777 Speedbrake Deployment
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.