system for low-visibility landings. These categories define exactly what the aircraft and the crew are certified to do when the weather closes in. Let me walk you through them one by one, because each one is a precise legal and technical limit, not a vague description. We begin with Category 2. A Category 2 approach is a precision instrument approach and landing with a decision height lower than 200 ft but not lower than 100 ft, and a runway visual range not less than 300 m. So you have two hard numbers here. The decision height — that’s the altitude at which you must have the runway in sight or you execute a missed approach — sits somewhere between 100 and 200 feet. And the runway visual range, which is the horizontal distance you can see down the runway, must be at least 300 metres. That’s the minimum visibility for a Cat 2 operation. Now Category 3A. This is a precision instrument approach and landing with a decision height lower than 100 ft and a runway visual range not less than 200 m. Notice the decision height has dropped below 100 feet, and the runway visual range has come down to a minimum of 200 metres. So we’re getting into much lower visibility territory. Next, Category 3B. This one is a precision instrument approach and landing with a decision height, if any, lower than 50 ft and a runway visual range less than 200 m but not less than 75 m. Two things to flag here. First, the phrase “if any” — that means in Cat 3B there may be no decision height at all, or if there is one, it’s below 50 feet. Second, the runway visual range is now between 75 and 200 metres. So the visibility band has narrowed considerably. And finally Category 3C. This is the most demanding. It’s defined as operating to and along the surface of the runway and taxiways without external visual reference. That means the entire landing and the subsequent taxiing are done with no visual reference to the outside world at all. The aircraft must be guided all the way to the gate, essentially, on instruments and guidance systems alone. Now, before we go further, I want to make sure you understand the term decision height. It’s the altitude on the approach at which, if you cannot see the runway environment, you must initiate a missed approach. In these categories, the decision height is the key threshold that separates one category from the next. And runway visual range, or RVR, is the distance over which the runway markings or lights are visible — it’s a measured value, not a pilot’s guess. Now let’s move to a concept that’s absolutely central to autoland operations: the Alert Height. This is a specified radio height, based on the characteristics of the aircraft and its fail-operational landing system. Let me unpack that. Radio height is the height above the ground measured by the radio altimeter, not barometric altitude. And a fail-operational landing system is one that, if a single failure occurs, can still complete the landing automatically without the crew having to take over manually. Here’s how the alert height works in practice. In operational use, if a failure occurred above the alert height in one of the required redundant operational systems in the aircraft, the approach would be discontinued and a go-around executed, unless reversion to a higher decision height is possible. So above the alert height, a failure in a critical system means you abort the approach and go around — unless you can raise your decision height and continue as a less demanding approach. But if a failure in one of the required redundant operational systems occurred below the alert height, it would be ignored and the approach continued to complete the autoland. Why? Because below the alert height, you’re committed. You’re too low and too close to the runway to safely execute a go-around, so the system is designed to tolerate that failure and carry on to touchdown. So the alert height is essentially the decision point for system redundancy. Above it, you have the option to abort. Below it, you’re committed to the autoland. That’s the logic, and it’s a critical concept for understanding how autoland systems are designed and operated. Now, one thing I want to point out — the figure I have here, , shows the categories of low visibility landing capabilities. It’s a visual summary of exactly what we just covered, so it’s worth studying alongside these definitions. That’s the core of the Autoland chapter’s opening. We’ve got the four categories — Cat 2, 3A, 3B, and 3C — each with their decision height and runway visual range limits, and we’ve got the alert height concept that governs when a failure is cause for a go-around versus when it’s ignored to complete the landing. These are the building blocks for everything that follows in the chapter.