Not ‘back on the elevator’ - it’s always on the elevator in FLCH, regardless of what thrust happens to be set. (Leaving aside whatever might happen now in low speed reversion, which is after my time)

Yes sorry. So this does go in the same way as the end of my reply.

Quote:

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- If the target can be reached in less than 2min, AT will compute a given amount of thrust. This will definitely lead to an output VS.

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THR sets a value below THR REF in order to command a ROC leading to our target altitude in less than 2min at MCP window speed. Once thrust is set, it doesn’t change. Speed is on elevator. I’d assume if target thrust is correctly computed (and we don’t change MCP speed), very few elevator input should be required.

Yes, it is just that simple. When you press FLCH, the ATS helpfully makes a guess at how much thrust you will need to get there in about two minutes, sets that, then drops to HOLD and leaves you to it. Speed is always controlled by the elevator.

Just out of curiosity - Airbus FBW pilot here - in that situation, and assuming a 2000' level change; does the B777 FCU (glare-shield control) display V/S 1000 fpm, and what happens if you press FLCH and also select a V/S other than 1000 fpm ?

Does the Auto-thrust deal with it or does the pilot have to ?

With the caveat that my Boeing time is a distant memory, so anyone more current please correct me if I am wrong!

No V/S is displayed on the MCP(FCU) when using FLCH.

FLCH is, crudely, the equivalent of open descent/climb with the added function that when pressed, it will calculate the thrust required to reach the level in the MCP window in 125 seconds and set that thrust. In the FCOM this was said to be for passenger comfort and to enable FLCH to be always the preferred non VNAV mode (managed to you and me!) mode. If the level will not be reached in 125 seconds even with full climb thrust or idle then the thrust commanded will be that and it will be broadly the same as open climb/descent.

Once “hold” is annunciated the autothrottle is armed but not active, you’re free to move the levers as you wish - and this is actually a really nice way to fettle a FLCH descent or climb without using V/S.

edits: for clarity.

when you select FLCH it will give you an approximate thrust setting to maintain approximately 1000fpm, but that will not be on the MCP (glare shield). The speed window will open but not the VS window. If you subsequently press V/S then the speed window will remain open and the V/S window will open at your current V/S: now vertical path is on the elevator and speed is on the AT.

During climb, if it can’t reach the target altitude in 125 seconds, it sets climb thrust, displayed as THR REF on FMA.

Any insights during descend ?

I have never seen it go IDLE - FLCH SPD. Even when it can’t reach it in 125 sec, FMA shows THR but actual thrust is at Idle.

Why so? IDLE - VNAV SPD/PATH exists. Why not display IDLE for FLCH when it’s really setting Idle thrust ??

I am aware it then transitions to HOLD but my question refers to the first AT mode displayed (THR) as we select FLCH during descent.

I’m guessing if it can reach the target within 2 mins, it’ll show THR, regardless of what the actual thrust level is.

HOLD is a fairly stupid mode IMO. They way they’ve programmed it just leads to higher workload on my part.

I started preparing the following a while ago, and I think a few of the posts in the interim should have clarified things a little for you, but I will post this for you anyway.
With regard to this last post of yours. When on descent, when thrust is reduced to idle, it is my recollection that FMA transitions to HOLD pretty much simultaneously with idle stops on the levers.
But in reality does it matter when it transitions? Is it going to lead you to change your inputs if doesn't tell you what you expect? Are you going to close the already closed thrust levers? Are you going to adopt some alternate procedure to achieve what you have already asked the AFCS to do for you?

pre prepred respose to earlier.

Consider these points;
1) If FLCH controls V/s, why then do we need a separate and distinct VS mode. On your reading, they are doing exactly the same thing?

2) Why can you NOT select FLCH and VS modes simultaneously?

