777 THR mode logic
 

Hello everyone,

Another question on the 777! Trying to understand THR mode coupled with FLCH.

If I get it correctly, the aircraft will try to compute a given value of thrust (leading to a target VS) in order to get to the target MCP altitude in 120 seconds. If it can’t, it will apply Thrust climb during climb (THR REF) or Idle thrust during descent.

1/ I am having trouble understanding how the speed target can still be on the elevator (FLCH SPD) if the given amount of thrust is commanding a vertical speed in order to reach the target MCP ALT in 2min.

Let’s suppose we are climbing from 5000 to 6000ft in FLCH. FMA will display:
THR FLCH SPD.
However are we really pitching for speed here? Isn’t the amount of computed thrust calculated to provide us a target VS ?

When making big changes, of course, we get full climb thrust and speed is on elevator. No vertical speed is targeted. FMA displays THR RÉF - FLCH SPD.

2/ Now for descent in FLCH, from what I see on the real 777, AT always commands THR (followed by HOLD).
It still displays THR even though thrust is at idle. Is it the case or am I mistaken somewhere ?

Let’s suppose we are at FL150 and want to descend to FL060. We can’t reach it in 2min. Therefore AT provides us idle thrust…but FMA still displays
THR - FLCH SPD.
Followed by
HOLD - FLCH SPD.

So does IDLE - FLCH SPD display on FMA not exist?

3/ Let’s suppose we are descending from FL150 to FL140 in FLCH. The airplane will use the same logic as during climb. As we can reach the target altitude in less than 2min, it will compute a given amount of thrust.

FMA displays THR FLCH SPD.
But is it followed by HOLD FLCH SPD? Or does the system not have enough time to revert?

Any input will be highly appreciated, thanks.

To climb or descend at a particular forward speed and vertical speed requires a fixed amount of thrust and a fixed pitch angle - whatever 'modes' you are using. With V/S you set the path with the elevator and thrust controls your speed. With FLCH you set the speed with elevator and thrust controls the rate of climb or descent. In both cases the plane follows the same flight path. The difference between modes is all about priority.

FLCH is considered a 'safe' mode because it prioritises speed. If you're descending in FLCH with THR HOLD and add power you'll reduce the rate of descent. Add enough power you will climb. For small climbs or descents achievable within 125 seconds it calculates and sets an appropriate power setting, but speed is always controlled by elevator.

V/S prioritises V/S which means with insufficient power you can underspeed and with insufficient drag you can overspeed.
You can often see the difference in flight - FLCH tends to hunt the speed by gently pitching up and down - one minute you're at 1500fpm then you're at 600fpm. V/S tends to be smoother as the autothrottles do all the work.

Thanks for your answer! I get your point, but having speed on the elevator and thrust controlling rate of climb/descent does not make sense to me. Let’s take climb as an example, in flight school we are all taught to just apply full power and pitch for target speed. Vertical speed is a parameter that is never controlled, it’s always incurred.

Talking about a 777 that is trimmed for speed, starting from a level flight position at a given speed…simply adding thrust will put the aircraft in climb at the same speed. No (or very few - to damp oscillations) elevator input will be required. Having thrust controlling rate of climb and elevator controlling speed…won’t both interfere with each other ? Pitching for speed will change rate of climb/descent thus changing the commanded rate of climb by our new thrust setting.

Thanks !

In the scenario you just mentioned, a 777 trimmed for speed the elevator IS controlling speed. Add thrust and you climb - that's FLCH.
When you're flying straight and level the autopilot is keeping altitude with pitch and the autothrottles as controlling speed. If you want to climb at 500fpm, imagine a 1° line climbing away. Use the elevator to fly along that line and you'll need to add a bit of power to keep your airspeed - that's V/S. Anything that involves flying a PATH (vnav path, straight and level or a defined VS) - then elevator is the primary control. You'll notice in a geometric descent in VNAV path the autothrottles control the speed - because the priority is maintaining the path. If you open the speed window the priority changes to VMAV speed and you will diverge from the path. Going backcto your flight school plane you always have the option to reduce the thrust and reduce the rate of climb, its just that you normally wouldn't. All phases of flight are a combination of pitch and power. You say the interfere with each other, but you could also say they compliment each other. Boeing logic is all about priority. FLCH proritises speed above all else and generally in a long climb/descent it does behave exactly like your flight school plane. Also, it tends to get you started with thrust management then it leaves you to it. Once it goes THR HOLD the the thrust levers are yours to control in the descent.

If ATC ask me to maintain 1000fpm descent rate I could use FLCH and fiddle with the thrust levers for the next 30 seconds trying to fine tune the correct ROD - but its a ****load easier to just dial in -1000fpm and let the autothrottles fine tune the thrust for me.

