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.