Page Links: First Previous 1 2 3 4 5 6 7 8 9 10 11 12 Next Last Index Page
wheelsright
2025-06-20T04:57:00 permalink Post: 11906594 |
The Indian Express is carrying a story (
https://indianexpress.com/article/ci...lues-10077117/
) that includes the following:"Investigators probing the June 12
crash of Air India flight AI-171 from Ahmedabad to London Gatwick
are taking a close look at a February 2020 incident in Gatwick, involving an Airbus A321, in which both engines malfunctioned immediately after takeoff. It led to a Mayday call before the aircraft returned to Gatwick 11 minutes later after a turnaround." ...
..."it was \x93clear from visual observation and wreckage\x94 that the flight suffered a power failure." ... ..."The black boxes and the DVR have been recovered but the officer said that the devices were damaged and file extraction would \x93be a complicated process\x94." ... ... "We will check the technical logs to see if any of the engineering teams or pilots of the previous flight left comments on the performance of both engines" Assuming there is some credence to the article, dual engine failure due to water contamination is the leading theory. It certainly fits much of the speculation in this thread. It may be that the flight data was not captured and much more reliance will be on forensic examination of the CCTV footage and the wreckage. Those waiting for the flight data to be published may be disappointed. If the original CCTV footage was made available, together with a detailed survey map of the airport, it will be possible to accurately estimate the takeoff speed and altitude during all the critical periods. My guess is thrust was reduced or lost very early and perhaps before the aircraft left the tarmac. Then shortly after becoming airborne, power was lost resulting in the deployment of the RAT. It is doubtful that the pilots shut down the engines or the wrong engine. Likewise flap/slat misconfiguration is unlikely. 1 user liked this post. |
Lookleft
2025-06-20T05:20:00 permalink Post: 11906598 |
Assuming there is some credence to the article, dual engine failure due to water contamination is the leading theory.
Last edited by Lookleft; 20th Jun 2025 at 05:57 . |
Musician
2025-06-20T05:30:00 permalink Post: 11906603 |
TCMA things, imagination and evidence
You may be surprised to know that TCMA doesn't require that, it just requires a differential between commanded and actual thrust.
It has never triggered during takeoff until now. Maybe it still hasn't been. We'll see. Given there is an actual example of a 787 in the wild shutting down both of it's engines when it shouldn't (ANA), I'm surprised how complacent people are that this couldn't be the cause..Software can always have weird corner failures that could never have been thought of or tested. Note that the thrust lever actuators are wired to the FADECs, and that the TCMA gets the T/L position from that. For TCMA to trigger, it has to determine that its FADEC (on that engine) failed to achieve a commanded reduction in thrust. So we're either looking at a weird, unprecedented edge case, or a FADEC failure, or both.
Just so I have this clear, are you saying that the implementation of the TCMA functionality involved
no
new components being added to the pre-existing FADEC? Are you saying, in effect, that the two switch relays described in the TCMA patent application, which relays and their configuration achieves the described two channel redundancy, were already there as components or are mere depictions of what the software does itself?
Originally Posted by
Lead Balloon
I am not suggesting you are wrong and, as I've said before, the descriptions and schematic in the patent application are just 'big hands / small maps' concepts. However, if TCMA functionality "is simply a bit of software in the FADECs", merely sending a 1 or 0 or other signal into a point in the pre-existing FADEC that already had control over fuel cutoff (with the TCMA software merely monitoring data busses, rather than direct sensor outputs, to work out thrust lever position and whether or not the aircraft is 'on the ground' for TCMA purposes) I for one would really like to know that for sure and get my head around the implications.
