777 THR mode logic

[Capn Bloggs]

Just as stupid, Maui. It might be new, but that doesn't mean it's safe either.

Surely, deliberately designing an ATS that will stay dormant when you're Vmin-30 simply because you were in the wrong modes doesn't make sense to me. Airbus doesn't do it, McD didn't do it with the 717 and MD-11, and I assume Bombardier didn't do it with the C Series/A220.

What about an oddball emergency where the crew had to get back ASAP or were forced into a rapid arrival due to say fuel, like that 777 in Singapore/Indo. A simple mode stuff up crashes the jet. Where's the logic in that?

[TheBusFlyer]

Maui, I think Vessbot of course assumed we were talking about small altitude changes only, ie. those we can reach in 125 seconds. In that case, THR does command a given rate of climb/descend (or vertical speed). What I get from this discussion is that this command is simply set INITIALLY. Once thrust set, elevators pitch for speed if required.

For large altitude changes, it’s a no brainer. Full climb thrust during climb (THR REF), Idle during descent (still shows as THR on FMA - who knows why - but quickly followed by HOLD).

[punkalouver]

Originally Posted by Uplinker

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

There was CWS on the 737 that had you hand flying with the autopilot engaged, why not the HOLD mode with autothrottle engaged?

Originally Posted by Uplinker

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 !!

Pilots see HOLD on every takeoff from 65 knots to 400 feet above the airport. So, they should easily know what the term means as it is critical knowledge about what it means for the takeoff both before and after 80 knots.

Originally Posted by Uplinker

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 thought that Boeing updated the logic on this a few years ago to give autothrottle wake-up. Perhaps it is only optional and not all aircraft have the update.

Originally Posted by maui

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?

It would be foolish to try and use FLCH for a desired vertical speed when precise descent rate is required, as it involves too much guesswork when one wants to minimize erroneous descent rates. For example, if you have a glideslope that was fluctuating significantly on approach(as happens sometimes) leading to the aircraft pitching up and down as the automatics chase it, quickly selecting a VS value of 800 fpm(or whatever you deem appropriate) can allow you to continue while returning to a stabilized descent. Then you can make minor adjustments(as low as 100fpm increments), and the aircraft will adjust precisely to the new desired descent rate. Trying this sort of recovery with FLCH would have a high likelihood of chasing a proper descent value as you vary thrust and try to set up the desired, stabilized descent rate. Or maybe VS for a descent to MDA on a non-approach(visual or instrument) where for some reason, vertical nav is unavailable.

VS is precise. FLCH is not very precise (more suitable to a situation such as a descent from ten to five thousand when VNAV is no longer desired and you don't care if the descent is at 2000fpm or 2500 fpm. FLCH is typically used once on vectors for the approach and VNAV is no longer desired. Frequently, you are brought in a bit high by ATC. Therefore, FLCH used on a lot of flights during descent once VNAV is no longer needed as in most cases it quickly brings the thrust to idle. Then you can decide if you need speedbrakes.

VS used much less often in my experience. Maybe if you are low too early, you might use it to reduce the rate of descent. Or one might want to reduce a high descent or climb rate when approaching a level off altitude where there is proximate traffic as this reduces closure rate preventing a TA or perhaps even an RA(select VS at 1000fpm).

[maui]

Punkalouver

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

Punka, thanks for your input, we are on the same page. Further to your explanations and to address Uplinkers problem with the concept of half and half; he may not understand that in days of yore when auto throttles had hands and sat behind the pilots that was how it was done. Autopilot would look after path and pilot or FEO would handle the thrust. Not commenting on the efficacy of philosophies, just pointing out that nothing much in this world is out of the ordinary or necessarily weird,

Maui
​​​​​​​

[maui]

TheBusFlyer and Vessbot

In FLCH SPD mode, the rate at which the thrust levers move depends on the difference between the current altitude and the target altitude .The A/T is limited by the thrust limit at the forward range of thrust lever travel and by idle at the back range of travel. During a FLCH descent, HOLD mode will engage when the thrust levers reach the aft stop or if the pilot manually overrides the A/T. During a FLCH climb, HOLD mode will engage only if the pilot manually overrides the A/T. When the HOLD mode engages, the annunciation for the A/T mode will change from “THR” to “HOLD,” and the annunciation will be surrounded by a green box for 10 seconds. The A/T will remain in HOLD mode until one of the following conditions is met:

(1) The airplane reaches the MCP target altitude.

