It's my observation, not backed up by a deeper understanding of the Hardware/Software, that in the Cruise phase using VNAV PATH, the B777 Auto Throttle has only limited authority above and below a pre-determined thrust required for the speed. There seems to be more authority when using Speed Intervention, but this may be subjective on my part.
I recently posted on another forum (Turbulence) a B777 event of mine where rapid loss of tailwind (+160 Kt to +80 Kt in 21 seconds) lead to Mmo exceedance in cruise. Prompt crew action to fully close the Thrust Levers and fully deploy the Spoilers could not prevent the Mmo exceedance. That was at the initial stage, but on 2 smaller 'decreasing tailwind' shears over the next 4 to 5 minutes, observed the Auto Thrust to move the Thrust Levers aft by about an inch, and no more, even though the Speed Trend Vector was still beyond Mmo, again requiring direct crew intervention.
Does anyone have a deeper understanding of the logic of the Auto Throttle system to back up my observation that the A/T SEEMS to have limited authority in Cruise? I hope so.
Changeing direction slightly, I enjoyed sharing dinner in Beijing recently with a British Airways B777 crew, and one made the observation that the Vmo at 330 KIAS was deliberately placed artificially low. I didn't have the opportunity to follow up his remark as they had to be on their way. It always seemed damned low to me, does anyone have any information on this, or was he just having a gripe? It always surprised me that Boeing simplified Vmo to a constant CAS, as opposed to variable CAS in line with a constant EAS as on some other Boeing aircraft.
Regards,
Old Smokey