I have no source for my posit, as I am the owner of a fried motherboard that has locked away all my previously owned manuals and documentation. My comments are based on considerable observation of the way things work, over a lengthy period of service on type. I have not been in a control seat for over 6 years, so apologies for any holes in my recollection.

Request for confirmation can run both ways. Can you produce a credible reference for your position that FLCH will calculate and target a specific ROC/ROD in compliance with your assumption of method of operation. Or Vessbots for that matter.

The fairy story below may help develop a perspective of how it works.

******************

Hi my name is FLCH. I live in a box in the E&E. Living with me are multiple modes, VS, VNAV to name a couple. We each have different roles, sometimes utilising different parameters that are pertinent to our allotted tasks. We normally do not all work together at the same time, but we harmonise to produce the required outcomes. Our environment is populated with PFC’s ACE’s and a plethora of electronic gizmos and sensors, who help us keep in touch with all the parameters and environmental factors we encounter in our daily work. Some call our home PFM, which roughly translates to Pure Magic and refers to our output.

Our role is to take instructions from our bosses who sit in the comfy chairs upstairs. We task allocate then analyse the requests/instructions. We formulate a response, translate them into electronic form and communicate them to the components on the outer extremities of our environment, with a view to accommodate the bosses wishes. We are each and collectively obedient servants. At all times we verify the effect of our actions and relay important status info to our bosses

As an example: If someone upstairs presses my call button (FLCH) I will instantly jump into action. I am not the smartest kid in the room but I do well that which is asked of me. Some wrongly endow me with credit for calculations far more sophisticated than those for which I am qualified. I call them overthinkers. Sometimes those overthinkers get frustrated with me because I do not behave the way they think I should. I will behave the way I am programmed at build, in compliance with the instructions I have received and within the programmed limitations of the combined systems.

When those in the comfy seats upstairs ask me to climb or descend, I will adjust the output of the noise makers, appropriately. Those upstairs will tell me what speed they want to maintain (lock) throughout, and also tell me what level they want to go to.

Go up - increase noise

Go down - decrease noise

As I reduce or increase the noise and maintain the locked speed, I will constantly monitor the consequential variation in vertical speed and calculate the approximate time to level. If that time is 2 minutes from my call button press, or less, I will cease reducing/increasing noise. If that 2 minute time window is unachievable, it will be ignored and I will continue the movement of the noise levers until they reach their limits. Failure to comply with the time restriction is no biggie, it was only an arbitrary thing anyway. Just to keep everyone in the loop, I will let the supervisor, my cousin FMA, know that I have stopped fiddling with the noise makers. In the case of descent he will then put out a HOLD message. That is to let those upstairs know that I have done my bit and gone for coffee, if any variation of noise level is required /desired, they are going to have to do it themselves, or call on one of my other cousins to have some input. Cousin VS could step in but he and I cannot work together so to maintain the harmony I will step out. Regarding the HOLD message I prefer to think of it as an instruction to those upstairs to hold on to the noise levers because if, in their infinite wisdom, they decide some variation in profile is required, they are now in charge and any variation is on their head (or more precisely, in their hands). I am happy to sit and watch, and I am here if they call but I will not interfere. My job is now done.

I will sit back with my coffee and wait for cousin Capture, to take over. When he comes in he will resume responsibility for noise level and the attitude to maintain the requested speed and altitude. So, I am simple, but I am good at my job.

PFM.

As operational footnotes.

1) For short level change, cabin comfort will be enhanced by the use of VS rather than FLCH due to the smoother and lesser application of the noise levers.

2) To see what actually goes on when you press FLCH, pull up FCTL on one of your MFD’s and then set up a level change. May not give much value in the sim but could be enlightening in real life. Don’t know, haven’t done it. It would also be accessible through the maintenance terminal, but that is a complex beast to get into when unfamiliar.

