Autoland — Page 393, Lesson 482

We’re starting the Autoland chapter, and the very first thing I want to put on the table is the idea of low-visibility landing categories. These are the ICAO definitions that tell you exactly what weather conditions you’re allowed to fly an approach and landing in, and they’re built around two numbers: decision height and runway visual range. Let me define those two terms first, because everything else hangs on them. Decision height is the height above the runway, in feet, at which you must have the runway environment in sight. If you don’t see it at that height, you go around. Runway visual range, or RVR, is the horizontal distance, in metres, that a pilot can see down the runway from the cockpit. So decision height is vertical, RVR is horizontal. Now, Category 2. That’s a precision instrument approach and landing with a decision height lower than 200 feet but not lower than 100 feet, and a runway visual range not less than 300 metres. So the decision height window is between 100 and 200 feet, and you need at least 300 metres of RVR. Category 3A. Decision height lower than 100 feet, and RVR not less than 200 metres. Notice the decision height has no lower bound here — it just has to be below 100 feet. Category 3B. Decision height, if any, lower than 50 feet, and RVR less than 200 metres but not less than 75 metres. That phrase “if any” is important — it means there may be no decision height at all, or one below 50 feet. And the RVR band is 75 to 200 metres. Category 3C. This one is different. It says: to and along the surface of the runway and taxiways without external visual reference. So there’s no decision height and no RVR requirement at all — you’re landing and taxiing with zero external visual reference. That’s the most demanding category. Now, the key concept that ties all of this together is the alert height. The alert height is a specified radio height, based on the characteristics of the aircraft and its fail-operational landing system. Let me unpack that. Radio height means height measured by the radio altimeter, not barometric. And fail-operational means the landing system can survive a failure and still complete the landing automatically. Here’s how the alert height works in operation. If a failure occurs 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, you still have time and altitude to break off and go around safely. But if a failure in one of the required redundant operational systems occurs below the alert height, it would be ignored and the approach continued to complete the autoland. Below the alert height, you’re committed — you don’t have enough height to safely go around, so the system carries on and lands automatically. So the alert height is the dividing line between “we can still abort” and “we’re committed to the autoland.” That’s the whole logic of the chapter. The categories tell you what weather you’re certified for, and the alert height tells you when a failure forces a go-around versus when you press on. One thing to note — the excerpt also shows the start of the next chapter, Autothrottle, with its contents: introduction, autothrottle system, turbulence, FADEC, and thrust control. But that’s a separate chapter we’ll get to. For now, we’re on Autoland, and you’ve got the categories and the alert height.