Shep69
2025-06-20T18:37:00
permalink Post: 11907194

Quote:

Originally Posted by nachtmusak

How does a non-faulty autothrottle pull power to idle at the critical time we are witnessing (i.e. just before or just after takeoff)?

I don't mean to keep asking this, but I still haven't gotten an answer. Unless the allegation is that the crew did not arm the autothrottle properly, it just doesn't make sense to me.

An auththrottle malfunction makes as much sense as anything else at this point. But even if not I wonder if the low altitude capture scenario would dramatically pull off power initially assuming the aircraft was going to level off before restoring it as the aircraft slowed down. Didn`t fly the 78 but on the 777 the auththrottles were laggy sometimes.

Subjects: None

Shep69
2025-06-20T19:23:00
permalink Post: 11907231

Quote:

Originally Posted by nachtmusak

I am specifically asking about the case without a malfunction because my understanding is that the autothrottle explicitly does not affect thrust between passing 80 knots on the runway and passing 400 ft AGL.

Neither does (or should?) any autopilot mode (except LNAV? at 50 feet?) be active under 400 feet. Armed yes but not actually engaged.

So how would either of these things have any effect on an aircraft that's just lifting off unless they're faulty? Everything I can turn up seems very clear that the autothrottle, VNAV, etc only engage at 400 feet AGL on departure. In fact, both altitude capture incidents I've been able to turn up were only triggered when the autopilot was engaged. Why would any pilot be engaging the autopilot bare seconds after lift-off?

As has been gone over before, the auththrottles normally stay in HOLD until 400 AGL at which point they engage in THR REF xxx VNAV SPD to whatever value was programmed in the RTOW.

BUT

Say the MCP is mis-set to 200’ or so. VNAV never engages and the expected mode (from others; I`ve never tried it) would be SPD xxx ALT.

The modes still engage regardless of whether or not the airplane is on autopilot; the autopilot itself just follows the flight director commands (and the PF certainly woudn`t follow the FD in this case). Autothrottles are independent of autopilot.

And the autothrottles would initially be expected to reduce thrust to whatever value the squirrels think is appropriate to level flight. They WILL advance (how fast I don`t know) to maintain speed on auto throttle but this would be confusing. That`s the scenario I`m wondering about as well as a failure of the auththrottles themselves; it`s as plausible as anything else they engaged to reduce thrust for some wild reason.

Subjects: VNAV

Shep69
2025-06-20T21:52:00
permalink Post: 11907348

Quote:

Originally Posted by nachtmusak

To be clear, are you stating categorically or guessing (or neither, and I'm misreading you and you mean something else) that the 787's autothrottle will come out of HOLD and into SPD mode below 400 feet by design in response to the altitude that's set in the mode control panel, entirely by itself, without the autopilot engaged? Not that this is the way it would behave in normal flight - that this is the way it is designed to behave while it's in HOLD mode for takeoff?

I am actually looking for an answer from a pilot (or at least someone with an FCOM who can share any relevant passages) because nothing I have been able to look up on my own suggests that this is the case. All secondary sources I can find just say that the autothrottle is inhibited under 400 feet on takeoff, with my impression being that the crew is expected to manage thrust manually during that phase of flight. The incidents I was able to turn up involving an aircraft attempting to capture a target altitude at takeoff specifically don't help either:

- G-ECOE: Flight was completely normal until the crew engaged the autopilot at 1350 feet, at which point the aircraft started diving to the target of 0 feet. However it involved a Dash 8 and not only is that not a Boeing aircraft, it doesn't have an autothrottle to begin with. Nothing to conclude from this.

- F-WWKH: Again automated deviation was triggered by the crew engaging the autopilot a few seconds after takeoff as part of a test flight. However again also not a Boeing aircraft (an A330) and the selected target altitude was 2000 feet so the autopilot tried to pitch up to capture it. Not sure if anything can be concluded from this.

- A6-EQI: The most relevant, being a Boeing 777, with wide speculation being that the selected altitude was left at 0 feet. However the preliminary report is very thin so there's little to go on in the way of factual information, but the problem seems to entirely have been that the pilot flying was following the flight directors without question. There's zero indication of any loss of thrust, in fact they seem to have nearly entered an overspeed condition partly due to the shallow climb angle implying that the engines were doing just fine. So it doesn't seem like the selected altitude caused the autothrottle to do anything.

