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| Shep69
2025-06-20T21:52:00 permalink Post: 11907348 |
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? 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 . |
| nachtmusak
2025-06-20T22:24:00 permalink Post: 11907362 |
The autopilot would NOT be engaged below 400\x92 (or 200\x92 in the 78\x96although 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 \x97 and it`s been a few years). But think it probably would. As one goes through 50\x92 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\x97and 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\x97and 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 \x97 and they did not activate providing low speed protection in the case of Asiana. 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. 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. 1 user liked this post. |
| M.Mouse
2025-06-20T22:29:00 permalink Post: 11907367 |
he 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.
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.
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'
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.
2 users liked this post. |
| GroundedSpanner
2025-06-21T15:24:00 permalink Post: 11907841 |
Resubmitting following some Mod Feedback and a significant re-write. Yes, it is speculative
I have a theory that I'd like to share. It brings together various pieces of known information, along with 30+ years of my experience as an aircraft engineer that forms a plausible (IMO) explanation of what may have happened. We Know - From the Video's and the ADSB Data: That up to and for the first few seconds after take-off appears relatively normal. The AC appears to lose thrust without e.g. birdstrike or other spectacular smoke /fire producing event. That the RAT deployed. That the pilot reported 'Thrust not achieved' [Edit - We dont 'know' this - it is heavily reported] We can see that the AC had a relatively busy schedule in the few days prior to the accident flight, so there was no significant downtime for maintenance activities that could have caused incident. The AC flew DEL-CDG on 11 Jun with quite a racy turnaround in CDG of 1h12m. The centre tank would have been empty at CDG on arrival, and would have been partially filled for the return CDG-DEL. CDG-DEL Arrived 01:47 am IST. Again the Centre Tank would have been empty. But quite a bit of fuel in the wings. 8 Hrs later, at 09:48 am IST the AC departed DEL-AMD. For such a short-hop, Fuel upload would have been minimal, merely a 'topping up' if at all. Certainly nothing into the Centre Tank. DEL That night was fairly hot and humid - 57% at 02:30, 54% at 05:30, 44% at 08:30. That wing tank fuel could have picked up a fair amount of water. The flight DEL-AMD would have only used the wing pumps. Thus any water in that 'overnight' fuel would have been vigorously stirred and evenly suspended. At concentrations that would cause no ill-effect at all. The AC was on the ground at AMD for 2 Hrs, from 11:17am to 1:17 pm IST. The AC would have re-fuelled, first filling up the wing tanks to the top, then filling the centre-tank to whatever quantity necessary. There was enough time for water in the wing tanks to settle out. The B787 Fuel system has pumps in the wing tanks, and pumps in the centre tanks. The Centre Tank pumps are also known as 'override' pumps because they output a higher pressure than the wing tank pumps, thus ensuring that with all pumps running, the centre tank fuel is used first. Should the centre tank pumps stop, due to either filure or running out of fuel to pump, the wing tank pumps then produce the pressure. In the event that all pumps stop running (e.g. an electrical failure), the engines will suck the fuel from the wing tanks. The 'sucked' fuel comes from a dedicated pipe in each tank through the 'Suction Feed Check Valve' (so that pumped fuel doesn't just exit through the suction tube). The suction tube pickup is in a slightly different position to the wing pump pickups. It is conceivable to me that the suction tube pickup could have been immersed in water, settled out from the fuel in the wing tanks. Then - at start-up of the aircraft in AMD, The engines would have been supplied with fuel from the centre tank. Fresh Fuel. All OK. Wing pumps running and doing not much. But, I speculate, the suction pick-ups immersed in water. Waiting. Start up and taxi out was all normal. Runway acceleration up to v1 appears normal. V1 - Rotate - (positive rate - Gear up? - Not my debate). But somewhere around that time, I speculate that a significant electrical failure occurred. Enough for the RAT to deploy. Enough for the fuel pumps to stop. I'll not speculate on the cause. We know that it can occur, that's why the RAT was designed to operate. The engines at that point were at TOGA thrust. In a significant electrical failure, the engines will keep on doing what they were last told. Keep that thrust stable. So the AC climbed for a few seconds more. The pilots did what they were trained to do for a power failure, manage that, thankfully the engines were still going well... But there was only so much 'good' fuel in the lines. The engines sucking fuel themselves, the fuel would now be coming from the suction pickups, a different supply. A supply likely heavily water contaminated. It would take a few seconds for that contaminated fuel to actually reach the engines, but when that contaminated fuel hit, Thrust would have been significantly reduced. The EEC's would have been doing their best to maintain the thrust, firewalling the throttles would probably have little effect at that exact moment. The engines would have likely worked through that bad fuel in a shortish period of time, but a period of time that our crew did not have. A fully loaded aircraft producing less than take-off thrust, is not sustaining enough thrust for continued flight. The rest - is down to the skill of the crew in deciding exactly where to hit the ground within the very narrow range of choice they had. Last edited by GroundedSpanner; 21st Jun 2025 at 17:52 . Reason: Thrust not achieved comment is not proven. 14 users liked this post. |
