PPRuNe Forums - Boeing (787) speed control and FMAs

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- - Boeing (787) speed control and FMAs (https://www.pprune.org/tech-log/611153-boeing-787-speed-control-fmas.html)

Fly3	20th Jul 2018 03:27

BuzzBox.
Can you please identify the page number in the FCOM which says the speed automatically reduces with flap selection if done prior to the flap profile point? I'm having trouble finding it. Thank you.

BuzzBox

20th Jul 2018 04:12

Quote:

Originally Posted by Fly3 (Post 10201104)

The paragraph I quoted is a note at the bottom of page 11.31.34 of Volume 2 (Systems Description > Chapter 11-Flight Management, Navigation > Vertical Navigation (VNAV) > Approach).

Freddie-M

20th Jul 2018 07:05

Quote:

Fly Airbus like and Airbus and fly the Boeing like a Boeing!

The best bit of advice I can give, having spent 16 years on the Bus and 4 on the 787, is never try and compare anything that the Bus does to what a Boeing does.

FullWings

20th Jul 2018 07:12

I dimly recollect from my 737 days that it depends on which FMC is installed. The Smiths that we had on some of our 737s respected the manoeuvring speeds so you could leave the window closed as it wouldn’t reduce the commanded speed until you had the appropriate flap setting. Seemed like a no-brainer. You could also put in a destination QNH so that VNAV approaches through transition level didn’t end up with a vertical discontinuity.

Why the 777 and 787 FMCs can’t/won’t do this has always been a puzzle to me. Why would you design a system that requires intervention on most approaches to avoid a low-speed state? Maybe it was overseen by the same guy who did the autothrottle logic...

Uplinker

20th Jul 2018 14:09

I find the Boeing ‘logic’ very unlogical compared to the Airbus.

The Airbus will state on the FMA that it is climbing or descending. The Boeing (737) does not - you have to interpret whether it is doing what you commanded, by looking at the engine thrust and airspeed settings.

Goldenrivett

20th Jul 2018 16:24

Quote:

The Airbus will state on the FMA that it is climbing or descending.

It needs to because there is no tactile feed back on the thrust levers with autothrust engaged.

Vessbot

20th Jul 2018 16:34

I dunno about y'all, but on my airplane I look at the VSI :D

Uplinker

20th Jul 2018 17:21

Without wanting to cause too much thread drift or get too close to the eternal ‘moving thrust lever’ debate; Airbus pilots get used to including the N1/EPR gauges in their scan, and it becomes second nature to know exactly what the A/THR is doing, even though the levers remain static. Honestly, it is very easy.

My point about the Boeing (737 classic) is that is does not tell you that it is deliberately climbing or descending. You can see from the instrumentation that it is going up or down obviously, but there is no direct confirmation of the intention on the FMA; only an inference related to a change in engine thrust and speed reference.

Goldenrivett

20th Jul 2018 20:30

Quote:

but there is no direct confirmation of the intention on the FMA;

It's not necessary, because if the Thrust Levers advance then the aircraft intends to climb, retard then it intends to descend. The autopilot uses the elevators to control the speed and the thrust change controls the rate of climb or descent.

Quote:

Airbus pilots get used to including the N1/EPR gauges in their scan,

- so do Boeing pilots.

Long Haul

20th Jul 2018 23:21

I’ve flown only the 747, 777, and 787 in the last 25 years. I know that each one behaves differently at low speeds and with the speed window closed, but It’s not exactly clear to me which one does what, when, and I couldn’t care less. That’s because in all that time I’ve never flown at a speed below flaps up maneuvering speed without the speed window being open (other than during a go-around). Even when flying a VNAV approach, the third thing you are supposed to do, after setting the MDA on the MCP and selecting or verifying VNAV, is select speed intervention. It’s just the way the airplanes are designed to be flown, in my opinion. I do understand your desire for knowledge, of course, but in normal practice it’s just not an issue.

Uplinker

21st Jul 2018 08:04

Quote:

Originally Posted by Goldenrivett (Post 10201846)

It's not necessary, because if the Thrust Levers advance then the aircraft intends to climb, retard then it intends to descend.

Unless it is changing speed while on a level segment.

So you have to interpret what it is doing, but it’s not worth arguing over. :ok:

salcrocker

31st Jan 2021 03:12

Hey Superpilot,

From my manual...

During approach maneuvering with IAS/MACH window closed, VNAV sets command speed to flap maneuvering speed as the aircraft passes the flap profile point indicated on the ND. The command speed change occurs even if flap selection is not made. If flaps are not extended before passing the flap profile point, the IAS/MACH window should be opened and the desired speed set to avoid an under-speed condition. When flaps are extended prior to the flap profile point, VNAV commands flap maneuvering speed for the flap position selected. This is very similar to the Airbus. Remember, F1=S speed, F2=F speed, etc. It's been a couple of years, but if memory serves me correctly that was the scheme on the Bus.

Cheers!

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