With a MCAS crash, it required a hardware problem with an AOA sensor, used as input to a correctly working MCAS, to cause the aircraft to behave erratically. With a correctly working TCMA, I believe it'd require two hardware problems to get TCMA to shut down the engine, as there'd have to be an implausible thrust lever reading, and a FADEC/engine failure to process it within the TCMA allowed range ("contour"?). On both engines, separately and simultaneously. That leaves a software problem; it's not hard to imagine. The issue is, at this point it's just that: imagination. I could detail a possible software failure chain, but without examining the actual code, it's impossible to verify. We simply don't have the evidence. I could just as well imagine a microwave gun frying the electronics on both engines. An escaped hamster under the floor peeing on important contacts. A timed device installed by a psychopathic mechanic. There's no evidence for that, either. This process is a way to psychologically cope with the unexplained accident, but because it lacks evidence, it's not likely to identify the actual cause. We've run the evidence down to "most likely both engines failed or shut off close to rotation, and the cause for that is inside the aircraft". Since the take-off looked normal until that failure, we have no clues as to the cause hidden inside the aircraft. We need to rely on the official investigation to discover and analyse sufficient evidence. The post-crash fire is going to make that difficult. "Both engines failed or shut off close to rotation" explains all of the evidence : it explains an unremarkable take-off roll, loss of lift, absence of pronounced yaw, loss of electrical power, loss of the ADS-B transponder, RAT deployment, the noise of the RAT banging into place and revving up, emergency signs lighting up, a possible mayday call reporting loss of thrust/power/lift, and a physically plausible glide from a little over 200 ft AAL to the crash site It explains what we saw on the videos, what the witness reported, where the aircraft ended up, and the ensuing sudden catastrophe. I don't believe we have evidence for anything else right now—I'd be happily corrected on that. ----- Edit: the evidence of the crash photo with the open APU inlet door, and the main gear bogeys tilted forward, are also explained by the dual engine failure/shut off. Last edited by Musician; 21st Jun 2025 at 06:48 . Reason: more evidence 17 users liked this post. |
skwdenyer
2025-06-20T06:18:00 permalink Post: 11906620 |
TCMA can't be disabled electrically. It's just software, and all of the hardware involved serves other functions which are still needed while in the air. For example, the FADECs would command the HPSOV closed in case of N2 overspeed. That would have the exact same effect as TCMA.
4 users liked this post. |
amsm01
2025-06-20T08:36:00 permalink Post: 11906715 |
The Indian Express is carrying a story (
https://indianexpress.com/article/ci...lues-10077117/
) that includes the following:"Investigators probing the June 12
crash of Air India flight AI-171 from Ahmedabad to London Gatwick
are taking a close look at a February 2020 incident in Gatwick, involving an Airbus A321, in which both engines malfunctioned immediately after takeoff. It led to a Mayday call before the aircraft returned to Gatwick 11 minutes later after a turnaround." ...
..."it was “clear from visual observation and wreckage” that the flight suffered a power failure." ... ..."The black boxes and the DVR have been recovered but the officer said that the devices were damaged and file extraction would “be a complicated process”." ... ... "We will check the technical logs to see if any of the engineering teams or pilots of the previous flight left comments on the performance of both engines" Assuming there is some credence to the article, dual engine failure due to water contamination is the leading theory. It certainly fits much of the speculation in this thread. It may be that the flight data was not captured and much more reliance will be on forensic examination of the CCTV footage and the wreckage. Those waiting for the flight data to be published may be disappointed. If the original CCTV footage was made available, together with a detailed survey map of the airport, it will be possible to accurately estimate the takeoff speed and altitude during all the critical periods. My guess is thrust was reduced or lost very early and perhaps before the aircraft left the tarmac. Then shortly after becoming airborne, power was lost resulting in the deployment of the RAT. It is doubtful that the pilots shut down the engines or the wrong engine. Likewise flap/slat misconfiguration is unlikely. https://www.gov.uk/aaib-reports/airc...-february-2020 Last edited by Senior Pilot; 20th Jun 2025 at 08:51 . Reason: Remove oversized image and sort out hyperlink 1 user liked this post. |
ignorantAndroid
2025-06-20T08:53:00 permalink Post: 11906736 |