(2) The pilot engages a new AFDS pitch mode or new A/T mode.

(3) The A/T arm switches are turned off

(4) The thrust is manually commanded to increase past the thrust limit.

(5) The A/P is disconnected, and both F/D switches are turned off.

Note: When the pitch mode is FLCH or TOGA, or the airplane is below 400 feet above the airport on takeoff, or below 100 feet radio altitude on approach, the autothrottle will not automatically activate.

An event analysis provided by Honeywell indicated that the accident flight did not meet the criteria to trigger any EGPWS alerts.

The above are quotes from the NTSB report on the analysis of the Asiana 214 Accident in SFO some years ago. I suggest to you that this supports, though not explicitly, my contention that FLCH controls speed and does not compute and call for a particular ROC. Rather it applies thrust and then regulates IAS on elevator to achieve selected altitude and maintains selected IAS. This contention is supported by years of watching how it actually happens.

Vessbot.

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.

In this case the THR will not respond because it is on the limit set by FLCH and it is thinking about getting to altitude, not how rapidly it will get there. As it pitches up THR will increase to the limit, where it will stay until ALT CAP or manual intervention. In VS, different story. But VS is not in discussion.


Vessbott.

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.

It is my contention that issue/possibility has been resolved by Boeing and Honeywell by making FLCH responsible only for thrust and IAS.



Uplinker.

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

Is that referring to the comment in the earlier FairyTale?
If so, perhaps it would be useful for you to delve into the meaning of the expression FAIRY TALE.

Maui

[maui]

Captain Bloggs

The statistics are not in your favour.

How many millions of sectors would 777’s have operated? Approx 2300 aircraft in the fleet and 30+/- years of service. Rough guess, in excess of 20 million. Of those 2300 one came to grief due to engine/fuel component design and abnormal temperature operation (BA), One had a major disassembly after someone got angry (MH). One mishandled a go around (EK). One operated to Nirvana (MH) . My point is, that this is a crazy good aircraft that is able to be successfully operated by pilots of all calibres and backgrounds, with a high degree of success. To my simple mind it must have been well designed. It may not be the newest nor the most technically able but it is a good, effective, bit of kit, when operated in accordance with the instructions on the packet.

If FullWings is correct (and I have no reason to doubt him), the anomaly would appear to be resolved.

But in the retrospect, you have sought to malign a great product, in service for 30 years, designed closer to 40 years ago of which the NTSB says

“… the 777 remains one of the safest aircraft of all time,…”.

And you impugned all the pilots and operators of those aircraft.

There has been an issue with a lack of comprehension of the functionality and control of, some of the systems, but of the over 2300 built and delivered there have been none of the losses that have been attributed to system failures.

The NTSB stated in respect of Asiana 214 ;

3.2 Probable Cause The National Transportation Safety Board determines that the probable cause of this accident was the flight crew’s mismanagementof the airplane’s descent during the visual approach, the pilot flying’s unintended deactivation of automatic airspeed control, the flight crew’s inadequate monitoring of airspeed, and the flight crew’s delayed execution of a go-around after they became aware that the airplane was below acceptable glidepath and airspeed tolerances

Further

I believe setting the stage for the crash was expectancy; the pilot flying expected the airplane to do something that it wasn’t designed to do. Specifically, he expected the autothrottle system to provide speed control for him, but unbeknownst to him, the system would not do so while in a HOLD mode.

And

The NTSB Board members unanimously determined that “the complexities of the autothrottle and autopilot flight director systems that were inadequately described in Boeing’s documentation and Asiana’s pilot training, which increased the likelihood of mode error.”

And

In spite of the unintuitive design feature from a pilot’s perspective, and in spite of the determination that an autothrottle wake up feature would probably have prevented the crash, the majority of the Board voted (3 to 1) against a recommendation proposed by staff and supported vigorously by me to require Boeing to redesign the system.

And

Guidance for Asiana pilots on use of flight directors during a visual approach. During the accident flight, after the A/P was disconnected, the PM loosely followed Asiana’s informal practice, which was to turn both flight directors (F/Ds) off and then turn the PM’s F/D back on when conducting a visual approach. However, the two F/D switches were not both in the off position at the same time. If they had been, the A/T mode would have changed to speed mode and maintained the approach speed of 137 knots. In addition, during a visual approach, F/D pitch and roll guidance is not needed and can be a distraction.

To use the outcome as a reason to s**t can the aircraft and it’s designers is disingenuous.