Most of the time you'll be in VNAV anyway. FLCH is useful for small altitude changes which don't need full power as well as flying when you're on vectors and don't have a path, as well as being a general get-out-of-jail-free button. V/S is useful to reduce the rate of climb to avoid TA/RA, for bringing the nose up to slow down quickly and for getting a nice CDA. Ultimately you can use VNAV, FLCH or VS whenever you like - you could pick any one and do the whole flight from start to finish if you wanted to it's personal preference.

Not having flown the 777 I was gonna sit this one out, but seeing that people aren't understanding your question (boiled down to this sentence) I'm gonna go ahead and speculate. (A FCOM I have a copy of, is no help either). Probably one of 2 tings is happening:

- It's not responding to rate of climb with thrust actively in a closed loop, like every other autopilot mode responding to its parameter, but rather the required thrust is calculated from altitude, weight, and temperature, and set at the result of the calculation. So it's just a fixed value of thrust like THR REF or CLB or whatever, but less.

or

- It's responding to rate of climb with thrust but extremely slowly. So, on the time scale of an elevator response to speed, the thrust is essentially fixed. The two frequencies are so different from each other that they don't interfere.

I suspect it's the first.

Overthinking. It's a common fault.
Try thinking of FLCH as a speed lock.
Like the IAS button on MD series. Once the FMA goes to HOLD. the thrust levers are yours, speed stays locked V/S follows thrust.
Simples.
Maui

It appears you are underthinking, a more common fault.

For "THR," the FCOM says "The autothrottle applies thrust to maintain the vertical speed required by the pitch mode."

This means that the airspeed and vertical speed are simultaneously affected by both the autothrottle (per the statement above) and the pitch channel, as it pitches to maintain airspeed in FLCH.

Like any physical system, the target speed is not held perfectly, but rather constantly overshot and corrected unless having had some time to settle down in completely smooth air.

Let's say the VS went below target. If the THR behavior as described above makes a correction for it, and bumps the thrust up a little, the airspeed wont be "locked" to the airspeed target but will go up a little bit, until the pitch channel in FLCH responds to that and pitches up, bringing the speed back down to target. AKA, the phugoid cycle.

While it's pitching up, what's happening to the VS? It's also going up, to which THR will respond by pulling the thrust back. That will also pull the speed back, causing FLCH to pitch down, reversing the original correction. As a result of this, the VS is below target, bringing us back to step one.

Hopefully you can see now, how one triggering event can cause both thrust and pitch channels to reverse themselves in responding to each other's responses. Just how much do these reversals have a potential to cause even bigger reversals next time around, and diverge in amplitude as more time passes and they keep feeding each other? Who knows, with the interaction of multiple overlaid cycles now we're entering the black magic world of imaginary numbers, Laplace transforms, and who knows what else.

Obviously it is a solved problem as the 777 is not known for anything like this happening. But this is what the issue is, in the first place.

Vessbot.
My comments was not directed at you specifically. It was a generalised statement. Too often on these pages we see explanations so detailed that the original, or supposed, point gets lost. This is such a case.

The essence of FLCH is quite simple. If you want to go down (the most common use of the mode); wind down the selected altitude. Press FLCH.

From memory; (I no longer have access to FCOM of other relevant data), speed window will be at, or will open at, selected speed, (if it’s not what you want, change it). AFCS will endeavor to get the A/c down to that altitude in (I think) 120 seconds. It will maintain selected speed (speed lock) and adjust thrust to suit, vertical speed will be whatever, as it is a function of the other parameters, not a controlling function. FMA will show FLCH and THR until AFCS is done with adjusting thrust, when it will then revert to HOLD.

HOLD will be maintained until either altitude capture or G/S capture, (or ground contact hello ASIANA). It will then wake up and do its thing as appropriate. Throughout the procedure, whilst in HOLD pilot input to thrust levers is possible and appropriate for vertical speed adjustment.

Knowledge of what is happening in the relationship of trim and elevators as has been referred to in this thread, are outside of the control of the pilot, is inconsequential to the operational outcome, and only serves to confuse. The system is well designed and normally works as advertised, provided the inputs are appropriate. In 17 years on type, I never once thought about which control surface, is doing what, to achieve the outcome that I am requesting of the whole. That’s designer ****. Not pilot ****.



Looking at your second post.

Para 1. Sometimes it is a virtue rather than a fault

Para 2. In FLCH, V/s is not a parameter It is a consequence.

Para 3. Airspeed is “locked” (speed window) V/s is a consequence.

Para 4. I deal with what I can see. There is no target V/s. Speed is “locked”. Pitch will vary to maintain speed. Speed is “locked”.

Para 5. Once the FMA goes to HOLD pitching will only occur to restore Speed. The Auto throttle is out of the equation until Alt Cap or G/S cap, when it will wake up and do its speed thing. There is no target V/s.

Para 6. Que?

Para 7. Exactly. This issue is one generated by overactive minds.

As I scribe this, it has become abundantly obvious that you have missed the point that the Auto throttle is out of the equation once the FMA transitions to HOLD.

