Boeing 777 flight controls

[seagull967]

Watching the below video, I am left wondering. My understanding is at this stage, in normal law, the pilot command would go to the ACEs, then to the PFCs, then back to the ACEs to command the elevator position. In this case there are very rapid movements, but would not the elevator actually be responding to the PFCs, with the pilot commands being, I would believe, theta-dot, at this stage? Video here. https://youtu.be/EuVlbVr_jrs?si=qRdGtGTNjFat7x-N

[FlyingRoland]

When I did my transition from captain 747-400 to 777 the most interesting difference I noted was aileron control. On the 747-400 on finals you could move the aileron control wheel quickly from neutral to full left, then to full right and back to neutral again without major roll resulting from this input. On the 777 however a similar roll input would flip the whole aircraft from 30 degree left to 30 degree right bank, setting the stage for overcontrolling the roll axis. The only way to stop this is just holding the controls steady neutral for a couple of seconds. Why this happens is the question but my guess is fly by wire sensitivity and larger deflection of the control surfaces compared to the 747 design.

[BraceBrace]

I would believe, theta-dot, at this stage? Video here. https://youtu.be/EuVlbVr_jrs?si=qRdGtGTNjFat7x-N

What is theta-dot? Aren't you confusing Airbus and Boeing? Fly-by-wire laws on a Boeing do not change during the flare maneuver and you stay in "normal".

For any fly-by-wire aircraft it is very dangerous to look at flight controls deflections and draw conclusions on pilot inputs, as the fly-by-wire will make many corrections to make the aircraft do what it "thinks" the pilot wants to do. In general you have to think of "path" more than actual corrections when making inputs to the control yoke.

However, (I was told this during training long time ago) and this is very true for the 777 and cause of many PIO especially in the roll during the flare: once yoke deflections become "large", the FBW system thinks the pilot really needs the aircraft to move quickly and will react in a much more "direct" way, hence much bigger aircraft reaction to stick inputs. As FlyingRoland mentions, in turbulent weather, when the pilot makes too big corrections, the FBW will also kick in hard to make the aircraft roll quickly in the desired direction. There are many videos out there of PIO on the 777 during the flare in gusty weather.

In this specific video, if the elevator is deflecting downwards that much, it is indeed probably an overcorrection of the pilot. And yes, the best is to simply let go of the control yoke (if correctly trimmed).

[Fursty Ferret]

Fly-by-wire laws on a Boeing do not change during the flare maneuver and you stay in "normal".

This isn’t strictly true. They do change. The pitch law includes landing flare compensation, in order to preserve desirable handling characteristics during the flare. Any kind of C* law is challenging to land without a nose-down bias (Airbus used to introduce a gradual pitch down command during the flare, but this was removed from the Neo - presumably due to the natural pitch-power couple from the larger engines).

My understanding is that the 777 is effectively direct wheel-to-aileron control, moderated by the PFCs for flight envelope protection. 787 is a more advanced roll-rate command akin to the Airbus.

[seagull967]

What is theta-dot? Aren't you confusing Airbus and Boeing? Fly-by-wire laws on a Boeing do not change during the flare maneuver and you stay in "normal".

The flare modification was already responded to. Theta-dot is a pitch rate control, as opposed to the g-rate at higher speeds. C*, yes, same as Airbus pretty much in that regard!

However, (I was told this during training long time ago) and this is very true for the 777 and cause of many PIO especially in the roll during the flare: once yoke deflections become "large", the FBW system thinks the pilot really needs the aircraft to move quickly and will react in a much more "direct" way, hence much bigger aircraft reaction to stick inputs. As FlyingRoland mentions, in turbulent weather, when the pilot makes too big corrections, the FBW will also kick in hard to make the aircraft roll quickly in the desired direction. There are many videos out there of PIO on the 777 during the flare in gusty weather.

In this specific video, if the elevator is deflecting downwards that much, it is indeed probably an overcorrection of the pilot. And yes, the best is to simply let go of the control yoke (if correctly trimmed).

This is what I am trying to find out, but have yet to confirm. Nothing published says this!

[seagull967]

This isn’t strictly true. They do change. The pitch law includes landing flare compensation, in order to preserve desirable handling characteristics during the flare. Any kind of C* law is challenging to land without a nose-down bias (Airbus used to introduce a gradual pitch down command during the flare, but this was removed from the Neo - presumably due to the natural pitch-power couple from the larger engines).

My understanding is that the 777 is effectively direct wheel-to-aileron control, moderated by the PFCs for flight envelope protection. 787 is a more advanced roll-rate command akin to the Airbus.

That's interesting. The 777 is, as you state (to my knowledge), but not familar with the 787.

[BraceBrace]

This isn’t strictly true. They do change. The pitch law includes landing flare compensation, in order to preserve desirable handling characteristics during the flare.

The question refered to the ’architecture’. Even though the laws can change, I’m not aware the signal path changes (ACEs are not ruled out afaik). But then again, fcoms are mainly ’user interface manuals’, not a technical publication.

[Sim25]

The question refered to the ’architecture’. Even though the laws can change, I’m not aware the signal path changes (ACEs are not ruled out afaik). But then again, fcoms are mainly ’user interface manuals’, not a technical publication.
ACEs can't be ruled out, as they provide the interface to the hydraulic servo controls and the interface to the yoke.
For further reference/reading:
http://www2.coe.pku.edu.cn/tpic/20119263710178.pdf

[FullWings]

In this specific video, if the elevator is deflecting downwards that much, it is indeed probably an overcorrection of the pilot. And yes, the best is to simply let go of the control yoke (if correctly trimmed).
I’d agree with that as a current 777 pilot with my fair share of less than perfect landings.

Having flown a variety of aircraft for work and pleasure, I would say that the FBW on the 777 leans very much towards a facsimile of how a non-FBW aircraft would behave on landing, given the same control inputs. It feels “natural”, like you have control over all three axes without noticeable contribution from the electronics.

I find it fascinating to watch people flying the takeoff, approach and landing using sometimes quite different techniques. One school seems to favour constant jabbing at the controls, while the other more deliberate application with periods of inactivity. “Low gain” seems to get better results and you are less likely to end up in PIO territory, which actually applies to most aircraft...

[seagull967]

“Low gain” seems to get better results and you are less likely to end up in PIO territory, which actually applies to most aircraft...

Low gain is always the better choice. High gain is a great way to flesh out if there is any PIO tendencies!