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Someone Somewhere
2025-06-20T09:14:00 permalink Post: 11906758 |
I would, of course, presume, that take-off roll performance was within expected limits, otherwise they would have aborted by V1. They reached VR before running totally out of runway, and achieved a short-lasting climb. What one single point of failure occurred very shortly after aircraft went nose-up and would it be possible that the fuel feed in some way affected by virtue of that angle in the context of some failure?
Runway performance seems to be still under some speculation but I thought it was fairly solidly shown that they rotated in about the normal position. Two events that I see are rotate (g-forces/deck angle causing fuel sloshing) and weight-on-wheels going false due to lift-off. The engines would be fed separately by the left and right pumps in the centre tank as there's >34t of fuel onboard; one pump per engine. I am not sure how physically separated they are in the tank. |
Propellerhead
2025-06-20T14:06:00 permalink Post: 11907000 |
Sorry if this has been covered in the previous 1 million posts, but do we know if the training captain or trainee was handling the takeoff? The one thing that does change at V1 is PF\x92s hand comes off the thrust levers and joins the other hand on the control column. How long until people then put their hand back on the thrust levers varies a lot. Especially if turbulent or there is a perception of difficult handling ie) engine failure which often delays it. If your hand is on the thrust lever then should be able to feel them moving - unless gripping them so firmly it over rides the clutches.
Oh, and the FDRs haven\x92t been read yet as they were damaged in the fire. |
wheelsright
2025-06-21T04:06:00 permalink Post: 11907468 |
Just to summarize. There appears to be fairly wide consensus as to what happened:
On that basis, there is still some mileage in establishing the aircraft speed in the last moments before takeoff. There is definitely mileage in identifying a single point of failure that would cause the engines to shut down; other than fuel contamination/vapour issues. I suspect that the official investigation is not all that further ahead of this thread. Without useful data from the EAFR they have to rely on forensics and history. Enough has been leaked to know the engines were no more than windmilling at impact. A high level of interest will continue given there are still remaining questions whether the reliability of Boeing machinery is implicated. That is not to mention the hundreds of people closely affected by this tragedy that are looking for reasons why it happened. Perhaps an interim report is now overdue? Last edited by wheelsright; 21st Jun 2025 at 04:24 . 4 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 . |
First_Principal
2025-06-28T05:47:00 permalink Post: 11912344 |
In keeping with PilotDAR's request, here is some possibly useful information regarding the type of Flight Data Recorder (FDR) and Cockpit Voice Recorder (CVR) likely installed in the Air India B787 (using long names etc for those who may be unfamiliar with various acronyms).
The B787 was one of the first aircraft to receive a new type of FDR/CVR, known as an Enhanced Airborne Flight Recorder (EAFR). Some detail of the time may be found here: https://www.flightglobal.com/boeing-.../67970.article https://www.militaryaerospace.com/po...for-boeing-787 I don't believe it's absolutely confirmed yet but earlier posts (thank you V1... Ooops et al ) indicated that the unit in question may be a GE Aviation Model 3254F, the document here gives some good detail, along with another doc from Skybrary that has some relevant information. And this one discusses more on the 'new' ARINC-767 protocol in use with these later units. Some broader background into ARINC's role in determining the standards applicable to aviation can be seen in this video. While it doesn't specifically cover ARINC-767 (used by the B787 EAFR) the history and associated detail is interesting and may give a little useful background: In terms of actual data recovery I had a look around but couldn't find any video that showed this from an EAFR, however this NTSB YT link gives insight into how data was recovered from an earlier FDR unit (mounted in a Bombardier CRJ700 ). While it's different to that installed in the 787 the nature of the work and how it's carefully carried out may give some insight. Finally, this is a link to a short report re data recovered from an EAFR in a B787-9: https://data.ntsb.gov/Docket/Documen...ort-Master.PDF I claim no specific knowledge here, just providing references to detail I read while trying to learn more about these newer units. FP. 3 users liked this post. |
DaveReidUK
2025-06-28T07:27:00 permalink Post: 11912371 |
I don't believe it's absolutely confirmed yet but earlier posts (thank you
V1... Ooops
et al
) indicated that the unit in question may be a GE Aviation Model 3254F,
the document here
gives some good detail, along with
another
doc from Skybrary that has some relevant information..
