View Full Version : B777 single button A/P?

4th Jun 2002, 15:11
Just been thinking, what is the idea behind having only a single button to engage the A/P on B777, rather than the three found on the B744?

The B777 must still have 3 A/Ps like the B744 but why remove the other two buttons, its not as if they need to save space on the MCP.

gas path
4th Jun 2002, 21:35
Simplicity!! Pushing either of the A/P engage switches will (usually) engage all three AFDCs, left right and centre. The AFDC selection changes each time the autopilot engages - left or right the centre AFDC is used for monitoring and comparison, also the centre is not connected to any of the backdrive actuators.
Pushing the A/P disconnect switch on either column will of course disconnect all 3 AFDCs as will the disconnect bar on the MCP.
The MCP disconnect bar has three toggle switches underneath one for each AFDC.

max motor
4th Jun 2002, 22:16
Just to add a point to what gas path has said:

The 777 fly-by-wire system requires triple channel primary flight computers and servos to be engaged at all times (regardless of autopilot engagement). Prior to autopilot engagement the PFCs respond to signals from the yoke/pedals. This effectively means that the 777 is flying with triple channel autopilot computers and servos engaged even before the autopilot engage button is pressed. When the pilot engages the autopilot the yoke/pedal signals are replaced by the autopilot command signals. Since triple channel PFCs and servos are already engaged triple channel command signals are required to drive/monitor them. So one button is used to engage all three autopilots command channels simultaneously.

This is fundamentally different to the way typical non fly-by-wire aircraft like the 747 behave. In normal operation only a single autopilot is engaged. Triple channel is only used when increased redundancy is required for automatic approach/landing.

10th Jun 2002, 06:26
Single button??!?. Our 777s have two!!

10th Jun 2002, 20:05
Jetboy..thanks a lot! I looked on my MCP too..and I found another one! I never reach over there to the right to push it though...forgot it was there. These Boeings are amazing. The guys thought of everything!:)

11th Jun 2002, 08:47
Well, not quite EVERYTHING......they forgot the sidesticks!!!!

11th Jun 2002, 11:28
"Well, not quite EVERYTHING......they forgot the sidesticks!!!!"

No... I think I saw them in the cargo bays... where they belong :D