In my view your attitude in declaring this aircraft and it’s systems as dangerous and unsafe (stupid) is unhelpful, it is not reflective of the facts and importantly, it is dismissive of all it’s crews as you are inferring that all are knowingly operating dangerous equipment in a dangerous manner and abrogating their duty of care to their passengers. In other words you are assigning culpability. It is my view that you are probably not qualified to do so. If you genuinely feel that the aircraft is “stupid” and operation thereof is “unsafe”, you have a moral obligation to make your concerns known, to the relevant authorities. Have you done so? If not, you are part of the problem, not part of the solution.

I would suggest that rather than shouting about stupidity, you should, rather, take a sip of the ‘aide, sit, and quietly reflect on whether or not your intemperance is appropriate or justified. A good read for you while you contemplate your navel, would be the NTSB report on Asiana 214.
Maui

[Vessbot]

Originally Posted by maui

FLCH cannot command a V/s

You make this point a few times and yes it's of course true... for the pitch channel. But we're talking about the throttle channel.

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.

If we're descending and it's in HOLD, then it's not in THR (other than the transient few seconds it's pulling back to HOLD) so there's no question of how it acts in THR. Clearly, the question of how it acts in THR is only meaningful when we're in in that mode (i.e., climbing).

If climbing outside of 125 seconds then yes it's in full noise. Inside of 125 seconds, the FCOM's "THR - The autothrottle applies thrust to maintain the vertical speed required by the pitch mode" seems to apply, and in doing so contradicts your categorical "the THR will not respond because it is on the limit set by FLCH and it is thinking about getting to altitude, not how rapidly it will get there."

[maui]

Vessbot

Is it my use of “locked” that has caused you technical offence? That term is derived from days of old. 727’s and DC9’s had a rotary selector that among other things had an IAS lock. MD80 series had an IAS button which “locked” IAS. From memory, on an MD 11 audit fight I think I might have seen the same. And from memory 747 200 and 300 had a rotary variety of speed control selection. The purpose of these controls was to effectively “lock” in, the existing IAS at the time of selection. For convenience and commonality I have used the same nomenclature and inference. If that caused confusion I apologise.

Vessbot # 31

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 (your interpretation of) 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.

Vessbot #47

You make this point a few times and yes it's of course true... for the pitch channel. But we're talking about the throttle channel.

I was under the impression that we were talking about the functioning and output of the entire box of tricks, as a unit. I would posit that the “throttle channel” has control of exactly that, throttle/thrust. It doesn’t command anything but thrust.

NTSB say this;

In FLCH SPD mode, the rate at which the thrust levers move depends on the difference between the current altitude and the target altitude .The A/T is limited by the thrust limit at the forward range of thrust lever travel and by idle at the back range of travel. During a FLCH descent, HOLD mode will engage when the thrust levers reach the aft stop or if the pilot manually overrides the A/T. During a FLCH climb, HOLD mode will engage only if the pilot manually overrides the A/T. When the HOLD mode engages, the annunciation for the A/T mode will change from “THR” to “HOLD,” and the annunciation will be surrounded by a green box for 10 seconds. The A/T will remain in HOLD mode until one of the following conditions is met:

(1) The airplane reaches the MCP target altitude.

(2) The pilot engages a new AFDS pitch mode or new A/T mode.

(3) The A/T arm switches are turned off

(4) The thrust is manually commanded to increase past the thrust limit.

(5) The A/P is disconnected, and both F/D switches are turned off.

FCOM Section 6. says

The performance database supplies all the necessary performance data to the flight crew. It supplies the FMC with the necessary data to calculate pitch and thrust commands. All necessary data can be shown on the CDU. The database includes: (FCOM 6.11.69)

And

Autothrottle Thrust Lever Operation The autothrottle system moves either or both thrust levers to provide speed or thrust control, depending on the mode engaged.

Vessbot # 31

​​​​​​​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.

As previously stated. It doesn’t because the thrust is already on a limit.

Functionally, the amount of thrust required is computed by the FMC utilising the pilot inputs, the existing environmental factors, and the FMC performance database, and then transmitted to the TLC.

The upshot of all of this is that we have 4 possible conditions to consider. Long descent, Long Climb, Short climb, and short descent.

