View Full Version : B777 fly-by-wire

6th May 2004, 17:39
the B777 fly-by-wire system has a feature listed under 'Normal Control Mode' (pitch, roll and yaw) which is 'Autopilot Backdrive'. Can anybody tell me what is meant by this?

6th May 2004, 22:01

Might not be correct so don't hesitate to correct me!

I'll try and explain using the difference between the 777 and the Airbus FBW system.

In the airbus, if one pilot moves the sidestick, the other one doesn't move. Same for the rudder pedals.

The 777 is different in the sense that if you move one yoke, the other one will have the same deflection.

It's works as well with the autopilot ON. If a turn is executed by the 777's AP, the yoke will move accordingly (Autopilot Backdrive)whereas in the Airbus the sidestick won't.

But please please guys... Don't start a new Boeing/Airbus war!!!


gas path
6th May 2004, 23:51
The 777 has a 'normal' control column unlike the Airbus which uses a side stick and Boeing elected to have the column respond to autopilot inputs like a conventional cable controlled aircraft.

Position transducers change the pilots' manual commands of the controls to analog electrical signals. These signals go to four actuator control electronics (ACEs). The ACEs change the signals to digital format and send them to the primary flight computers (PFCs).

The PFCs calculate the flight control commands based on control laws and flight envelope protection functions. The control laws supply stability augmentation in the pitch and yaw axes and flight envelope protections in all three axes. The digital command signals from the PFCs go to the ACEs.

The ACEs change these command signals to analog format and send them to the power control units (PCUs). The ACEs and the PCUs form control loops which control the surfaces based on the PFCs commands.
The PCUs each contain a position feedback transducer.

When commanded, PCUs move the control surfaces. The position transducer sends a position feedback signal to the ACEs. The ACEs then stop the PCU command when the position feedback signal equals the commanded position.

Autopilot Operation

The PFCs receive autopilot commands from all three autopilot flight director computers (AFDCs). The PFCs use the autopilot commands in the same manner as the pilots' manual commands. In addition, the PFCs supply the backdrive signals to the backdrive actuators through the AFDCs. The backdrive actuators move the control wheels, control columns, and rudder pedals. The movement of the flight deck controls supplies visual indications to the flight crew.

PFCS Operation

The PFCS have three modes of operation: normal, secondary, and direct.

Normal mode operates when all the necessary data is available for the PFCs and the ACEs. All the control laws, protection functions, and the AFDCs operate.

When the PFCS detects the loss of important air and attitude data, the PFCS operation changes to secondary mode. The PFCs and the ACEs operate but the PFC control laws and protection functions downgrade. The autopilot cannot operate in secondary mode.

In direct mode, the PFCs are not used. The ACEs set the position of the control surfaces in direct response to analog pilot input.

Also, if an attempt is made to fly the aircraft outside of the normal flight envelope (in normal mode) a 'clunk' will be heard as the backdrive actuators attempt to fightback for envelope protection. It can be overridden but the effort required increases.

7th May 2004, 01:25
FBW Feel

Does anyone know the Stick Force per g for the Airbus and Boeing systems.

For the F16 side stick it is 3 pnds per g which means a one handed pull of 27 pnds to reach max g of 9.

For a yoke control I would guess it to be around the minimum design specs of around 11 pnds per g for heavies.

Also is there any intentional break out force around centre to prevent spurious inputs?

7th May 2004, 05:39
Great answer, thank you gas path!

7th May 2004, 08:26
Just a small point DII, rudder pedals on Airbus FBW types are linked, move one lot and the others move too.

7th May 2004, 08:48
Thanks for the info VC813.

Guess it makes sense.


gas path
7th May 2004, 08:54
The feel and centering mechanism for the B777 for both the wheel and column is a simple cam and roller mechanism.
For the aileron it is something in the order of 2kgs at breakout to 6 kgs at full travel.
For the column it's 3kgs to 19kgs at full travel low speed and 19kgs to 90kgs at high speed.
A bit of a simple description but I hope it helps!

12th May 2004, 17:34
gas path,
just another question came up yet. Beside the stability augmentation and flight envelope protection which I guess includes stall, overspeed, bank angle and TAC is there also a load factor limitation to protect the pilot from overstressing the aircraft?

gas path
13th May 2004, 09:01
In short no!
The aircraft can be flown outside of the flight envelope but it will let you know, besides the aural 'bank angle' for instance the backdrive actuators engage (with an audible thump!) and significantly increase the forces.
I think Mr.Boeings idea was to let the pilot have the last say!