I am sorry if it seems like I'm banging on about this autothrottle point a bit much but as an engineer it just seems completely backwards to me. What exactly is the point of the HOLD mode or of setting those specific gates (80 knots, 400 feet) if the autothrottle can so easily come out of it? The design might as well not have it at all and just leave the autothrottle in THR REF for takeoff then - what would be the difference?

The autopilot would NOT be engaged below 400’ (or 200’ in the 78–although I doubt anyone engages it that low. The autopilot and autothrottles are separate systems but do interact. The autothrottles typically WOULD be engaged from the start of the takeoff roll; using the TOGA levers to set takeoff thrust).

I am guessing because although I flew the 777 I never tried a low altitude capture before VNAV engaged — and it`s been a few years). But think it probably would. As one goes through 50’ LNAV engages; VNAV is normally armed prior to the EFIS check if it`s to be used (which it usually is). So in this scenario LNAV would have been engaged but since VNAV is armed but never engages my guess is that the automatics would engage in SPD/LVAV/ALT. I could be wrong. The PF would have been hand flying (and obviously not following the flight director with autothrottles engaged).

HOLD is present in many other regimes of flight; all it means is that the auththrottle (right now) is not controlling the throttles and they stay where they are—and the PF can move them if desired at will. Fr` instance, when descending in FLCH or even VNAV SPD the throttles will usually be in HOLD. (To me this usually meant `hold` the throttles—and tweek them in descent as required). Thrust can be modulated to adjust rate of descent (the throttles become vertical speed levers). On altitude capture in the case of FLCH or path capture in the case of VNAV SPD (in descent) the auththrottles kick in and it becomes SPD/xxx/ALT (or VPTH or VALT as the case might be).

Most everyone knew the autothrottles would not engage below 400` and that FLCH in descent at very low altitudes was not an appropriate mode — and they did not activate providing low speed protection in the case of Asiana.

IIRC our throttles went into HOLD at 60 knots and stayed there until VNAV activated (THR REF—takeoff thrust). It was also possible that the autothrottles under some environmentals wouldn`t fully achieve takeoff thrust setting (EPR or N1 depending on which engines) and they could be manually moved in HOLD to achieve it. Although I don`t remember that as ever happening.

But at this point it`s a guess because I never did it (MCP set at low altitude on takeoff with VNAV never engaging). Perhaps someone else has.

Last edited by Shep69; 20th Jun 2025 at 22:05 .

Subjects: FCOM Preliminary Report TOGA Takeoff Roll VNAV

Shep69
2025-06-20T22:29:00
permalink Post: 11907366

Quote:

Originally Posted by nachtmusak

I suppose an informed guess is the best answer I can get here, short of someone having access to an actual sim or a Boeing engineer chipping in. Though I am curious that you are guessing that it would engage, when in the example you give to point out the autothrottle being inhibited at low altitudes (the Asiana accident at SFO) the autothrottle didn't engage - not even to provide low speed protection, a potentially life-and-death matter. If that accident was my only datapoint my guess would be that if it's strict enough to not engage for stall protection, it's strict enough to not engage for altitude capture.

Also to be clear I do know that the autopilot and autothrottle are independent - I have been talking about the autopilot because as I listed, in the incidents I could find where the aircraft automatically tried to capture a target altitude on takeoff, the autopilot was first engaged. So my impression was that until then the aircraft might provide guidance (like in the Emirates case) but will not actually do anything to change the thrust, pitch, etc parameters that have been set.

FWIW, They`re actually somewhat different scenarios as far as the auththrottle behavior situation. In FLCH below 400` in a descent, they are deliberately inhibited (to include low speed protection) because the jet thinks you`re landing. And very clear in the FCOM.

Subjects: FCOM Parameters

Shep69
2025-06-20T22:59:00
permalink Post: 11907385

Quote:

Originally Posted by M.Mouse

Both the B777 and B787 have a minimum autopilot engage height of 200'. I would suggest it is often the best course of action after an engine failure to engage the autopilot at 200' to dramatically reduce the workload!