On the long cases I think you agree, that thrust levers will be at upper or lower limits. On long climb the thrust levers will be at full noise and the FMA will show THR, until ALT capture mode or pilot intervention whereupon it will transition to HOLD. Whilst in THR any negative speed excursion will be rectified by pitch down, no thrust change, cos its already on the stops. Any speed excursion up will be countered by a pitch up to regain “locked speed”. If the pilot has intervened on thrust it will have gone to HOLD, and thrust will remain as set, until further intervention, such as ALT cap or selection of another mode i.e. VS.

Long Descent is handled the same way. THR will reduce the noise until the stops. FMA will show THR initially then HOLD after the noise is minimum. The thrust will remain at min until something intervenes be that ALT cap or an alternate mode selection, or pilot intervention. Any speed variation will be responded to by pitch alone, as the thrust levers are at the back stop and the FMA is showing HOLD.

In both these scenarios, IAS is controlled by pitch. Thrust is at limit. ROC and ROD is a consequence of pitch changes to maintain at “locked” IAS.

So then we come to the short climb or descent. According to the NTSB quote above, the FMC using the current environmental conditions and the pilot commanded action, will determine if this to be a long climb/descent (actions as above) or a short climb/descent (less than or equal to approximately 2 minutes. For the short It will then calculate an appropriate thrust setting, which it will command from TLC. From observation it will only make one command and the live with it for the next minute or so (2minutes minus, initial power adjustment time plus flare time). It will not hunt, it will sit, unless a major upset invokes stall protection. Highly unlikely.

In each of the 4 scenarios the controlling factors are THR and IAS. ROC/ROD would only complicate a simple equation. If you get the hots for a ROC or ROD use VS mode. At all times in the above scenarios, there is no TARGET ROC or ROD. What you see is what you get THR and IAS. When you select FLCH the speed window will open if it is not already. The VS/FPA window will remain closed. If you want to nominate a VS you will have to change modes.

Vessbot I entered this conversation because whilst I see this as a great forum for those desiring to expand knowledge, It is incumbent upon us all to ensure that the knowledge imparted is valid AND useful. I firmly believe that those following us should be afforded useful information unburdened by minutae of limited use. Make a simple concept complicated and you are sure to end up with confusion.

I would be surprised if anything discussed in this thread will result in any reader modifying their mode of operation.

Maui

[Captain Sherm]

The B777 was designed to be operated by two qualified, experienced, well trained and current pilots in the two front seats. Pilots following SOPs (and not modifying them with following a bit of hangar talk or "it seemed a good idea at the time your Honour quick fix.) Pilots watching, one watching the aeroplane, the other watching the PF, the TMA and the various flight path parameter indications. If the preceding conditions are not met then there may well be tears. Little has changed in aviation.

It is true that to deeply understand the pitch, speed and thrust systems and modes takes a bit of delving. Hardly tough duty. Your airline SOPs as practiced in the sim and LOE will probably cover the bases. Nevertheless it is true that the B777 Systems FCOM is a bit thin. But a bit of Sherlock Holmes stuff can unlock a lot of secrets. Your airline will probably have an in-house Boeing rep with all the deeper books. None of the "secrets" wiill be life-changing but will add a bit of gloss to your life as a pilot and avoid splinters in the bum. Good airmanship does rest on a bit of effort to build knowledge. How many people really did understand, (or remember) as the Emirates crew found out sadly in DBX on a rejected landing, that the A/T ceases to be active at 2 ft RA. Otherwise you could never land. Problem is if you reject the landing below 2 ft then pressing the TOGA buttons will not make the thrust levers active, You actually need, as you should have been trained and every pilot back to the Wright Brothers was trained, to push the bloody thrust levers forward as you announce "Going Around". Or if you are, probably unwisely, going to cancel a rejected landing after initiation, then remember that using the thrust levers forward disarms the auto-spoiler and auto brakes. Think QF1 at BKK. Knowledge is key.

(pause to think back to the days when you practically had to be able to build an aeroplane to get a type rating.)

Having said all of the above there are probably a few things the manufacturer could put in place to avoid traps. Just as not that many years ago the ILS arm/capture logic functions were changed to prevent G/S capture before LOC capture. Thumbs up. And the entry fields in the FMS were altered to prevent entry of ZFW in the TOW line. Thumbs up. Though its been a while since an aging Sherm had the privilege of commanding a B777 I think a good case could be mounted for preventing the use of FLCH below, as 3,000 ft on approach. Years back I recall an incident in Milan where a 744 crew, a bit high, selected FLCH on a NPA after crossing the FAF, Not a good idea.