| violator
2025-06-21T15:42:00 permalink Post: 11907854 |
Resubmitting following some Mod Feedback and a significant re-write. Yes, it is speculative
I have a theory that I'd like to share. It brings together various pieces of known information, along with 30+ years of my experience as an aircraft engineer that forms a plausible (IMO) explanation of what may have happened. We Know - From the Video's and the ADSB Data: That up to and for the first few seconds after take-off appears relatively normal. The AC appears to lose thrust without e.g. birdstrike or other spectacular smoke /fire producing event. That the RAT deployed. That the pilot reported 'Thrust not achieved' We can see that the AC had a relatively busy schedule in the few days prior to the accident flight, so there was no significant downtime for maintenance activities that could have caused incident. The AC flew DEL-CDG on 11 Jun with quite a racy turnaround in CDG of 1h12m. The centre tank would have been empty at CDG on arrival, and would have been partially filled for the return CDG-DEL. CDG-DEL Arrived 01:47 am IST. Again the Centre Tank would have been empty. But quite a bit of fuel in the wings. 8 Hrs later, at 09:48 am IST the AC departed DEL-AMD. For such a short-hop, Fuel upload would have been minimal, merely a 'topping up' if at all. Certainly nothing into the Centre Tank. DEL That night was fairly hot and humid - 57% at 02:30, 54% at 05:30, 44% at 08:30. That wing tank fuel could have picked up a fair amount of water. The flight DEL-AMD would have only used the wing pumps. Thus any water in that 'overnight' fuel would have been vigorously stirred and evenly suspended. At concentrations that would cause no ill-effect at all. The AC was on the ground at AMD for 2 Hrs, from 11:17am to 1:17 pm IST. The AC would have re-fuelled, first filling up the wing tanks to the top, then filling the centre-tank to whatever quantity necessary. There was enough time for water in the wing tanks to settle out. The B787 Fuel system has pumps in the wing tanks, and pumps in the centre tanks. The Centre Tank pumps are also known as 'override' pumps because they output a higher pressure than the wing tank pumps, thus ensuring that with all pumps running, the centre tank fuel is used first. Should the centre tank pumps stop, due to either filure or running out of fuel to pump, the wing tank pumps then produce the pressure. In the event that all pumps stop running (e.g. an electrical failure), the engines will suck the fuel from the wing tanks. The 'sucked' fuel comes from a dedicated pipe in each tank through the 'Suction Feed Check Valve' (so that pumped fuel doesn't just exit through the suction tube). The suction tube pickup is in a slightly different position to the wing pump pickups. It is conceivable to me that the suction tube pickup could have been immersed in water, settled out from the fuel in the wing tanks. Then - at start-up of the aircraft in AMD, The engines would have been supplied with fuel from the centre tank. Fresh Fuel. All OK. Wing pumps running and doing not much. But, I speculate, the suction pick-ups immersed in water. Waiting. Start up and taxi out was all normal. Runway acceleration up to v1 appears normal. V1 - Rotate - (positive rate - Gear up? - Not my debate). But somewhere around that time, I speculate that a significant electrical failure occurred. Enough for the RAT to deploy. Enough for the fuel pumps to stop. I'll not speculate on the cause. We know that it can occur, that's why the RAT was designed to operate. The engines at that point were at TOGA thrust. In a significant electrical failure, the engines will keep on doing what they were last told. Keep that thrust stable. So the AC climbed for a few seconds more. The pilots did what they were trained to do for a power failure, manage that, thankfully the engines were still going well... But there was only so much 'good' fuel in the lines. The engines sucking fuel themselves, the fuel would now be coming from the suction pickups, a different supply. A supply likely heavily water contaminated. It would take a few seconds for that contaminated fuel to actually reach the engines, but when that contaminated fuel hit, Thrust would have been significantly reduced. The EEC's would have been doing their best to maintain the thrust, firewalling the throttles would probably have little effect at that exact moment. The engines would have likely worked through that bad fuel in a shortish period of time, but a period of time that our crew did not have. A fully loaded aircraft producing less than take-off thrust, is not sustaining enough thrust for continued flight. The rest - is down to the skill of the crew in deciding exactly where to hit the ground within the very narrow range of choice they had. |
| lighttwin2
2025-06-21T15:46:00 permalink Post: 11907858 |
TCMA continues to be one of the few (very unlikely) causes of/contributors to simultaneous shutdown of both engines. So far, though, I don't think we've seen a credible scenario explaining the possibility that TCMA was triggered in this accident. I'm not sure I understand your speculation.
In the scenario you are considering, it's clear that the air/ground state would be wrongly "understood" by the TCMA function. But we don't have, AFAIK , a credible theory for how that might happen. Surely it would have to result from either incorrect signals from the relevant sensors or a failure of the related logic in the FADEC TCMA function, or a combination of those. Indeed, I don't think we yet know exactly which sensor readings that logic depends on or how those readings are fed to the FADEC. Does your speculation include any thoughts about this? Also, the FADEC TCMA function has to "believe" that the engine is operating at high power and not responding to thrust lever operation. In your proposed scenario, is this also a logic failure — in both FADECs? Or false inputs from both TLs? Or are both engines actually operating at higher than commanded power levels? Or do I misunderstand your post?
