777 auto throttle off in tcas Ra

[mothergoose1]

What’s the logic behind turning autothrottle off in tcas Ra on 777.?Also why does full rudder not disconnect with ap in cruise.
whereas pitch and roll it disconnects and further it only disconnects when in rolllout flare land 3 etc mode armed .Sorry those are two questions

[rudestuff]

If it's anything like other Boeings - A/P and A/T are simply to slow to initiate a climb or descent, as the software is coded for comfort. If you want to it to do something immediately you have to do it manually.

[FlyingStone]

If 777 is anything like the 737, then the A/T will maintain climb thrust during the climb and idle thrust during descent. If you disconnect the autopilot alone during climb with TCAS RA, and initiate descent, the A/T would probably still maintain climb thrust and the airplane would start to accelerate... a lot. Same goes for initiating climb with idle thrust. Probably not a good idea.

[wiggy]

If it's anything like other Boeings - A/P and A/T are simply to slow to initiate a climb or descent, as the software is coded for comfort. If you want to it to do something immediately you have to do it manually.

(Warning, no longer rated)…I thought one reason for knocking out the A/T was to give commonality with some of the other time critical maneuvers, another reason was to avoid any danger of falling into the FLCH trap if the RA happens during a descent being flown using FLCH…

Flyingstone..from what you describe it doesn’t sound like the A/T functions in same, 737/777.

[vilas]

In 777 which FD mode is used in TCAS? Is the AP disconnected?

[Smokey Lomcevak]

@ Vilas- none. However the procedure does not call for them to be switched off - at odds with the 320 The pitch target is that required to satisfy the RA. On the 777 we have the benefit of the red trapezoid on the attitude display, in addition to the red band on the VS tape, which again is a difference to Airbus.

To the OP: the reason is to provide some means for controlling/protecting aircraft speed. The autothrottle may be in a fixed thrust mode like Idle/Hold/ Thr Ref that would be inappropriate. If the question is why we don’t switch the FD’s off, thus forcing the autothrottle into speed mode, I think that comes down to Boeing and Airbus’s interpretations of what the easiest/simplest and least risk solution looks like. In addition, there may (!) be situations where simply turning the FD’s off would not force autothrottle into speed mode, as alluded to above. It also helps to further avoid the slightly awkward scenario where one pilot turns his/her own FD off, meanwhile the other does not. Given - this should not happen, but, speaking frankly, it does.

[BraceBrace]

QRH Traffic avoidance maneuver
TCAS RA: if maneuvering is needed, disengage autopilot, disengage autothrottle. Smoothly adjust pitch and thrust to satisfy the RA command.

In pitch you DISREGARD COMPLETELY the flight director. You focus on the RA command. The autothrottle engaged would react in accordance with APFD modes. You DON'T want that. That's the whole point.

(I'll leave the door a little bit open if you wan't to discuss lateral navigation... :-))

[FullWings]

As others have said, it’s because of the possibility of being in a mode that doesn’t have speed protection. I am led to understand that there is a software update coming that means the autothrottle (if armed) will always able to wake up in a slow/fast situation, avoiding the stall crash die scenario. Whether this will change the drill IDK...

[rudestuff]

I'm pretty sure it's because in level flight to initiate a climb or descent generally takes ages: It involves twiddling a knob, pushing a button then waiting at least 0.2NM for the airplane logic to initiate a climb or descent, moving the levers nice and slowly. Then there's the possibility of Alt-capping again just when you don't want it to. All of these things are avoided by disconnecting and flying manually.

[Commander Taco]

(Warning, no longer rated)…I thought one reason for knocking out the A/T was to give commonality with some of the other time critical maneuvers, another reason was to avoid any danger of falling into the FLCH trap if the RA happens during a descent being flown using FLCH…

Wiggy,

That was precisely the philosophy at my carrier.