"No F/D if A/P on same source"

[nobody]

Question: What´s the reason behind this feature? Under normal operations you (almost) always use the FD... so why deactivate it? I don´t see the reason behind this option. 

[Hardy Heinlin]

Although this has been discussed a couple of times in the past 17 years, I still don't know either. I think it's an invention of United Airlines and they're probably the only company that implemented it.

Maybe it's supposed to function as a reminder. The two pilots should use different instrument sources whenever possible.

[nobody]

ok, thx!

[United744]

I think it is to do with monitoring of the various systems (particularly IRS for attitude/position drift problems).

There was an incident many years ago where an aircraft starting pitching/rolling in response to a failed attitude system.

By hiding the FD bars, I think it encourages use of the center autopilot, which has the following effect:

* Capt FD bars are on the left system
* FO FD bars are on the right system
* Aircraft flies and reacts to center system

There are visual indications as to what these systems are doing on the PFD/ND.

By having the center system flying the aircraft through the center autopilot, if it starts to deviate, you can see it by both Capt and FO having the same offsets/problems.

If only one side has a problem (Capt or FO), then it is likely that side is the fault; if both sides have the same problem, chances are the center system failed (otherwise there is no other way to see the center system is operating correctly without switching instrument sources).

If you're flying the left or right AP with everything selected to NORM, you have no way of checking center system operation without switching instrument sources.

[Vnav]

I think it is to do with monitoring of the various systems (particularly IRS for attitude/position drift problems).

There was an incident many years ago where an aircraft starting pitching/rolling in response to a failed attitude system.

By hiding the FD bars, I think it encourages use of the center autopilot, which has the following effect:

* Capt FD bars are on the left system
* FO FD bars are on the right system
* Aircraft flies and reacts to center system

There are visual indications as to what these systems are doing on the PFD/ND.

By having the center system flying the aircraft through the center autopilot, if it starts to deviate, you can see it by both Capt and FO having the same offsets/problems.

If only one side has a problem (Capt or FO), then it is likely that side is the fault; if both sides have the same problem, chances are the center system failed (otherwise there is no other way to see the center system is operating correctly without switching instrument sources).

If you're flying the left or right AP with everything selected to NORM, you have no way of checking center system operation without switching instrument sources.

Well, I was given exactly this very explanation during a transition course on the 757/767 - that is, crosscheck of the AFDS. Some of our planes have this configuration and they don't come from UAL. Apparently several other operators had it...

[emerydc8]

I haven't been able to verify this, but I was told once that an advantage of using the center autopilot is that if you lose hydraulics in system 2 (right stab trim and right autopilot) or system 3 (left stab trim and left autopilot), if you are using the center autopilot, the center FCC will automatically switch to the operating stab trim system. So, if you are using the center autopilot and lose hydraulic system 2, the FCC will automatically switch to system 3 for the ASTM.

Maybe someone more knowledgeable about the system can verify if this is true or not.