777

[linksys]

Dear All,

Can anyone out there, explain why B777 autopilot
is so sluggish in maintaining speed in CRZ.?

Any bump ar turbulence will see the A/c going for the
'barber-pole'.
Of course we can leave and let the a/pilot recover.
Not before it generates a FDAP incident.

It is worse on the decend. It goes all the way into the
'barber-pole' ringing bells before a/pilot, positively
intiates a pull-up.

Curious.:cool:

[Vorsicht]

It has to do with the amount of 'G' the autopilot is allowed to apply. We had quite a bit of trouble with this a few years back, particularly when stowing the speedbrake on descent. Boeing introduced a mod that increased the amount of 'G' authority to the A/P, which improved the problem. It is still sluggish, but better than it used to be.

In cruise is a different matter. That is simply related to the autothrust.

[FunctionedSatis]

I have herd it metioned by pilots at my last airline that the autothrottle creeps the a/c to overspeed in cruise, bit of high speed buffet now and then. I actually watched it during a fligh back from barbadoes once and the captain eplained they just manually pull it back everso slighly. The servo loop controll in the autothrottle system dont seem to be responsive enough.

[contact-landing]

It's not only on the B777, 75 has this as well. Don't know about other types. Red somewhere in the book or CBT that it is an autothrottle software thing : when the speed is below the bug the throttle corrects faster then with the same speed deviation above the bug.
It helps also if you select descent, descent now +exe in the CDU just a little bit before the VNAV TOD. Instead of going a bit over this TOD and then "diving" to pick up the VNAVPTH and thus increasing speed the transition is smoother and the speed closer to the bug.

[BOAC]

In my limited Boeing experience (737 various) it is a 'typical' Boeing auto-throttle ie it does not react well in turbulence and is rubbish at speed control near the BP.

I was once told the turbulence symptom is due to the input from the alpha-vane which bounces around in turb, and the poor speed in descent is due to excessive 'smoothing' of the system to avoid 'chasing'.

Best to use CWS or manual near the BP in my experience.

[linksys]

THanks for the info.

Looks like , built in flaws are there
to remind us to be pilots.

I have also seen a 744F 'misbehave'
on the decend.
Probably because the landing weight was
299T.

Vnav decend, took the a/c deep into
'barber-pole' before a/p decided to
pitch up.
GOT a FDAP inquiry for that incident.

:ugh:

[BOAC]

As I said, 'too smooth'! It is also a well-known Boeing 'oddity' that when you call for a speed increase or decrease via the MCP, the autopilot INITIALLY, almost imperceptably, moves the nose the wrong way! It maybe, of course, that the software is written 180 deg out and it is only the ultimate (incorrect) speed change that puts the pitch right. IIRC, most of the early autopilots had an inbuilt 'initial' pitch input which corresponded to the demand. E.G. I seem to remember the Lightning a/p pitched 3 deg down at GP capture BEFORE the GP deviation signal took effect and 'flew' the GP?

[Old Smokey]

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