Which Autopilot is engaged
Join Date: Mar 2009
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I can't comment on the 777 specifically but when multiple systems are fitted to aircraft airlines often adopt a policy of alternating usage. i.e Unit 1 on outbound flights, Unit 2 on return flights. Or Capt flying Unit 1 and F/O Unit 2.
Does this help? Cheers
Does this help? Cheers
Join Date: Feb 2010
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Can you clarify what you are asking?
You should think of it as having 1 "autopilot" or auto flight control system, with 3 processors. If one or more fails, you'll have some annunciations to that effect, but it isn't important which individual processors have failed. They are identical. Nor is it necessary to know which one is active. Typically, computer systems like these will "power on self test" and then select the channel or processor that wasn't used previously.
Think of it like this: If you have a lamp with three light bulbs and one goes out, does it matter which one went out? It just isn't as bright, and you know you need to change a light bulb.
You should think of it as having 1 "autopilot" or auto flight control system, with 3 processors. If one or more fails, you'll have some annunciations to that effect, but it isn't important which individual processors have failed. They are identical. Nor is it necessary to know which one is active. Typically, computer systems like these will "power on self test" and then select the channel or processor that wasn't used previously.
Think of it like this: If you have a lamp with three light bulbs and one goes out, does it matter which one went out? It just isn't as bright, and you know you need to change a light bulb.
Join Date: May 1999
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Grounded27 is spot on. Whether you engage the A/P using the left or right hand side button merely affects which altimeter reference it uses and that's annunciated by the little L or R under the baro reference on the PFD.
Plus, whilst there are three channels, there are only two drive systems. The centre system is purely a monitoring system.
Join Date: Jul 2009
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Refer to the B777 Systems Description of the Automatic Flight Chapter
when the autopilot light is illuminated. it means all operating auto pilots are engaged. (L+C+R) assuming all are operative.
when the autopilot light is illuminated. it means all operating auto pilots are engaged. (L+C+R) assuming all are operative.
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As I recall, in normal circumstances, all 3 are engaged in cruise, but one is the master and the other two are slaves. The centre does not have any backdrive capability for the controls so can never be master, as someone else pointed out. I heard a rumour the right becomes the master the first time it is engaged after power up, and then it alternates each time it is re-engaged, but you'll never be able to tell in the flight deck.
