Boeing (787) speed control and FMAs

[Superpilot]

Hi all,

Currently doing the B787 type rating (2 weeks in) having flown the mini Bus for nearly 10 years and I'm having lots of issues understanding Boeing FMAs and Automatic speed reduction (for the approach) concepts. Not helped by the fact that the FPT instructors have never flown the type before and perhaps do not fully understand some of the modes themselves. The choice of words used in the FMA confuse me (3 FMA columns vs 5 from Airbus) and the behaviour of the aircraft also seems odd and against convention. Of course, I'm being unfair here and realise I still have a bias towards my old type. Could I get the help of someone who has undergone an Airbus to Boeing brain transplant in an attempt to join things up for me?

In Airbus, we have the 'managed' speed, altitude and navigation concepts. The managed speed function (where the speed automatically reduces with each flap selection) can be engaged regardless of the vertical and lateral navigation modes as long as an approach has been 'activated'. Never will an Airbus automatically reduce speed to below a min flap manoeuvering speed by itself unless you have manually planted a low speed constraint somewhere against a waypoint (I think)

First question. On a B787 on approach, with the speed window closed and when in VNAV PTH, will flap selection always automatically reduce the airspeed to flap min maneuvering speed? What's the logic/condition it relies on in order to do this? One of the instructors was surprised by this behaviour. Is this new to the 787? (Airbus have had it since 1987)

Secondly, on approach, can't remember the FMAs but sometimes after crossing the F1, F5, Fx markers on the ND, the speed automatically reduces without any flap selection and we have to select flaps. A bit dangerous isn't it? I.e. to not get caught out you should have the speed window open on approach? Why doesn't it open by itself? Or have I totally confused myself here? I hope the FPT is reflecting the real plane correctly!

Thank you

[Fly3]

With ATHR engaged, and in VAV PTH with the speed window closed the aircraft will reduce speed as it crosses the speed profile points regardless of the flap selection. Unlike the Airbus, flap selection has no affect on speed control in the B787 and it's not new. It was always like this. The ATHR should prevent you coming below Vmin but you can end up with pretty high nose up attitudes if you are not careful. Airlines usually have an SOP which states an altitude below which the speed window must be open to avoid this trap.

[Fursty Ferret]

On a B787 on approach, with the speed window closed and when in VNAV PTH, will flap selection always automatically reduce the airspeed to flap min maneuvering speed?

No, it won't. It'll happily reduce the speed to the top of the amber band. The autothrottle (*nervous tic*) system on the 787 - and all Boeings, really - is pretty rudimentary. As are the autopilot and horizontal / vertical navigation modes. The 787 will do wonky turns, crap holds, weird offsets, aggressive bank angles and fly an ILS that would fail an instrument rating. It always hands the aircraft back to you slightly out of trim, and as you've already noticed, the FMAs aren't really helpful.

Unless it's an ISA day, VNAV will always put you in the wrong place on the profile for an idle descent and the only saving grace you get on the 787 is the VSD, which tells you in advance that it's going to cock it up.

You still have the joy of "HOLD" mode to figure out, which isn't satisfactorily explained by anyone.

[eckhard]

You still have the joy of "HOLD" mode to figure out, which isn't satisfactorily explained by anyone.

I’ve always taken it to mean “HOLD the thrust levers, and don’t be surprised if you have to move them”.

[underfire]

Unless it's an ISA day, VNAV will always put you in the wrong place on the profile for an idle descent and the only saving grace you get on the 787 is the VSD, which tells you in advance that it's going to cock it up.
AND, there are no ISA days.
Pouring through the logic sequence for coded procedures, I am not sure what happened with this variant....would love to map the if/then sequence on this, but damn.

[Flyboy_SG]

Hi,



Boeing’s automation concept is very simple and less reliable than Airbus. The FMA modes is simple
ARP. Auto throttle Roll mode and Pitch mode in order. In the Boeing , the Pilot flies the aircraft ! So AFDS has to be monitored all the time, religiously.

1.it will not reduce to Flap manv speed. Speed window has to be opened and flap manv has to be manually set.
Although I was told some versions of FMC/AFDS had auto setting feature(not verified ). But surprising even a 737NG has this feature.
Alternatively you can enter the speed on vnav des page in FMC. Without opening the speed window.

2, It won’t reduce the speed unless there is a speed constraint. And speed window should be open for approach.
There is nothing like managed speed here.


Yes you can fly vnav all the way to Loc capture, but the des wind forecast has to be accurate too apart from ISA Devn. At the most you have to add drag by speed brakes to help vnav maintain its profile, that’s the only thing it can’t control.

And Hold modes simply holds the thrust lever in previous thrust setting , aka the AT takes his hands Off , so that the thrust can manually increased or decreased as per requirement Say on Take off roll and descent. Nothing fancy about it.
(And the thrust level will not move automatically till the next mode capture or selection.

[Superpilot]

Unlike the Airbus, flap selection has no affect on speed control in the B787 and it's not new

1.it will not reduce to Flap manv speed. Speed window has to be opened and flap manv has to be manually set.
Although I was told some versions of FMC/AFDS had auto setting feature(not verified )

Thanks all, but I don't know guys... This is not what me and my partner (and surprised trainer) are seeing. The FTD is consistently reducing aircraft speed to that flap's min speed upon flap selection with VNAV PTH. We see this behavior if we make selections 5 miles before reaching the Fx symbols on the ND. I think I'll wait until I get to the FFS!

[AtoBsafely]

I’m not familiar with the 787, but the newer Boeings WILL reduce the VNAV speed to flap manoeuvre speed on the flap selection. I’m old school, and maybe a control freak, so I always manage speed manually on the MCP by that point. In real life ATC are usually assigning speeds anyway.