When talking about low altitude capture below the 400' VNAV engagement height, although it has been talked about, would mean (mis) setting the MCP altitude to less than 400'. I really find that implausible. You are correct though that the modes would be SPD | LNAV | ALT should this occur.

All correct.

The lack of autothrottle automatic activation when the autothrottles are in HOLD mode and the speed has decayed below minimum manouevring speed has been addressed. The autothrottle will now automatically activate if, when in HOLD mode, the speed decays below minimum manouevring speed with FMC software after BP4 on the B787 and after AIMS V17B on the B777.

Since the Asiana accident many airlines prohibit use of FLCH below 1,000' and the lowest altitude setting when using FLCH is then, of course, 1.000'

Almost fully correct, the speed when autothrottles transition to HOLD on takeoff is 80kts. on both the B777 and B787.

Thanks much; been a few years with other A/C in between.

Subjects: Engine Failure (All) VNAV

1 user liked this post.

Shep69
2025-06-21T18:58:00
permalink Post: 11907986

Quote:

Originally Posted by Capn Bloggs

I would take that post by Crossky with a grain of salt. No part of his post made sense and I can only assume he is not a 787 pilot despite claiming to be. "Fuel starvation if pumps aren't turned off, not in my manual but I read about a procedure on the Internet", it's loony stuff.

.

My understanding of the 78 fuel system is it\x92s very similar to the 777; assuming center tank fuel all pumps are turned on for takeoff (the center tank override pumps are at higher pressure than wings so it feeds first). If all electrical power is lost at lower altitudes engines suction feed just fine (probably from the wing tanks).

Subjects: Centre Tank Fuel (All) Fuel Cutoff Fuel Pumps

Shep69
2025-06-22T13:13:00
permalink Post: 11908531

Quote:

Originally Posted by adfad

There is a bigger issue here. The general public are more and more concerned that Boeing is cutting corners, and perhaps that is debatable and a complex balance of "how much do you want to pay for a plane ticket" vs "how absurdly close to diminishing returns can statistical probability get when dealing with some of the most complex machines and industries humanity has ever created".

What isn't debatable is that MCAS was part of an initiative to save on regulatory and training costs. It was designed entirely because a regulatory environment existed where you could extend the fuselage to the point where you needed to mount engines in a way that would essentially make this no longer the same type, and probably not something that any aircraft designer would design fresh. Boeing, like everyone else, played within that framework, but Boeing didn't execute properly and the public optics are that they cut corners on top of cutting corners.

I agree that engineering is all about tradeoffs but I don't think anything is "no longer an issue" because we had 2 disasters within a couple of years and learned from mistakes. There is an issue somewhere, maybe a systemic corporate issue, a PR issue, or just a Boeing issue - the Air India crash has the potential to make it far bigger.

I do not agree at all that MCAS was \x91not debatable.\x92 It was required for certification of a stretched type and probably not something Boeing wanted to install at all (there are plenty of aircraft with less than desirable stall characteristics mitigated by staying out of that regime). The \x91regulation happy\x92 approach is often counterproductive and hangs boxes on airplanes that have hidden traps. Without necessarily mitigating risks (like those idiotic seat belt dingers on cars or auto-start stop. If my seat belt isn\x92t on there\x92s a damn good reason for it and I don\x92t want a distracting and uncancelable alarm).

I also think that saying \x91cutting corners\x92 is an unjustified indictment. They didn\x92t IMHO. What WAS wrong was not making it clear how the system operated and that it could be triggered by a single AOA probe failure. And making it clear that the crew handles it just like any other runaway trim scenario. So to me it was mostly a training issue.

IF \x97 and it\x92s a big IF \x97 this accident was caused by an FAA requirement to automatically cut engines with engine runaway due unresponsive throttle (which isn\x92t needed in the first place with FCS and fire handles available) then the primary culpability lay with the regulatory entity (FAA) requiring more silly boxes on airplanes. Without zero thought towards unintended consequences.

But at this point it\x92s wayyyyyyy too early to do any Boeing Bashing based on an accident none of us have any clue as to the cause.

