Control Laws — Page 413, Lesson 513

I want to walk you through the control laws of the autopilot, starting with the core idea that drives everything else. Think about how a human pilot flies. The autopilot must manoeuvre the aircraft logically and safely, in a similar manner to the way a human pilot should. That means it must never break aircraft limitations — speed, load factor, pitch, bank limits. But at the same time, it should use a satisfactory amount of the aircraft's performance, otherwise manoeuvres would take far too long to execute. Let me give you the example from the text. Imagine a light aircraft on a VFR navigational exercise. Overhead the next waypoint, the pilot needs an 80° left turn. The bank angle used will be determined by that turn. The pilot would not normally choose 5° of bank — that would make the turn take an inordinate amount of time. Conversely, the pilot would not normally choose a 90° banked turn — that would be excessively hard, possibly breaking load factor limitations and probably causing a loss of height. A turn using 30° to 45° of bank would be the most sensible. But if the required turn were only about 10° instead of 80°, then banking to 30° would generally be considered too harsh. So the autopilot needs to apply similar logic to all its actions. Here's the key principle: whenever the autopilot is required to make a correction — either for stability or when referring to a particular flight path — the control response will always be in proportion to the deviation or corrective action required. But only up to a limit that prevents the autopilot from attempting a manoeuvre that would cause an excursion from the safe flight envelope. That's the fundamental control law logic. Now let's look at the Boeing 737-400 specifically, and its autopilot limiting and reversion modes. To prevent the AFS — that's the Autopilot Flight System — from causing a flight envelope excursion, there is a system of command speed limiting and reversion modes. And an important point: AFS command limiting and reversion is independent of the stall warning and airspeed/Mach warning systems. So this is a separate layer of protection. Let's break down command speed limiting. The AFS provides speed, pitch, and thrust commands to avoid exceeding the following limit speeds: VMO/MMO — that's maximum operating speed and maximum operating Mach number; wing flap limiting speeds; landing gear speeds; and minimum speeds. The commanded speed can be equal to, but will not exceed, a limit speed. Now here's a subtlety. Speeds greater than VMO/MMO cannot be selected from the MCP — that's the Mode Control Panel, where the pilot sets the desired speed. However, speeds can be selected which exceed flap and gear limiting speeds, or that are less than the minimum flight speed. So the system allows you to select those, but then it steps in. What is minimum speed based on? It's based on an angle of attack, and it's approximately 1.3 VS for the current flap configuration — VS being the stall speed. It's sensed by the angle of attack vanes, one on either side of the forward fuselage. So here's the behaviour. If a speed greater than a placard speed — that's the published limit speed — or less than the minimum speed is selected, the AFS allows acceleration or deceleration to slightly short of the limit, then commands the limit speed. In other words, it lets you get close, but not past it. And when the commanded speed cannot be reached, the over-speed or under-speed limiting symbol appears in the MCP IAS/Mach display. IAS is indicated airspeed. That symbol tells the pilot the system is holding the limit. So the whole logic is proportional control up to a safe boundary, with a separate limiting system that catches you before you leave the envelope. That's the heart of the control laws.