[crwkunt roll]

I don't think I've ever flown a VNAV approach with the speed window closed.

[Superpilot]

Great, so we're getting somewhere! It does exist, it's just that most Boeing pilots are not familiar/comfortable with it. More than 75% of worlds controlled airfields do not have speed control on approach so using a "closed" speed window is standard practice on an Airbus and works well with the aircraft's performance on approach.

Thanks

[tttoon]

From what I remember from my MCC on A320 ten years ago, which was my only Airbus experience, VNAV PTH is equivalent to the managed modes in the Airbus. It does whatever is entered in the FMC as well slowing down for configuration (very conservatively). It doesn't only work on an ISA day, if you take the time to fill out the DES page with the forecast winds and temperatures.

If you leave the window closed, it'll indeed slow down to the low speed protection, expecting the crew to select flaps on schedule. Most people I know are not huge fans of using VNAV down low, but it's very useful for STARs with lots of altitude and speed limits. Once on the approach, I usually use the basic modes or open the speed window.

[Superpilot]

Thank you. I think me and my partner have become victims of a bit of 'look what this aircraft can do' and it seems to be at odds with how this machine is flown in real life and on the line.

[Jetjock330]

Fly Airbus like and Airbus and fly the Boeing like a Boeing! I have flown both and fly Boeing as a Boeing! Opinions by comparison won't help! and at least the Boeing won't call you a Retard��

[BuzzBox]

The FTD is consistently reducing aircraft speed to that flap's min speed upon flap selection with VNAV PTH. We see this behavior if we make selections 5 miles before reaching the Fx symbols on the ND. I think I'll wait until I get to the FFS!

The B787 FCOM states:

During approach maneuvering with IAS/MACH window closed, VNAV sets command speed to flap maneuvering speed as the airplane 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 underspeed condition. When flaps are extended prior to the flap profile point, VNAV commands flap maneuvering speed for the flap position selected.

In other words, if you extend the flaps before crossing the flap profile point (F1, F5, etc), VNAV will command the respective flap manoeuvring speed when you extend the flaps. If you cross the flap profile point and don't extend the flaps, VNAV will command the respective flap manoeuvring speed regardless of the actual flap position.

[Capn Bloggs]

Sounds like a great system... not. :rolleyes:

[wiggy]

If I dare ask: re the various comments about approaches and the MCP speed window not being open...on the likes of the T7 once VNAV has gone into approach mode (triggered by any flap being selected and/or a couple of other parameters that escape me ATM) the MCP window will automatically be open, you have no choice in the matter, so you should certainly never end up flying a VNAV approach with the MCP window closed ...does the 78 use similar logic or is it even more mysterious than I imagine?

As for the HOLD FMA...as has been said to a simple soul it means the autothrust backdrive to the thrust levers is disconnected and therefore you can “hold” the thrust levers and manipulate them as required, if e.g you are descending in VNAV PTH, below the profile and the dozy machine hasn’t put any power on you can do it yourself.

[BuzzBox]

If I dare ask: re the various comments about approaches and the MCP speed window not being open...on the likes of the T7 once VNAV has gone into approach mode (triggered by any flap being selected and/or a couple of other parameters that escape me ATM) the MCP window will automatically be open, you have no choice in the matter, so you should certainly never end up flying a VNAV approach with the MCP window closed ...does the 78 use similar logic or is it even more mysterious than I imagine?

As for the HOLD FMA...as has been said to a simple soul it means the autothrust backdrive to the thrust levers is disconnected and therefore you can “hold” the thrust levers and manipulate them as required, if e.g you are descending in VNAV PTH, below the profile and the dozy machine hasn’t put any power on you can do it yourself.


I fly Airbus these days and it's been a while since I flew the B777, but I don't remember the B777 VNAV behaving as you described. As I recall, the MCP speed window would close (if open) when the FMC transitions to 'on approach'. The speed window could then be opened and the VNAV would stay in VNAV PTH instead of changing to VNAV SPD. I think the B787 is similar. There was a discussion about the B777 case many moons ago: https://www.pprune.org/tech-log/221530-lnav-vnav-approach.html

[Monarch Man]

I’m going to reiterate what has been said here regarding Airbus v Boeing, over the years A300/320/321/330 and 75/76 and now 777.
A Boeing is designed to be flown by the pilot via the MCP or manually, it is much more of a ‘hands on machine’ in terms of how to get the job done, forget your Toulouse test tube philosophy, make the jet do what you want via the MCP, the 787 version of the FMC is there to help you plan, unless of course your company tries to make you fly a Boeing like an Airbus.
In terms of modes and speed control the modes you will see are dependant on what’s in the box, and if you have the speed window open.
Its really just a different way of doing things.....and trim for speed...then hand flying is even better that the electric death jet.

[wiggy]

I fly Airbus these days and it's been a while since I flew the B777, but I don't remember the B777 VNAV behaving as you described. As I recall, the MCP speed window would close (if open) when the FMC transitions to 'on approach'. The speed window could then be opened and the VNAV would stay in VNAV PTH instead of changing to VNAV SPD. I think the B787 is similar. There was a discussion about the B777 case many moons ago: https://www.pprune.org/tech-log/221530-lnav-vnav-approach.html

My mistake, it had been a long night...yes, agreed- speed window can be opened when “on approach”, you can then fly the approach in VNAV PTH controlling speed with the MCP.

[JM926]

Can’t speak for the 787 but the 737, even the classic 300 version will reduce the command speed to that of the manoeuvre speed for the selected flap setting whilst in VNAV PTH.

[Fly3]

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]

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.

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]

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]

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]

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]

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]

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

[Uplinker]

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]

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.
Airbus pilots get used to including the N1/EPR gauges in their scan, - so do Boeing pilots.

[Long Haul]

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]

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]

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!