Categories 1 and 2 would be common to both engines, so they both remain plausible. For category 2, it would be impossible to identify the issue without analyzing the complete source code. Since we don't have access to that code, this is a dead end. It could be the cause, but we won't be able to figure it out. Looking at how the FADECs are designed to work isn't going to be very useful here, since by definition, they'd be doing something they weren't supposed to. Category 1 is a bit different. There are 2 functions we know of that can close the fuel shutoff valve: TCMA and N2 overspeed protection. We don't have the complete specifications, but the basic logic of both functions has been described. If we assume that one of these was the cause, then the conditions for one of those functions must have been met. The conditions for TCMA, at least as it's been described in this thread, are:
As for the overspeed protection, as far as I know, there's only one condition: N2 greater than a certain value. That reading comes from sensors that are inside each engine and wired directly to the FADECs. I don't see any way this could affect both engines simultaneously either, but it still seems a bit more likely than something involving TCMA since it only requires 2 separate, simultaneous failures rather than 3 or more. For the sake of accuracy, I should also note that not everything fits neatly into one of my 3 categories. For example, let's say we have a machine that's programmed to shut down if any one of 3 parameters goes above a certain value. If one of those values gets corrupted by a faulty memory chip, the machine could shut down unnecessarily. If we add more parameters to the list, the probability of an inadvertent shutdown increases since there are more critical areas in memory. As another example, consider a case where corruption of the CPU's program counter causes it to inadvertently jump to a particular subroutine. If we add more subroutines that can trigger a shutdown, we make the machine more vulnerable, albeit to a very small degree. Changes like these are sometimes referred to as "increasing the surface area." Due to those types of scenario, I will admit that the existence of something like TCMA probably makes an engine ever-so-slightly more likely to fail. Whether the benefit is worth the cost could be debated. In any case, I still find it pretty unlikely that any of this will turn out to have been a factor in this accident. Last edited by ignorantAndroid; 20th Jun 2025 at 09:11 . 9 users liked this post. |
FullWings
2025-06-20T09:43:00 permalink Post: 11906781 |
Although it seems inconceivable that they did not firewall the thrust levers, it will be interesting to know if and when this happened. The aircraft clearly did not have enough thrust for the flight regime with the gear extended etc. But does this imply a total loss of thrust on both engines?
I would be very surprised if the thrust levers were not firewalled early on, in fact with such determination that they went through the instrument panel! On a wider observation, professional commercial pilots like the Air India ones in this accident go through regular simulator training according their own SOPs, which when dealing with things like thrust loss during or after the takeoff roll are likely pretty similar or even identical to the manufacturer\x92s guidelines; if they did differ it would be because they were more conservative in application. Boeing standard is to do nothing until 200\x92AGL other than control the aircraft in yaw, pitch and roll. Above 400\x92AGL you can start doing some drills, if applicable. This assumes, of course, that you can get to these heights in the first place. I would put forward that in this accident, the crew immediately found themselves in what Boeing call \x93Special situations\x94 or \x93Situations beyond the scope of normal procedures\x94. We don\x92t know yet whether there was a thrust loss or total failure at the outset; we don\x92t know if the RAT deployed due to sensed failures or control operation. As a trainer, the captain would have known the implications of actioning the dual engine failure memory items, especially near the ground, but if you\x92ve tried everything else and are still going down then what is there to lose? This is not to suggest this is what happened, just to fill in the blanks in terms of possibilities. Whatever did occur likely put them outside the realm of SOPs in short order, which is a difficult situation at the best of times, especially as for your whole flying career you have been trained and assessed at your ability to conform to those SOPs as accurately as possible in the takeoff phase. 9 users liked this post. |
BFM
2025-06-20T10:19:00 permalink Post: 11906806 |
I have read through all the first thread and much of the second before commenting; SLF here, albeit with a lot of flights in a lot of commercial, emergency and non-commercial aircraft but with a relevant background and expertise in collision reconstruction and looking for holes in Swiss Cheese.
