Thrust increase when V/S is selected - Boeing

[proxus]

Hi.

Maybe I'm to lazy but I can't remember ever seeing an explanation for this but what is the reason behind the thrust increase when vertical speed is selected. It is like that at least on the B757. Can't remember if the Airbus autothrust sys. does this as well.

[Intruder]

If the airplane is stable and level, or in a steady climb or descent, there should be no immediate thrust change. Depending on the previous pitch mode selected, the thrust may gradually increase or decrease to maintain the selected speed and vertical speed.

[proxus]

Sorry I need to be maybe a little bit more specific.
Let's say your're descending in FLCH and get a request to increase your descent or for example, I'm above the glideslope on approach and select 1500 FPM to increase descent, speed is good as soon as I select V/S the autothrottle adds power from maybe 1.00 EPR to approximately 1.05 EPR. Happens every time as it seems to be programmed into the system. Just a similar increase as you get when A/I is selected. Just wondering why..

[slam525i]

I'm not a big-airplane driver, but I seem to recall reading somewhere that it has something to do with reducing spool-up times, so the lowest thrust commanded by the A/T is higher than idle.

[SMOC]

Yep 744 bumps up with V/S selected. I'll go along with the above statement re spool up. Seems odd if it can catch an altitude with idle thrust yet needs to spool up with V/S though.

[aussiepride]

Hey correct me if im wrong but if the A/C was engaged in VNAV or FLCH with the A/T armed then SPD mode automatically engages meaning that when V/S is selected the A/T will adjust engine thrust to maintain the speed the aircraft was doing the moment you pushed V/S.

[stilton]

Not sure about that Apride but on the rare occasions I use VS in the descent, for example on a high IAS descent having just switched over from FLCHG (not using VNAV for whatever reason) approaching whatever altitude I have to slow to 250 KIAS at, but also have a crossing restriction to meet and am just barely going to meet this restriction.


The last thing I want is any additional power above idle at this point or to go above profile. It takes 3000 feet to decelerate from 340 KIAS to 250 at idle with no speed brake in a B757 for example in FLCHG so I will disconnect the autothrottles to avoid them being 'bumped up' in VS, make sure they are at idle then cancel the warning but remain in VS slowly reducing it while decelerating to 250kts, then increasing VS to hold this speed until stable in the descent then reselecting FLCHG.


A bit long winded and belabouring the obvious perhaps but I do make sure to tell the FO what I am doing.

[Tee Emm]

It takes 3000 feet to decelerate from 340 KIAS to 250 at idle with no speed brake in a B757


Surely it is quicker to decelerate in level flight? The B737 FCTM states;"Losing airspeed can be difficult and may require a level flight segment."

[stilton]

It certainly is but that whole time you decelerate in level flight you are going further and further above the flight path required to make a crossing restriction (which is what I was referencing) and you may not be able to meet
this restriction then.

[proxus]

The example I was thinking specifically about and can be used as an example.

We were descending in FLCH, can't remember the speed but let's say it was 280 kts. In the descent, and long before we reached our cleared level, we were given further descent with a minimum of 2500 fpm. The plane has now been descending on idle power in throttle hold. When V/S is selected A/T wakes up and advances the thrust to c.a. 1.05 EPR. At the same time, as we are doing a good rate down, the speed of course starts to increase above bug speed (280 Kts).

This is just one of many examples when the V/S mode increases speed, and I'm not talking about when the A/T increases thrust to maintain speed. As in the situtation above we were already above the bugged speed so no need for extra thrust.

Just wondering about the system logic behind it.