I know that the plane must have two autopilots for flying approaches, but why have individual buttons for each autopilot? Wouldn't it be easier to have both autopilots work at the same time for the entire flight?
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4$\begingroup$ It isn't necessarily to have both autopilots engaged to fly an approach unless that approach is below Category I minimums, i.e. lower than a 200' ceiling and 1800' of visibility (RVR). Down to Cat I, you can fly approaches with a single autopilot all day long, unless specific rules require something else. $\endgroup$– Ralph J ♦Mar 23 at 16:00
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2$\begingroup$ It's usually one autopilot and two Flight Directors. The FD actually calculates the flight path and generates the commends on the primary flight display on its side, and the autopilot is just a dumb single channel machine doing what it's told by one of the FDs. The buttons usually say FD 1 and 2, not AP 1 and 2. At least with conventional mechanical/hydraulic flight controls. FBW is a whole different kettle of fish. That must be a panel from a plane with FBW? $\endgroup$– John KMar 23 at 22:04
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$\begingroup$ @JohnK Yes, this is from the A320. $\endgroup$– Boeing787Mar 23 at 23:11
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1$\begingroup$ That makes sense. In that case the autopilot(s) are mostly software. $\endgroup$– John KMar 24 at 2:01
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2 Answers
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Just like real-life pilots, you only want one flying the airplane. The other is running in the background and can take control when selected. When you select which A/P is active, that just controls which A/P is actually flying the a/c.
Depending on the system architecture, how monitoring and fault detection are designed, the system can have one or more reversion modes that can be implemented. These can be automatic or require crew action, depending on the system and phase of flight. For example, in cruise you could fly on one A/P when the other is failed. But in approach, both A/P would need to be active to fly a coupled approach.
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Actually many planes have 3 autopilots. One actually flies the plane and others check the one flying the plane. with 3 autopilots if the results from one, differs from the other two, it gets shut down and the plane continues with two. If there are only two autopilots and one gets different results from the other, both will shutdown and the pilot has to resume hand flying the plane.
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4$\begingroup$ In many aircraft, the self-monitoring and automatic reversion you describe only applies to autoland operations. In other phases of flight, including cruise, only one flight control computer is active and the pilots select which one. There is a very broad spectrum of sophistication across aircraft currently in service and the specifics vary from type to type. $\endgroup$– TypeIAMar 23 at 23:30