The evidence is overwhelming that both engines lost power, and substantial that they were cut simultaneously. There is one matter that I have not seen adequately explored here, and that is the Christmas Tree hypothesis, where a plane not in use is used as a stock of spare parts, which then have to be replaced if it is then called for service. One of the engineering experts has noted that a specific engine control circuit board must be powered down every 51 days or can cause problems, and even more so at 249 days when it has the capacity to cut both engines simultaneously. As the same expert pointed out, there are very few single points of failure in an aircraft - but this would seem to be one of them and accordingly should receive significant scrutiny. Is it possible, that on a parked aircraft, this item could have remained powered, silently ticking off the days? It then returns to service and the counter overflows at an extraordinarily unfortunate moment causing catastrophe? This is the way statistics works. I ran over a very small rock yesterday in a car with reinforced tyre walls on the wheels. I thought nothing of it until the tyre pressure warning lit - and the tyre was indeed losing air. I have checked the CCTV (dashcam) and inspected the wheel. It was clear that I had touched the stone in the one place where it had compressed the tyre against the rim and cut the sidewall. That car has travelled 207,000km in my hands. The rock was about 5cm in diameter. The chances of that I would hit a rock that size at that moment in that precise way to destroy my tyre were over 4 billion to one. But it did. Last edited by Saab Dastard; 20th Jun 2025 at 10:37 . Reason: paragraphs 1 user liked this post. |
Lead Balloon
2025-06-20T11:17:00 permalink Post: 11906849 |
I have read through all the first thread and much of the second before commenting; SLF here, albeit with a lot of flights in a lot of commercial, emergency and non-commercial aircraft but with a relevant background and expertise in collision reconstruction and looking for holes in Swiss Cheese.
.... One of the engineering experts has noted that a specific engine control circuit board must be powered down every 51 days or can cause problems, and even more so at 249 days when it has the capacity to cut both engines simultaneously. ... |
MaybeItIs
2025-06-20T11:21:00 permalink Post: 11906857 |
The ADS-B datagrams sent by the aircraft show a much diminished climb rate with decaying speed, betraying insufficient thrust in that phase of the flight. That somewhat contradicts your assertions.
I also do not have faith in anyone's ability to watch the cctv video and confidently determine through mere eyeballing that the climb rate did not decay by 15% within the first 100 feet or so.
(The ADS-B data suggests the speed diminished 7% for ~50 ft of climb.)
And why all the wrong figures for the height attained, quoted in previous thread? Can't all be the atmospheric conditions.
Other than your stone, even a glider can convert speed to altitude.
![]() To be honest, i believe that taking a lot of the evidence into consideration, it is possible to arrive at a limited number of scenarios for what is most likely to have happened. One fact that alters things substantially is whether the survivor's impression is correct that possibly the engines started to spool up again just before impact. If that's the case then what does that do to the possibility or otherwise that the TMCA system caused a dual engine shutdown? To me, since the world seems to be watching this forum, and we are getting no feedback from the authorities, what is posted here might be useful in helping the investigators look at things they might not have considered. Besides, as Icarus2001 has kindof suggested, it's probably a very good thing that there are clearly lots of keen eyes on this. Last edited by MaybeItIs; 20th Jun 2025 at 11:29 . Reason: Missing [/QUOTE] 1 user liked this post. |
Musician
2025-06-20T12:13:00 permalink Post: 11906909 |
Flightradar24 and ADS-B
Thank you for your reply! There's a lot we agree on; unfortunately, I'll be cutting that from my response here.
![]()
Sure, actual data is usually more accurate than eyeballed stuff. But not always. In fact, it's often the eye that determines that something measured or calculated is "Off". How accurate is ADS-B data? I've seen FR24 tracks go way off course then suddenly get corrected / interpolated, frequently. The erroneous data seems to be "removed" by their algorithm, but where are the errors arising? Why this inaccuracy, and therefore, how accurate are the datagrams referred to? I know there were no datagrams received during the backtrack that I accept actually occurred, but that's completely different from receiving erroneous ADS-B data.
However, the datagrams that FR24 actually received were correct. They contain the GPS position of AI171 and its unadjusted barometric altitude, as determined by its onboard instruments. This data is as reliable as the instruments themselves are. (An example here is that the NTSB wasn't sure that the altimeter on the Blackhawk that crashed at Washington-Reagan was accurate; if that is the case, the ADS-B data would also be affected.) On their blog post at https://www.flightradar24.com/blog/f...rom-ahmedabad/ , FR24 have published the data that they actually received.