|
Innaflap
2025-06-28T14:14:00 permalink Post: 11912510 |
I think that it is understandable that the Indian accident investigation team leaders are being very cautious about when and what information to release. This is probably the first "really big" accident investigation they have done that has achieved so much international attention. I am sure that they want to be certain that when they do release information it is accurate and carefully presented, simply to avoid any embarrassment that might arise from a hasty or less than carefully thought out disclosure.
For that reason, I think we ought to give the Indian investigators the benefit of the doubt and assume that they simply want to make sure that when they do make a statement it is 'bulletproof' and reflects well on their skill, professionalism, and the processes that they have followed. |
Abbas Ibn Firnas
2025-06-30T13:32:00 permalink Post: 11913628 |
Thank you for that answer, edge cases do abound in complex systems, but would not moving the throttles forward by hand (as the thrust was beginning to reduce {for that strange reason}) overcome that and restore thrust?
(As I don't fly the 787, I may be missing something basic on how the systems work). THRUST Asymmetry PROTECTION. "For an engine-out condition, Thrust Asymmetry Protection (TAP) reduces thrust on the operating engine to ensure there is sufficient rudder for directional control. TAP reduces thrust when the airspeed decreases below approximately V2 on a takeoff or below approximately VREF on a go-around. Once speed is increased above V2/VREF, TAP increases thrust." From what we know so far, it does seem the engines were not producing sufficient thrust, during a period when it would also be crucial to maintain electrical output for the electro-hydraulic systems and critical electrical loads. Reduced electrical output could explain the failure of the gear to complete retraction, maybe caused by a generator failing at the worst possible moment. If there was an EFATO, the ability of the remaining generators to provide sufficient power might become questionable, as is highlighted with the load shedding system. Other features which are unique to the 787 could be contributing factors in explaining the accident. It is known the 787 will generally employ an extended take-off roll, and a relatively higher V1 and Vr, and also climb out less steeply than other aircraft. Using more of the runway would reduce the margin for aborted take offs. With the evident lack of thrust early in the climb out, and failure to retract the gear, if V2 had not been maintained, the TAP system would have reduced thrust even further. Manually increasing thrust will be inhibited. |
Sailvi767
2025-06-30T18:57:00 permalink Post: 11913849 |
I found descriptions on the systems of the 787 were easily discovered online, and while I have no hands-on experience of aircraft related matters, I do have experience in wider electrical theory and maintenance.
THRUST Asymmetry PROTECTION. "For an engine-out condition, Thrust Asymmetry Protection (TAP) reduces thrust on the operating engine to ensure there is sufficient rudder for directional control. TAP reduces thrust when the airspeed decreases below approximately V2 on a takeoff or below approximately VREF on a go-around. Once speed is increased above V2/VREF, TAP increases thrust." From what we know so far, it does seem the engines were not producing sufficient thrust, during a period when it would also be crucial to maintain electrical output for the electro-hydraulic systems and critical electrical loads. Reduced electrical output could explain the failure of the gear to complete retraction, maybe caused by a generator failing at the worst possible moment. If there was an EFATO, the ability of the remaining generators to provide sufficient power might become questionable, as is highlighted with the load shedding system. Other features which are unique to the 787 could be contributing factors in explaining the accident. It is known the 787 will generally employ an extended take-off roll, and a relatively higher V1 and Vr, and also climb out less steeply than other aircraft. Using more of the runway would reduce the margin for aborted take offs. With the evident lack of thrust early in the climb out, and failure to retract the gear, if V2 had not been maintained, the TAP system would have reduced thrust even further. Manually increasing thrust will be inhibited. 2 users liked this post. |