Subjects: FAA

6 users liked this post.

Shep69 2025-06-20T18:37:00 permalink Post: 11907194	Quote: Originally Posted by nachtmusak How does a non-faulty autothrottle pull power to idle at the critical time we are witnessing (i.e. just before or just after takeoff)? I don't mean to keep asking this, but I still haven't gotten an answer. Unless the allegation is that the crew did not arm the autothrottle properly, it just doesn't make sense to me. An auththrottle malfunction makes as much sense as anything else at this point. But even if not I wonder if the low altitude capture scenario would dramatically pull off power initially assuming the aircraft was going to level off before restoring it as the aircraft slowed down. Didn`t fly the 78 but on the 777 the auththrottles were laggy sometimes. Subjects: None
Shep69 2025-06-20T19:23:00 permalink Post: 11907231	Quote: Originally Posted by nachtmusak I am specifically asking about the case without a malfunction because my understanding is that the autothrottle explicitly does not affect thrust between passing 80 knots on the runway and passing 400 ft AGL. Neither does (or should?) any autopilot mode (except LNAV? at 50 feet?) be active under 400 feet. Armed yes but not actually engaged. So how would either of these things have any effect on an aircraft that's just lifting off unless they're faulty? Everything I can turn up seems very clear that the autothrottle, VNAV, etc only engage at 400 feet AGL on departure. In fact, both altitude capture incidents I've been able to turn up were only triggered when the autopilot was engaged. Why would any pilot be engaging the autopilot bare seconds after lift-off? As has been gone over before, the auththrottles normally stay in HOLD until 400 AGL at which point they engage in THR REF xxx VNAV SPD to whatever value was programmed in the RTOW. BUT Say the MCP is mis-set to 200’ or so. VNAV never engages and the expected mode (from others; I`ve never tried it) would be SPD xxx ALT. The modes still engage regardless of whether or not the airplane is on autopilot; the autopilot itself just follows the flight director commands (and the PF certainly woudn`t follow the FD in this case). Autothrottles are independent of autopilot. And the autothrottles would initially be expected to reduce thrust to whatever value the squirrels think is appropriate to level flight. They WILL advance (how fast I don`t know) to maintain speed on auto throttle but this would be confusing. That`s the scenario I`m wondering about as well as a failure of the auththrottles themselves; it`s as plausible as anything else they engaged to reduce thrust for some wild reason. Subjects: VNAV
Shep69 2025-06-20T21:52:00 permalink Post: 11907348	Quote: Originally Posted by nachtmusak To be clear, are you stating categorically or guessing (or neither, and I'm misreading you and you mean something else) that the 787's autothrottle will come out of HOLD and into SPD mode below 400 feet by design in response to the altitude that's set in the mode control panel, entirely by itself, without the autopilot engaged? Not that this is the way it would behave in normal flight - that this is the way it is designed to behave while it's in HOLD mode for takeoff? I am actually looking for an answer from a pilot (or at least someone with an FCOM who can share any relevant passages) because nothing I have been able to look up on my own suggests that this is the case. All secondary sources I can find just say that the autothrottle is inhibited under 400 feet on takeoff, with my impression being that the crew is expected to manage thrust manually during that phase of flight. The incidents I was able to turn up involving an aircraft attempting to capture a target altitude at takeoff specifically don't help either: - G-ECOE: Flight was completely normal until the crew engaged the autopilot at 1350 feet, at which point the aircraft started diving to the target of 0 feet. However it involved a Dash 8 and not only is that not a Boeing aircraft, it doesn't have an autothrottle to begin with. Nothing to conclude from this. - F-WWKH: Again automated deviation was triggered by the crew engaging the autopilot a few seconds after takeoff as part of a test flight. However again also not a Boeing aircraft (an A330) and the selected target altitude was 2000 feet so the autopilot tried to pitch up to capture it. Not sure if anything can be concluded from this. - A6-EQI: The most relevant, being a Boeing 777, with wide speculation being that the selected altitude was left at 0 feet. However the preliminary report is very thin so there's little to go on in the way of factual information, but the problem seems to entirely have been that the pilot flying was following the flight directors without question. There's zero indication of any