Sure, the CCTV footage I've seen is very poor, a video, moved about and zoomed, of the CCTV screen. Not easy to judge, but still useful and could be analysed frame-by-frame to compensate for all the extraneous input. Anyway, it's obvious to me that the rate of climb dropped abruptly just before the flight attained its apex, as if thrust was suddenly cut off.
Knowing the momentum to altitude conversion, it might be possible to estimate whether that's true or not. The evident RAT deployment supports engine shutdown, not just engines to Idle, doesn't it? In that case, it would be useful to know at what altitude the engine shutdown took place.
(The ADS-B data suggests the speed diminished 7% for ~50 ft of climb.)
And why all the wrong figures for the height attained, quoted in previous thread? Can't all be the atmospheric conditions.
2) people adjusting for airport elevation (189 ft), but not for pressure: 437 ft 3) people adjusting for pressure, some adjusting for temperature, get 71 to ~100 feet for the last recorded altitude. But while ADS-B reception was lost then (or the transmitter lost power), the aircraft continued climbing; examine the cctv video, knowing the wingspan is ~200 feet, we see that the aircraft reached 200 feet but not much more.
One fact that alters things substantially is whether the survivor's impression is correct that possibly the engines started to spool up again just before impact.
If that's the case then what does that do to the possibility or otherwise that the TMCA system caused a dual engine shutdown?
[Now I just hope your post is still there as I post this. ![]() Last edited by Musician; 20th Jun 2025 at 12:26 . 3 users liked this post. |
MaybeItIs
2025-06-20T13:47:00 permalink Post: 11906986 |
Indeed, thanks to you for your most informative reply! Great to know we're much on the same page.
I'll strive for brevity here. [Fail, sorry!]
However, the datagrams that FR24 actually received were correct. They contain the GPS position of AI171 and its unadjusted barometric altitude, as determined by its onboard instruments. This data is as reliable as the instruments themselves are.
It's uncertain because the 787 rounds all altitudes it sends to the nearest multiple of 25. The altitudes sent were from 575 ft to 625 ft.,
The RAT deploying is a consequence of a dual engine shutdown. It says nothing about whether the TMCA was involved.
If the TMCA did activate and shut off the fuel for whatever reason, what causes the TMCA/FADEC Hardware (and Software) to Reset, since it's independently powered off the engine-driven PMG after engine start? There is so much here that is just so unclear. I haven't seen anything about a Reset input anywhere, and since it's supposed to work only when on the ground, that's not really necessary, as the engine will eventually spool down. At some point before that, the PMG output voltage will go to low enough that the FADEC/TMCA should be forced into a Hardware Reset. That's all fine on the ground, but in the air, the engine will windmill, potentially until.... Is the PMG output fed through a switch/relay that cuts the FADEC/TMCA supply at low (i.e. windmill) RPM, so that a Pilot-activated Engine Off/On cycle can reconnect the Aircraft FADEC Supply link, thus Rebooting the FADEC so that it reopens the Fuel Shutoff valve(s)? It all seems so "awkward". And potentially fatal. Is this a scenario that the designers considered? (Who can answer that one? ![]() Just now, I realise that if this is roughly what happens, then maybe the engines did commence a restart just before impact, due to the plane being deliberately mushed/stalled to the ground as softly as possible, thereby reducing the windmill RPM. And maybe the engines restarting interfered with that planned landing. Or maybe I've got this all wrong. I'm hoping someone will tell us all.
[Now I just hope your post is still there as I post this.
![]() ![]() 1 user liked this post. |
Shep69
2025-06-20T13:55:00 permalink Post: 11906990 |
One fact that alters things substantially is whether the survivor's impression is correct that possibly the engines started to spool up again just before impact. If that's the case then what does that do to the possibility or otherwise that the TMCA system caused a dual engine shutdown? There may have been other electrical and systems malfunctioning. But if whatever happened, let’s say the auto throttle simply pulled power to idle —or a low power setting—at a critical time. Perhaps on its own perhaps with other systems failures. We like to think it basic that we’d slam the throttles forward. Right away. But Asiana didn’t. And neither did Air Florida years ago. |
ams6110
2025-06-20T16:51:00 permalink Post: 11907131 |
... The generally held theory for each engine\x92s FADEC failure (due to a common software error related to a ground-air sensing failure) is not supported by the cumulative hours over many years without such a failure. So, if you then look around for another component that can shut down two engines simultaneously you end up with the fuel shut-off valves (FSOV). Why? On each engine It is fail-safed to close off fuel-feed flows by a spring that is held
open
by a solenoid. If that solenoid loses electrical power, the FSOV closes and the engine shuts down after a short period.
