B777 response to Speedbrakes

[FCeng84]

Part of the discussion during a recent thread (B777 airspeed in turbulence) touched on concerns regarding the pitch response of the 777 resulting from speedbrake extensions and/or retractions. The nature of that discussion also pointed toward differences with and without the autopilot engaged. I assume that there would also be differences depending on the autopilot mode selected. As this topic does not fit with the title of the earlier thread, I am raising it here as its own thread. My apollogies for rehashing part of the earlier discussion, but I am very curious with regard to any undesirable response characteristics that B777 crews may be experiencing when extending or retracting the speedbrakes with or without the autopilot engaged. I am very familiar with the associated controls logic and thus interested in learning from the past so that the future is even better.

Regards,

FCEng84

[Aileron Drag]

Regarding my post about the pitch response, I have been thinking that there were two problems. This situation only occurred when we were held high, and way above the ideal Vnav profile. FLCH was selected, and a grab made for the speedbrakes.

Problem one was minor, and avoidable. There was a rapid pitch down reaction to speedbrake deployment, which of course could have been avoided with a less heavy-handed approach.

Problem two was, I think, more of an issue. The aircraft would pitch down, but the autopilot seemed to have no selected speed input. That is, the aircraft would pitch down and ignore the selected speed in FLCH. It would bust the barber's pole quite happily - I wondered just how far the A/P would allow the airspeed to increase. On the occasions I experienced this phenomenon I had the impression that the A/P had lost the plot, and we would have ended up beyond Vmo/Mmo before the computer woke up. I hope that's not the case, but we had to disengage the A/P in FLCH in order to regain control of the speed.

On reflection, that is not strictly to do with speedbrake pitch response, but more with a remarkably sluggish response from the A/P.

I must say that this was all a few years ago, but of the types I flew in my career it was the the 777 I trusted least, on autopilot.

[STBYRUD]

Interesting, I thought the 737NG FCCs were slow, but what you describe sounds identical to what I've seen on the baby boeing. I doubt the 777 autoflight system is as basic as the 737 one, but just for comparison the 737 autopilot has no idea what the airplanes configuration is for the purposes of giving pitch commands fo LVL CHG or VNAV SPD, just that the control response is proportional to the deviation from the target speed - some colleagues are clever enough to set a speed 40 knots higher than the present speed (after an intermediate level off for example), hit LVL CHG and yank the speedbrake out - felt like the navkit was going to go airborne on the flight deck, but we (barely by the way it felt) maintained positive Gs ;)

[DJ77]

Never had any problem using the 777 speedbrakes in flight. Always used them smoothly though. They were much more efficient than those of other types I flew on.

Never felt betrayed by the autopilot neither.

However during sim conversion sessions I used to demonstrate what would happen if the speedbrakes were quickly retracted in a descent close to VMO/MMO. Noisy experience! The reason, I think, is the autopilot is not wired to change its 0.2g vertical acceleration limit unless, perhaps, a substantial enough overspeed has occured.

[misd-agin]

Reminds me of the carpenter vs. tools discussion.

[777AV8R]

With over 8000 hrs on this airplane and having worked in numerous environments with it, I can only say that the speedbrake/overspeed issue only surfaced while in VNAV, fast and in the process of stowing the speedbrakes. As I previously stated, the autopilot calculates a profile to intercept the path and once speedbrakes are retracted, VNAV works to regain the vertical angle, which means increasing speed. The other variable is the fact that this airplane is so clean and once the drag is gone, the airplane wants to accelerate.

I have come to trust this autopilot like no other. I have hit turbulence so strong, crossing the North Pacific, that our flight bags came out of their stowage areas on our sides. The autopilot flew through it like it was supposed to.

One area that does 'irk' me with the speedbrakes, is the initial extension. It seems that the panels seem to 'pop' up from their stowed position and one can feel a very definite-uncomfortable pitch change. I've never really gotten a 'smooth' extension from a fully retracted speedbrake.

How on that, over?

[FCeng84]

All,

Thanks for your responses to this thread. The overspeed issue seems to me to be related to the fact that the speedbrake response (while rate limited and compensated to avoid a large lift bump) is much faster than the autothrottle response. Stowing the speedbrakes over the course a few seconds significantly reduces drag, but it will take much longer for the autothrottle to respond and reduce thrust accordingly.

777AV8R's comment on difficultly with smooth speedbrake deployment, is new to me. Does "pitch response" refer to an abrupt change in pitch attitude or do you experience an objectionable bump in normal load factor? When speedbrakes are deployed the pitch attitude is intentionally increased to avoid a sharp change in lift. The compensation provides two to three degrees of pitch attitude increase as the speedbrakes are extended from stowed to full travel. That pitch change means that the cockpit rises and the aft galley descends as speedbrakes are deployed. Rate limiting speedbrake position changes to no faster than a couple of seconds stop-to-stop keeps the cockpit / aft galley response from being objectionable.

Another consideration is the peculiarity with wing spoilers whereby they tend to be quite non-linear in their response during the first few degrees of deployment. Small spoiler angles can energize the flow over the wing causing it to stay attached further aft than normal. In that event, net lift is actually increased. As spoilers are deployed further, they become effective at dumping lift. This characteristic can be seen when looking closely at flight data for a slow extension of the speedbrakes in calm air, but I am not aware of flight crews noticing this effect - at least not finding an objectionable response.

One more feature of speedbrakes on the 777 is that the associated surfaces do not deploy during extension until the speedbrake lever has move slightly past the armed detent. There is no tactile cue to the flight deck crew as to when during motion of the level the surfaces begin to move. This may contribute to difficulty with deploying speedbrakes very smoothly.

[777AV8R]

Thanks for the great input on this. Your remarks regarding activation begins after the detent might have something to do with the apparent bump of the spoilers. This could be my technique and not a 'fault' at all. I'll have to observe what happens next time, although this could be a while as I am now focused on the B787 project.

[stilton]

Interesting. The 757 seems to have many of the same characteristics.


And it is always tricky to extend the spoilers smoothly without an initial 'lurch'


If you are very smooth and deliberate on extension you can deploy them smoothly but it takes a little bit of care to do so !