Capn Bloggs.
Have you missed the memo regarding this being a full time AT? The aircraft is designed to be operated with it armed at all times when in flight mode.
It is armed before takeoff and disarmed with WOW and/or ( I think) reverse thrust after landing. If it is selected off during flight, that could be legitimate in compliance with a Non Normal Procedure, or illegitimate in the case of non compliance with SOP's.
If it is not turned off, it can be tricked by the use of some bloody stupid inputs. For instance FLCH with altitude zero set, and no G/S armed or below G/S,
FLCH should not be used inside the FAF. If it is used make darned sure the G/S is armed and below you, or it will really mess up your day.
If it is turned off, as with most systems, it will not wake up.
For a competent operator in a normal environment, it will always be there to help.


On another note: you still haven't answered my question of a couple of years ago which asked, inter alia, "do you still have an IAS mode in your Boeing 717"

Maui

1) If FLCH controls V/S, it is the thrust channel doing it. In the VS mode, it is the pitch channel doing it. Not the same redundant thing.

2) Because they do contradictory things - the former controls speed with pitch and the latter controls path with pitch - and vice versa for the thrust channel.

I already did, from https://www.ameacademy.com/pdf/boein...g-777-FCOM.pdf : "THR - The autothrottle applies thrust to maintain the vertical speed required by the pitch mode."

Of course it's an overly condensed and vague statement that could mean different things, but a plain reading of it certainly suggests that THR (as part of FLCH) controls vertical speed. And if you think it doesn't, then you're in agreement with me as I wrote in post 5.

You made a point before about vertical speed being a consequence, and not controlled target, of pitch controlling airspeed. While that is true, it is a physical event in the chain of events all the same. And, if the FCOM quote above is true, then the pitch and thrust channels in FLCH are both resulting in vertical speed changes, which creates the potential for interference as THR responds to it.

Perhaps I'm just tragically incapable of escaping my faults in thinking carefully about what I read, but his complaint was specifically based on the AT being on full time.

FLCH is typically used on descent and frequently, the thrust levers go right to idle when FLCH is selected as the attempt to change altitude in 2 minutes will be exceeded. That means the aircraft is at the commanded speed in the window, (which opened when you selected FLCH) with idle thrust and frequently, a fairly significant rate of descent.

HOLD means you can adjust the thrust manually as the thrust levers have been released and are now free to be manually moved by you. One sees it when FLCH is selected. After pressing the FLCH button, you will immediately see THR and FLCH displayed on the FMA(and hear the PF say “thrust flight level change” and see the thrust levers move to idle. Within a second or two, you will see the FMA change from THR to HOLD and hear the PF say “hold”.

You are now in command of the thrust levers while in HOLD, so your hand should be on them as pilot flying.

One might find this HOLD mode useful during descent. For example, you may be approaching the tops of some buildups and want to avoid going through them. Depending on the positioning of those buildups, you may be able to simply add some thrust manually and reduce the rate of descent from say 2500 fpm to around 1000 fpm for a minute or two and once clear, reduce the thrust to idle again and continue your idle descent with the cabin crew unaware of the turbulence you just avoided while they are still preparing the cabin for landing.

This HOLD mode capability actually reduced your workload as you didn’t have to change modes for your vertical deviation. Such a deviation won’t be quite as easy while in VNAV PTH, where a slow down in descent rate had you switching to VS and once clear back to VNAV.

My preferred technique to avoid that buildup would be to go to VS to shallow the descent.

Obviously yours works as well, but I’d just find selecting VS easier.

Agree. Pushing the throttles up to get over cloud was always a backwards way of doing it. If you want/need 1000fpm, instead of guessing how much to push the throttles up, just set it on the VS and be done with it.

Vessbot

Thank you for the link to Continental’s FCOM. It has been a trip down memory lane to pick through that tome. A useful reference in this discussion.

With regard to your last post:

My Questions 1 & 2 and your reply 1 & 2.

They were rhetorical questions directed to TheBusFlyer. Upon reflection maybe I should have been less obtuse, so here is a more direct reframing of the questions.