The B777 is a wonderful aeroplane. And it will pretty much always tell you exactly what it's doing or soon will do. But just as no-one should ignore smoke in a dynamite factory, or a dying canary in a coal mine, no-one should ignore it when a sophisticated MCP and Thrust Mode Annunicator tells you exactly what's happening or will soon happen. And most important of all, if you are in any doubt, go back up there further into the sky where it's safe.

[Uplinker]

Originally Posted by maui

Uplinker.

Is that referring to the comment in the earlier FairyTale?
If so, perhaps it would be useful for you to delve into the meaning of the expression FAIRY TALE.

Maui

We seem to have touched a nerve ! but no need to be quite so aggressive.

As I have said, I am not B777 type rated, (I am Airbus FBW rated). I flew B737 Classics for a season, and have flown a B777 SIM for half an hour. So I don't know how it works, and my comment about HOLD you have quoted is not my knowledge but refers to the fourth point down on the poster I saw in that B777 SIM briefing room - shown in post #12.

I don't know who wrote and published that poster, but they obviously had good reason to do so to remind their fleet pilots about certain B777 auto-thrust modes and traps. It was only just over a year ago that I saw that poster fixed to the wall - and it also refers to the B787, so I assumed it was still current advice.

I might be recalling it wrongly, but reading the report of the B777 crash at KSFO - caused by truly abysmal piloting by both PF and PM, (despite the jump-seat F/O trying to warn about the decaying speed) :- was, as I understood it, not helped by the auto-thrust remaining in HOLD and not doing anything to stop the speed decay ?

I will refresh my memory of that crash and also read all your subsequent posts about how the B777 auto-thrust works, when I get a chance.

[Uplinker]

I have read the NTSB report on Asiana flight 214 that crashed at KSFO, which includes a good technical description of the B777 auto-thrust system.

(My bold throughout)

The PF disconnected the A/P and moved the thrust levers to idle, which caused the autothrottle (A/T) to change to the HOLD mode, a mode in which the A/T does not control airspeed. The PF then pitched the airplane down and increased the descent rate. Neither the PF, the pilot monitoring (PM), nor the observer noted the change in A/T mode to HOLD.

The safety issues discussed in the report relate to the need for the following:

 Adherence of Asiana pilots to standard operating procedures (SOP) regarding callouts. The flight crew did not consistently adhere to Asiana’s SOPs involving selections and callouts pertaining to the autoflight system’s mode control panel. This lack of adherence is likely the reason that the PF did not call out “flight level change” when he selected FLCH SPD. As a result, and because the PM’s attention was likely on changing the flap setting at that time, the PM did not notice that FLCH SPD was engaged.

 Reduced design complexity and enhanced training on the airplane’s autoflight system. The PF had an inaccurate understanding of how the Boeing 777 A/P and A/T systems interact to control airspeed in FLCH SPD mode, what happens when the A/T is overridden and the throttles transition to HOLD in a FLCH SPD descent, and how the A/T automatic engagement feature operates. The PF’s faulty mental model of the airplane’s automation logic led to his inadvertent deactivation of automatic airspeed control. Both reduced design complexity and improved systems training can help reduce the type of error that the PF made.

The autothrottle can support stall protection when armed and not activated. If speed decreases to near stick shaker activation, the autothrottle automatically activates in the appropriate mode (SPD or THR REF) and advances thrust to maintain minimum maneuvering speed (approximately the top of the amber band) or the speed set in the mode control speed window, whichever is greater. The EICAS message AIRSPEED LOW displays.

Note: When the pitch mode is FLCH or TOGA, or the airplane is below 400 feet above the airport on takeoff, or below 100 feet radio altitude on approach, the autothrottle will not automatically activate

Note: During a descent in VNAV SPD, the autothrottle may activate in HOLD mode and will not support stall protection.

Obviously I am not an aircraft designer or aircraft systems designer, but the terminology still seems slightly ambiguous to me, because it will annunciate 'HOLD' even at idle thrust. Since there are buttons on the MCP to select HOLD of the Heading and HOLD of the Altitude, the word 'HOLD' would seem to suggest the automatics are actively modulating some parameter in order to hold it, i.e. maintain it. However, as I understand it; 'HOLD' as applied to auto-thrust is simply where the back-driving motors are just switched off and the thrust levers stay wherever they last were - including flight idle.

I personally think that 'IDLE' or 'THR IDLE' - ideally in pulsating amber - on the PFD mode annunciators would be a much more informative caption for better crew awareness whenever the thrust levers were at the flight idle stop, no matter how they got there.

Interesting.
.