loss of thrust, in fact they seem to have nearly entered an overspeed condition partly due to the shallow climb angle implying that the engines were doing just fine. So it doesn't seem like the selected altitude caused the autothrottle to do anything. I am sorry if it seems like I'm banging on about this autothrottle point a bit much but as an engineer it just seems completely backwards to me. What exactly is the point of the HOLD mode or of setting those specific gates (80 knots, 400 feet) if the autothrottle can so easily come out of it? The design might as well not have it at all and just leave the autothrottle in THR REF for takeoff then - what would be the difference? The autopilot would NOT be engaged below 400’ (or 200’ in the 78–although I doubt anyone engages it that low. The autopilot and autothrottles are separate systems but do interact. The autothrottles typically WOULD be engaged from the start of the takeoff roll; using the TOGA levers to set takeoff thrust). I am guessing because although I flew the 777 I never tried a low altitude capture before VNAV engaged — and it`s been a few years). But think it probably would. As one goes through 50’ LNAV engages; VNAV is normally armed prior to the EFIS check if it`s to be used (which it usually is). So in this scenario LNAV would have been engaged but since VNAV is armed but never engages my guess is that the automatics would engage in SPD/LVAV/ALT. I could be wrong. The PF would have been hand flying (and obviously not following the flight director with autothrottles engaged). HOLD is present in many other regimes of flight; all it means is that the auththrottle (right now) is not controlling the throttles and they stay where they are—and the PF can move them if desired at will. Fr` instance, when descending in FLCH or even VNAV SPD the throttles will usually be in HOLD. (To me this usually meant `hold` the throttles—and tweek them in descent as required). Thrust can be modulated to adjust rate of descent (the throttles become vertical speed levers). On altitude capture in the case of FLCH or path capture in the case of VNAV SPD (in descent) the auththrottles kick in and it becomes SPD/xxx/ALT (or VPTH or VALT as the case might be). Most everyone knew the autothrottles would not engage below 400` and that FLCH in descent at very low altitudes was not an appropriate mode — and they did not activate providing low speed protection in the case of Asiana. IIRC our throttles went into HOLD at 60 knots and stayed there until VNAV activated (THR REF—takeoff thrust). It was also possible that the autothrottles under some environmentals wouldn`t fully achieve takeoff thrust setting (EPR or N1 depending on which engines) and they could be manually moved in HOLD to achieve it. Although I don`t remember that as ever happening. But at this point it`s a guess because I never did it (MCP set at low altitude on takeoff with VNAV never engaging). Perhaps someone else has. Last edited by Shep69; 20th Jun 2025 at 22:05 . Subjects: FCOM Preliminary Report TOGA Takeoff Roll VNAV
Shep69 2025-06-20T22:29:00 permalink Post: 11907366	Quote: Originally Posted by nachtmusak I suppose an informed guess is the best answer I can get here, short of someone having access to an actual sim or a Boeing engineer chipping in. Though I am curious that you are guessing that it would engage, when in the example you give to point out the autothrottle being inhibited at low altitudes (the Asiana accident at SFO) the autothrottle didn't engage - not even to provide low speed protection, a potentially life-and-death matter. If that accident was my only datapoint my guess would be that if it's strict enough to not engage for stall protection, it's strict enough to not engage for altitude capture. Also to be clear I do know that the autopilot and autothrottle are independent - I have been talking about the autopilot because as I listed, in the incidents I could find where the aircraft automatically tried to capture a target altitude on takeoff, the autopilot was first engaged. So my impression was that until then the aircraft might provide guidance (like in the Emirates case) but will not actually do anything to change the thrust, pitch, etc parameters that have been set. FWIW, They`re actually somewhat different scenarios as far as the auththrottle behavior situation. In FLCH below 400` in a descent, they are deliberately inhibited (to include low speed protection) because the jet thinks you`re landing. And very clear in the FCOM. Subjects: FCOM Parameters