1) If FLCH is capable of direct control of V/s in the manner you are proposing, what could an independent VS input contribute that the FLCH cannot?

2) Is it possible to select FLCH and VS mode simultaneously. (note V/s is a parameter and VS is a command)

To end the pain I will supply the answers. 1) Nothing 2) You can’t

To separate the various worms in this can, Continental FCOM Section 6.4 is useful.

Nothing there about V/s.
And

The FCOM is silent on what happens if the 125 second time limit can’t be achieved.
What happens in the real world is that the thrust levers will be advanced or retarded to their limit be it idle for go down or full loud ( as determined by Thrust Limit Computer (TLC)) for go up. At about the point of max travel FMA will announce HOLD on the way down or THR or CLB (??) on the way up.

In your previous quotation of the FCOM, you omitted the part about the 125 seconds. Doing that eliminates one conditional element, and in my view, invalidates your assertion about control over V/s. FLCH cannot command a V/s that is the role of mode VS. It (FLCH) can only command auto throttle. You are correct in your contention that VS and FLCH are conflicting modes, thus in the following statement, the only mention of vertical speed is in the context of achieving the 125 second, soft, target.

Then, your

There is no potential interference there because the THR is out of it until some other input occurs viz Alt cap or physical intervention such as mode change. This is so ‘cos thrust is either at HOLD or full noise until an intervention. And it will be telling you that with HOLD or THR annunciations on the FMA.

With the other comment, which I directed to Capt Bloggs, don’t sweat it. It refers to a long ago intercourse I had with him in which he was similarly highly critical of the design of the 777 AFCS. A system that if understood and operated as designed, is enormously competent and versatile. Note that there is a significant difference between the A/T being asleep as opposed to being switched off.

Maui

I think that Boeing use very confusing terminology: "HOLD" suggests to me that some parameter is being held by the automatics, e.g. altitude, speed; not that, in fact, the auto-thrust has gone to sleep.

What Boeing actually meant was that the pilot must hold the thrust levers with their hand !!

This seems a weird way of controlling an aircraft with the autopilot flying - a sort of half and half. "I will do this bit but you have to do that bit".

And if, as I understand it, this "HOLD" mode can cause a major elephant trap such as that experienced by the B777 into KSFO where the auto-thrust stayed in HOLD - i.e. asleep - with decaying speed and crashed, then it seems to be very odd 'logic' to me - as a non B777 pilot.

(I do realise that piss-poor piloting by company management pilots was also a factor in that crash).

Uplinker.

Maybe it was a poor choice of terminology. I am ambivalent in that regard.
But the functionality is clearly explained in the documentation and training.
It is clearly annunciated what state the A/T is in. Just got to look at the FMA to know what you have or not, got. Is that difficult?
Just because it is/was a new/different way of doing things doesn't make it weird. It's called evolution.
Has Airbus not also adopted new, evolutionary equipment and procedures. Operating the A350 is vastly different to operating the A300, does that make it weird?
WRT to 777. This is not new technology. It has been in service now for near enough three decades with few major issues.
The manufacturers have moved forward. Perhaps it it long gone time we as a pilot group should be moving forward with them, rather than resisting change.
. HOLD did not cause the Asiana crash. Ignorance and inadequate training featured largely.

Maui[

Automatics are an aid. How hard is it to monitor the instruments and take over if necessary?

Maui, you need to get off the koolaid. This is the 21st century. The 777 might have been designed by "you do the right thing and you'll be OK" aces-of-the-base, but to, in any way, justify or try to explain away a state where a perfectly serviceable system stays dormant while the aeroplane crashes due to low speed in this day and age is just complete and utter nonsense. Get a grip on yourself.

Bloggs.
How does AF 447 fit with your philosophy. That is/was newer super safe technology.
Just saying.

Maui
PS I have an new load of Koolaid on order. I'll set aside some for you if you like.
M