Q: Would the a/c have enough kinetic energy a 184kts to climb to 100-150ft agl and then reach its final position if the engines had failed at, or just, before rotation? A: Theoretically possible - see calculation here . NB, the a/c actually flew 1.5km from the end of the runway and 2.3km from the likely point of rotation. Q: Doesn't the forward position of the gear mean that power failed after the pilots had selected gear up? A: Inconclusive - had hydraulic power had been lost prior to rotation, the gear could also be in this position - explanation here Q: If the throttle levers were brought to idle during take-off, would the A/C have applied autobrake, reversers and speedbrake? A: Yes, although there is a built in delay before reverser and speedbrake actually deploy - see here . Q: Is the ADS-B data consistent with this scenario? A: Yes, e.g. the Flightradar data shows the aircraft decelerating rapidly (12 knots in 4.2 seconds) from close to rotation. However, it's not clear how accurate this data is. For one, the altitude data is +/- 25 feet, second, while I was under the impression FR would have received airspeed data from the a/c sensors, this post suggests maybe not. Q: Does TCMA activation require the thrust levers to be at idle or does it function when the thrust levels are above idle, but where the actual thrust is above that commanded? A: No, the latter is true (i.e. idle is not required) - confirmed here - there are of course many protections against false activation Q. Did AI171 have the same software version / logic paths as NH-985 A. Unknown. That a/c had Trent 1000s so to some extent the software is different, but we understand the TCMA logic is broadly the same regardless of engine. I have not seen a post clarifying whether the TCMA software has been updated /changed via SB since 2019 to account for this incident. Be grateful if posters could refrain from speculative responses "e.g. I think this is unlikely because I feel x". I am not opining on how likely this sequence of events is, simply trying to summarise whether or not this theory has been ruled in or out. I also recommend this post for a summary to read before posting. . Last edited by lighttwin2; 21st Jun 2025 at 16:13 . |
| skwdenyer
2025-06-30T14:04:00 permalink Post: 11913652 |
Posters may like to read this old (2016) pprune thread on 787 engine failure procedures:
787 engine failure procedure
Some interesting comments about how a combination of ATM and derate can lead to some pretty surprisingly poor outcomes, coupled with Boeing advice not to advance thrust levers or engage TOGA. (edited for poor spelling) Last edited by skwdenyer; 30th Jun 2025 at 14:40 . |
| Capn Bloggs
2025-06-30T16:26:00 permalink Post: 11913745 |
Richard Godfrey has completely ignored that fact that the speed was reducing all the way down the descent. He has hypothesised a steady descent angle at around 3.5:1 and therefore concluded that one engine had stopped and the other was on ~10,000lb of thrust because, at the initial speed they needed that thrust to counter the drag. But the AOA clearly increases markedly as the aircraft approaches the ground, which means that the speed was reducing significantly.
As for statements such as:
One Engine Inoperative (OEI) just before take-off(Assumption).
The Take-Off Speed at rotation is estimated at 160 KIAS (167 KTAS, 170 GS), which is higher than normal and expected in an assumed OEI take-off.
The rotation point requires a runway ground roll of 2,540 m, which is longer than normal due to an assumed OEI take-off.
The climb gradient is estimated at 5.09%, which is slightly above the normal range of 3% to 4%at TOGA for a Boeing 787-8, but within limits
True airspeed was under-indicated due to low density, in other words the aircraft was moving faster through the air than the airspeed indicator showed, because there were fewer air molecules hitting the pitot tube in the hot, thin air
The aircraft peaked early, then levelled off and started descending while passing over obstacles. It barely cleared Obstacle B by ~50 feet, which is tight, but sufficient. This explains the visibly profile in video and supports the notion of degraded climb due to hot/high conditions, heavyweight, and limited thrust margin
In a nutshell, he's saying they had an engine failure before rotation, then got all the way up to 300-odd feet (following what looked to me like an initially standard low-angle/flat 787 takeoff, then ran out of puff and crashed with the other engine running at 15-18% Thrust. Pure speculation aka WAG. 11 users liked this post. |
| GroundedSpanner
2025-06-30T21:18:00 permalink Post: 11913900 |
It would go against every aspect of training for a professional crew to deploy the rat below 400 feet. Further if you put 100 crewmembers in a simulator on takeoff and said deploy the rat manually I would bet not a single crew member could find the rat deploy switch and activate it in under 5 seconds and maybe 2 or 3 in 10 seconds.
It can auto-deploy in a significant electrical failure. The theory I'm working is that, given the configuration of the aircraft, if there is a significant electrical failure (sufficient for the RAT to auto-deploy). The aircraft fuel pumps are lost. Approx. 7ish seconds after those fuel pumps are lost (if the engines are at TOGA), the fuel being received by the engines is arriving from a different source, a potentially contaminated source. I believe that the engine failure is a secondary result of the power loss (with specific circumstances). But this all hinges on whether the engines were trying to run, or commanded off. Hence the request for interpretation of the sound. 1 user liked this post. |