Shep69 2025-06-20T22:59:00 permalink Post: 11907385	Quote: Originally Posted by M.Mouse Both the B777 and B787 have a minimum autopilot engage height of 200'. I would suggest it is often the best course of action after an engine failure to engage the autopilot at 200' to dramatically reduce the workload! When talking about low altitude capture below the 400' VNAV engagement height, although it has been talked about, would mean (mis) setting the MCP altitude to less than 400'. I really find that implausible. You are correct though that the modes would be SPD \| LNAV \| ALT should this occur. All correct. The lack of autothrottle automatic activation when the autothrottles are in HOLD mode and the speed has decayed below minimum manouevring speed has been addressed. The autothrottle will now automatically activate if, when in HOLD mode, the speed decays below minimum manouevring speed with FMC software after BP4 on the B787 and after AIMS V17B on the B777. Since the Asiana accident many airlines prohibit use of FLCH below 1,000' and the lowest altitude setting when using FLCH is then, of course, 1.000' Almost fully correct, the speed when autothrottles transition to HOLD on takeoff is 80kts. on both the B777 and B787. Thanks much; been a few years with other A/C in between. Subjects: Engine Failure (All) VNAV 1 user liked this post.
Shep69 2025-06-21T18:58:00 permalink Post: 11907986	Quote: Originally Posted by Capn Bloggs I would take that post by Crossky with a grain of salt. No part of his post made sense and I can only assume he is not a 787 pilot despite claiming to be. "Fuel starvation if pumps aren't turned off, not in my manual but I read about a procedure on the Internet", it's loony stuff. . My understanding of the 78 fuel system is it\x92s very similar to the 777; assuming center tank fuel all pumps are turned on for takeoff (the center tank override pumps are at higher pressure than wings so it feeds first). If all electrical power is lost at lower altitudes engines suction feed just fine (probably from the wing tanks). Subjects: Centre Tank Fuel (All) Fuel Cutoff Fuel Pumps
Shep69 2025-06-22T13:13:00 permalink Post: 11908531	Quote: Originally Posted by adfad There is a bigger issue here. The general public are more and more concerned that Boeing is cutting corners, and perhaps that is debatable and a complex balance of "how much do you want to pay for a plane ticket" vs "how absurdly close to diminishing returns can statistical probability get when dealing with some of the most complex machines and industries humanity has ever created". What isn't debatable is that MCAS was part of an initiative to save on regulatory and training costs. It was designed entirely because a regulatory environment existed where you could extend the fuselage to the point where you needed to mount engines in a way that would essentially make this no longer the same type, and probably not something that any aircraft designer would design fresh. Boeing, like everyone else, played within that framework, but Boeing didn't execute properly and the public optics are that they cut corners on top of cutting corners. I agree that engineering is all about tradeoffs but I don't think anything is "no longer an issue" because we had 2 disasters within a couple of years and learned from mistakes. There is an issue somewhere, maybe a systemic corporate issue, a PR issue, or just a Boeing issue - the Air India crash has the potential to make it far bigger. I do not agree at all that MCAS was \x91not debatable.\x92 It was required for certification of a stretched type and probably not something Boeing wanted to install at all (there are plenty of aircraft with less than desirable stall characteristics mitigated by staying out of that regime). The \x91regulation happy\x92 approach is often counterproductive and hangs boxes on airplanes that have hidden traps. Without necessarily mitigating risks (like those idiotic seat belt dingers on cars or auto-start stop. If my seat belt isn\x92t on there\x92s a damn good reason for it and I don\x92t want a distracting and uncancelable alarm). I also think that saying \x91cutting corners\x92 is an unjustified indictment. They didn\x92t IMHO. What WAS wrong was not making it clear how the system operated and that it could be triggered by a single AOA probe failure. And making it clear that the crew handles it just like any other runaway trim scenario. So to me it was mostly a training issue. IF \x97 and it\x92s a big IF \x97 this accident was caused by an FAA requirement to automatically cut engines with engine runaway due unresponsive throttle (which isn\x92t needed in the first place with FCS and fire handles available) then the primary culpability lay with the regulatory entity (FAA) requiring more silly boxes on airplanes. Without zero thought towards unintended consequences. But at this point it\x92s wayyyyyyy too early to do any Boeing Bashing based on an accident none of us have any clue as to the cause. Subjects: FAA 6 users liked this post.

Posts by user "Shep69" [Posts: 27 Total up-votes: 27 Pages: 2]