18 users liked this post. |
MaybeItIs
2025-06-20T22:57:00 permalink Post: 11907382 |
Hello, this is my first post on pprune; as a 787 pilot I\x92m also puzzled by this accident. All seem to agree that for some reason there was a complete electrical failure and RAT deployment. With a complete electrical failure all six main fuel pumps fail. Each engine also has two mechanically driven fuel pumps. On takeoff, if there is fuel in the center tank, it will be used first, pumped by the two center tank pumps.
My airline\x92s manuals don\x92t go into much detail, but I read on another site that if both the center tank pumps fail, the engine driven pumps aren\x92t able to suction feed well enough from the center tanks to sustain engine operation. If there was fuel in the center tanks, a complete electrical failure would soon lead to center tank fuel pumps failure (all fuel pumps failure as stated previously) and fuel starvation of both engines. A rescue from this situation would be an immediate selection of both center tank fuel pumps OFF (not if my airline\x92s non-normal checklists) and waiting for successful suction feed from the L and R main tanks to occur, this would take a number of seconds. FWIW, according to earlier posts, the fuel load was about 50T, leaving about 18T in the centre tank, so (I think) about 25-30% full. A full centre tank might allow engine pump suction to work fine, but this might not? (Contrary to what some have said.) Anyway, FWIW, not everyone agrees with RAT Deployment - see recent post by shep69. Would love to know why he doesn't go with RAT deployment... |
Aerospace101
2025-06-21T00:41:00 permalink Post: 11907411 |
10 users liked this post. |
MaybeItIs
2025-06-21T01:21:00 permalink Post: 11907432 |
For those postulating the RAT was not deployed, what counter explanations do you have for the following clues?
1 user liked this post. |
Aerospace101
2025-06-21T08:56:00 permalink Post: 11907591 |
The issues with the "they shut down the wrong engine" theory:
1. No asymmetry evidence with flight path deviation. No roll, no yaw effects 2. No rudder inputs visible. 3. No crew should be doing memory items below 400ft. Boeing requires each crew member confirm together memory item switch/control selections. 4. Non-normal gear truck tilt position, a one engine failure should not affect the C hydraulics. As per (3) gear would be selected Up before any memory actions. The evidence so far is an almost simultaneous dual engine failure, which rules out alot of other theories. 7 users liked this post. |
lpvapproach
2025-06-21T13:50:00 permalink Post: 11907770 |
The issues with the "they shut down the wrong engine" theory:
1. No asymmetry evidence with flight path deviation. No roll, no yaw effects 2. No rudder inputs visible. 3. No crew should be doing memory items below 400ft. Boeing requires each crew member confirm together memory item switch/control selections. 4. Non-normal gear truck tilt position, a one engine failure should not affect the C hydraulics. As per (3) gear would be selected Up before any memory actions. The evidence so far is an almost simultaneous dual engine failure, which rules out alot of other theories. |
rachel1707uk
2025-06-21T17:38:00 permalink Post: 11907924 |
I'm sure this has been answered elsewhere and that I've just missed it by scrolling too fast through the backlog of (much appreciated) posts, but it's been buzzing round my mind and I wanted to try and get an answer if possible please.
Has it been confirmed that there was a dual engine shutdown and, if so, why weren't people commenting on this from the videos of the incident (if the audio was good enough to detect the RAT then surely it was good enough to tell whether the engines were running). Thank you for your patience! |
Page Links: First Previous 1 2 3 4 5 6 7 8 9 10 11 12 Next Last Index Page