View Full Version : 777 C*U Behavior

21st Feb 2017, 09:26
I want to confirm an assertion that I have, to see if I understand the 777 FBW logic.

Suppose you're in level flight at a constant speed and you manually pitch the nose towards 10 degrees nose up. While at the same exact time adding just enough thrust (imagine super pilot) to keep the trend of the airspeed exactly still, no gain, no loss.

Would you then get C* out of the C*U system and have an Airbus style auto trim, without you needing to trim or do anything else?

21st Feb 2017, 10:11
The 777 is trim for speed so if the speed doesn't change it stays in trim.

galaxy flyer
21st Feb 2017, 16:11
True, but inevitably, thrust would reduce as it climbed reducing airspeed (IAS); then it would start to pitch down to regain trimmed airspeed.

21st Feb 2017, 17:54
GF: okay so imagine glideslope intercept altitude and transitioning to a 3 degrees decent on it... something more simple and static.

Would lowering the nose to roughly 2.5deg nose up and reducing thrust appropriately give you an airbus style auto trim or rather a trimmed configuration as you're already trimmed for VREF+5 (let's say you called for landing flaps just a bit before GS intercept and got it trimmed in time for the new speed)

In other words, when minimizing the difference between Trim Ref Speed - Current Speed to 0 do you get only the C* pitch maneuver.

21st Feb 2017, 17:55
If this pitch up maneuver were performed with the autothrottle engaged to hold speed, needed thrust would be applied (and continuously adjusted as required) to keep the target speed.

When the 777 was first introduced, this configuration of manual path control with the autothrottle engaged was not recommended. Operational experience, however, showed that the C*U system is quite good at balancing pitching moment disturbances from autothrottle engine adjustments. Now Boeing recommends using the autothrottle during manual path control.

An important thing to remember, however, is that with this combination of manual pitch control and autothrottle speed control there are actually two separate systems closing the loop on speed. C*U and the autothrottle are not in communication with each other so the speed targets within each are not identically matched. As a result, over a long period of time (many, many seconds to minutes depending on how close the two speed settings are) the airplane will end up in either an idle descent or a max power climb at the specified speed if left alone (no input on the column). The bottom line here is that autothrottle commanding speed with manual pitch control is not a substitute for autopilot.

22nd Feb 2017, 16:45
DownWindAbeam - I think you have the right idea.

C*U (the variable controlled by the C*U control law) is essentially C* (similar to Airbus) plus a gain times the speed error (current speed - reference speed).

C*U works well at compensating pitch and lift disturbances associated with turns (to +/- 30 degrees), flap changes, speedbrake adjustments, and thrust changes. The only time that pitch trim input is needed from the flight crew is when changing the desired speed.

When the autopilot is engaged the speed stability of C*U is disabled. Some autopilot modes provide speed targets - those are retained. When autopilot is disengaged the current speed is captured as the initial C*U reference speed so no need for pitch trim input at that point unless you want further speed change.

Your glide slope capture example is correct. If flying level at the desired approach speed prior to GS capture and possible configuration for landing (flaps / gear) there is no need for pitch trim input as a result of GS capture, configuration adjustment, or the associated thrust adjustments. It will behave just like C* as long as speed is maintained. Boeing philosophy is that if speed is not being managed, the crew awareness should be increased by having the airplane tend to pitch up or down as required to capture the reference speed. Turn on the autothrottle and you essentially have a pure C* airplane.