757 auto throttle logic
 

One thing that I've wondered about for awhile:

Descending using FLCH, the throttle goes to idle. In the descend, select VS mode and speed bugged down, the auto throttle goes from fully idle to adding a little power (though still less than idle with engine AI on).

Anyone know the logic behind this? A quick scroll through the FCOM didn't provide the answer...

20 years on the airplane and I never knew why it does this. Boeing produced the 757 for 20 years and in a way still does the 767-300F. They never did any upgrades to the Auto-flight system (A/P, A/T). The 757 Autothrottle is a pretty poor system.

To improve the engine acceleration response time.

In FLCH, AT it is programmed to give either Climb or Idle and it does not expect to change until approaching the level set.
In VS + speed mode you might change the rate unexpectedly so the engines are set to respond quicker (higher idle).

Mr Ocean,
Based upon what theory do you make the statement "757 autothrottle is a poor system"?.Ive been flying it for 14,000 hours on type now (still do) and havent had any issues with it.Peter.

It's a matter of opinion.

Opinion: I don't like the 757 autothrottle system.
Factual statement (i.e, not opinion): The 757 autothrottle is a poor system.

The second statement deserves some kind of back up information such as, why is it bad? What does it do poorly that other systems do well? What is an example of a good system? Not just "that's my opinion."

Duh.
It does the job.
Spent a few years on the 757 and never paid much attention to the A/T.
Guess it works the way it was designed to do.
Certified for CAT III approaches.
What else do you want it to do?

I'm not saying the autothrottle is poor, I'm trying to explain to another poster that that kind of statement is not "opinion".

Ok, not far behind you there with about 11-12K hrs on it. It is poor in the respect that it is very slow reacting to airspeed changes not least in gusty wind conditions and needs a lot of over-riding in the wx conditions which are the norm in my part of the world 9 months out of the year. Even the A/T of the B737 Classics did a better job.

That's due in part to being common with the 767 A/T, which had to deal with the JT9D. The JT9D-7R4 on the 767 was prone to surge/stall with rapid throttle movements at altitude. Although they made engine changes to address this, they also slowed down the A/T rate at cruise.

Thanks tdracer. Always nice to know the "why's". Still don't understand while Boeing never offered an upgrade to the system, our latest757s were built in 2001/2002 IIRC and the JT9D was long gone out of the production system at that time.

True, but the avionic software needs to be downward compatible - so they may well have updated the s/w but it still had to work on the JT9D.

Although the 757/767 series had a major FMS change with the advent of the Pegasus, the fundamental aspects are a design circa 1980.

757 and 767 are the first of the "4th Generation" of jet transports, which the latest airbus and Boeing models all belong to as well.......but im sure you already know this fact.

Correct.

Not even sure what you mean by that? So the A350/B787 are the same generation as the 757/767? Doubtful. What were the 3 earlier generations?

The Curtis Condor, the Bristol Brabazon, and the Caravelle.

In that order.

Goldenrivett Actually FLCH is more clever than that. It utilises a time target of 120 secs to achieve required level change and adjust thrust to so do. i.e. increases/reduces sufficient thrust as required-just as a pilot would. If it cannot achieve that then it sets CLB or IDLE as required. This is particularly evident when being stepped down 500 ft or so by ATC during intermediate approach. Of course you simply manually override if its not doing it quickly enough.

Mind you this was a 100 or so years ago (80s-90s) and reading the various comments above, its just possible that's changed but I think unlikely.