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Autopilot and Manual Pilot

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Autopilot and Manual Pilot

Old 10th Sep 2009, 14:00
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QJB
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Autopilot and Manual Pilot

Hi everyone,

Just a query that came into my head the other day. On the big jets Boeing/Airbus. Do the pilots control columns move in accordance with autopilot inputs. I seem to remember that for airbus the auto throttle does not move the actual throttles but it does for boeings, but I was mainly thinking about the actual control column.

For instance a 777 has an engine failure with autopilot in command. For whatever reason the pilot does not take control. Autopilot puts in as much aileron as it can but still the plane rolls, pilot disconnects and the plane suddenly rolls sharply. I'm sure this specific example wouldn't happen but for arguments sake, is the autopilot system set up in such a way that the aircraft can surprise the pilot on disconnect.

Cheers
J
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Old 10th Sep 2009, 14:55
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I fly Boeing 757s.

When the autopilot is engaged, the control column moves in exactly the same way as it would if we were putting in the inputs manually. The thrust levers also move in exactly the same way when the autothrottle is engaged. On the Airbus the sidestick controller doesn't move at all with the autopilot engaged. Neither do the thrust levers.

When the autopilot is disengaged in the B757 it will give the aircraft to the pilot in a nice trimmed state (assuming it wasn't working hard to trim the aircraft upon disengagement, in which case you'd get a partially trimmed aircraft). A similar thing happens when the autothrottle is disconnected (99% of the time at the same time as the autopilot ) - the thrust levers will remain where they were on disconnect. This is why I always give the engine gauges a quick glance before I disconnect the autothrottle to make sure the power is about right for the phase of flight I'm in or else the speed will probably start to deviate away from what I want.

The autopilot can't control the rudder (unless we're in the latter stages of an autoland) so it only has the ailerons/roll control spoilers and elevators (plus the trimming tailplane) available for flying the aircraft.
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Old 10th Sep 2009, 15:07
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@QJB

You are right about airbus and auto thrust. The sidestick doesn't follow the autopilot command either. Yes, on A300/310 the control wheel/column follows the AP inputs.
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Old 10th Sep 2009, 15:58
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QJB

On the 777 the control wheels and thrust levers move to replicate what the autopilot/authrottle is doing is doing...as for your specific example of an engine failure the 777 also has a clever little feature ("TAC") which will in most cases apply rudder to at least partially compensate for an engine failure. In that case the rudder pedals do indeed move to reflect what TAC is demanding.
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Old 10th Sep 2009, 16:07
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In the 747 (all versions) the control column moves with the controls. The rudder does not respond automatically to an engine failure.
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Old 10th Sep 2009, 16:34
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QJ - on the 737, yes, it can 'surprise' in roll. Unlikely in pitch for the reasons stated above for the 757 (as long as it remains within the a/p trim capability) but any aileron input applied by the a/p 'disappears' completely at disconnect as it is not 'trimmed' and significant roll divergence can occur. Another cause would be 'un-noticed' lateral fuel imbalance which severely caught a colleague of mine a while back.

Again no rudder input (on current 737 a/c) although I believe it may now be available as a 'customer extra'.
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Old 10th Sep 2009, 16:42
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JUst a silly question..does the Airbus throttle stay put even if speed is selected instead of thrust. IF thats the case, when you take over...dont the engines suddenly increase or decrease power to meet the current throttle input? Ive always wondered....
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Old 10th Sep 2009, 17:28
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dont the engines suddenly increase or decrease power to meet the current throttle input?
Once u are airborne and selectet climb power, thrust levers remain in the climb notch until you will use manual thrust. Before disconnecting A/THR bring the levers to the donuts to synchronize otherwise it's getting loud
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Old 10th Sep 2009, 18:08
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Once u are airborne and selectet climb power, thrust levers remain in the climb notch until you will use manual thrust. Before disconnecting A/THR bring the levers to the donuts to synchronize otherwise it's getting loud
Ahhh i see! so u have to manually synchronise before disconnecting! the doughnuts are those little white circles representing throttle position on the upper EICAS right? Thanks Hetfield!
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Old 10th Sep 2009, 18:10
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Again no rudder input (on current 737 a/c) although I believe it may now be available as a 'customer extra'.
It is an option. Fail operational autoland requires autopilot rudder input. However it is limited to the latter stages of the approach and will drop out in the go-around when another roll mode is selected, which can surprise you the same as with the aileron in the engine-failure case as the rudder input is not trimmed, however rudder pedal displacement is there and you know whats going to happen.
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Old 11th Sep 2009, 04:06
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yes denti...

...and in the 737 the aileron displacement is also on display. I'm assuming the control wheel is firmly in-hand prior to a/p disengage selection.

...and for these reasons the prohibition of using aileron trim with the autopilot engaged!

Cheers...FD
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Old 13th Sep 2009, 17:30
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On the Airbus, Side Stick and Thrust Levers only move with a force is applied upon them. Autopilot on, nothing moves.
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Old 14th Sep 2009, 12:12
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Fil
 
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The autopilot can't control the rudder (unless we're in the latter stages of an autoland)
True for a Boeing but an Airbus can control the rudder with the Autopilot during all phases of flight.
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