Maui

The question is about THR, not HOLD.

Look it's everybody's decision how to spend their free time, and nobody is forcing you to deal with what you can't see. But if you don't like to do that, then don't purport to answer a question about that, fail to understand the question, and tell the person they're thinking too much.

The way I think of it is that for small changes, FLCH is essentially a vertical speed mode, but they don’t call it that.

I think on a recent flight, I selected FLCH for a 1000ft descent, and the thrust FMA stayed in THR. I could be misremembering that though. Standing by to be corrected if that’s the case.

I think issue is that the OP explains (reasonably well) how it works, then goes on to explain that they don't understand it... 🤔

1) yes, yes.
2) not always
3) yes

Does this help ?

Seen in a BA B777 SIM briefing room at LHR.

Sorry about the curvy poster and the glare, but you can just about read it.

What a nightmare. Regardless of the designer's (weird) ideas, surely there is no reason to allow the ATS to remain asleep when the speed falls below Vmin (or whatever it's called on the 777) under any circumstances.

Boeing should fly the MD11/717 to see how simple the ATS could be.

Vessbot. I have no problem with the latter part.
WRT the former, THR + HOLD.
On the way down THR will transition to HOLD so is relevant to the discussion. On the way up power will increase up to the selected limit with the annunciation remaining at THR (or maybe CLB, I can't remember), so no discussion needed there.
There is no target ROC/ROD in FLCH. There is a target time interval which if not possible, is ignored. There is selected speed, power, and selected altitude. If speed varies ROC/ROD will vary to suit. Talking of phugoids and closed loops etc. contributes very little.

The general philosophy of my input is that; where someone is trying to get their head around a procedure, it is often confusing to get too deeply into the nuts and bolts. What is important from an operational viewpoint, is that the enquirer comes away with the knowledge required to operate the system appropriately and that he/she comes away comfortable that the system will, if given the right parameters, deliver as advertised. Knowledge of which bit does what, and how, is not necessary, and sometimes introduces confusion. I can still remember the pain of learning about the operation of the "piston lift solenoid" on the Dowty Rotol propellor. Totally superfluous information. Try to avoid overthinking. If you want to go off and do a PhD in some esoteric aspects that's fine, fill your boots, but it does little to enhance the output in most circumstances.

Capt Bloggs

Firstly; the 717 is a Boeing as you well know. You are suggesting Boeing should go and talk to Boeing about how they do things differently. Que?

Secondly; Provided the ATS is armed the Auto Throttle will wake up at Vmin. Just as it will wake up when capturing set altitude or armed g/s. If you don't set either of those or set them in error it will not help. It is quite happy to do as you request, but it can't second guess you. If your request is to fly into the ground or you fail to give it parameters to work with, it will oblige. How is that a weird idea.

Maui

rudestuff said it best at post #11.

Maui

Rubbish. Why do you think I made the suggestion?

It's weird that a design would allow a perfectly serviceable ATS stay dormant while the aircraft reduced to Vmin -30 and then crashed. Why the hell should it need to be "armed" to wake up when things were obviously not right?

You said it all Vessbot. Maybe my English was not clear enough, but it’s exactly the problem I was thinking about.

Maui, by reading the FCOM you really get the feeling THR is trying to reach a target VS (function of time) by setting a given amount of thrust. And this of course, for small changes. If we can’t reach the target altitude by 2min, we get full climb thrust (THR REF) and speed is back on the elevator. Do you have any source that clearly says that no VS is targeted in THR (for small altitude changes)?

Check Airman gave an interesting point…for small changes, FLCH may be seen as a VS mode. This makes more sense to me…it would be interesting to see the behavior of the elevator during the climb.

To make it simple, would the system react in the following way:

- 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.
- Once the thrust is set, it won’t keep changing. In a perfect world, no elevator input would be required for a perfectly trimmed aircraft. The aircraft would climb at the MCP window speed at the target ROC set by thrust. But as we are not in a perfect world, elevator does pitch for speed once the thrust is set…as FLCH gives priority to speed. Our target VS (ROC/ROD) set by THR will change but the system disregards these changes…it’s just some initial math done to compute a given amount of thrust set for the climb.

Any thoughts?

.....or indeed the excellent Airbus FBW auto-thrust.

+1

I am astonished that this was ever allowed to be certified.

I was under the impression that the latest software update allows the A/T to wake up at any time if the speed is low (probably input from Boeing’s lawyers) but for obvious reasons haven’t tried it as our sims are still on the old version.

After AIMS-2 BP V17B it is the case.
If the pitch mode is FLCH SPD, VNAV SPD, or VNAV PTH and A/T is in HOLD or THR mode, and speed decreases into the amber band, the A/T will change from HOLD to THR mode if necessary, and thrust will increase proportional to the amount speed has decreased into the amber band.

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:

---

- 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.

---

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