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| lighttwin2
2025-06-13T14:32:00 permalink Post: 11900574 |
This was handled in this PPrune thread:
Just to clarify one point: the ANA B787 was powered with RR Trent 1000 engines while the Air India had GEnx-1B67 engines. So, the Air India thrust failure may still have its source in the TCMA system, however, if it's the case, the logical path must be somewhat different than for the thrust reversers of the ANA airplane. Note, the EEC module in GEnx itself is subject to periodic replacement under an AD due to risk of solder failure, but this seems unlikely to be related as would only impact a single engine. |
| TURIN
2025-06-13T23:49:00 permalink Post: 11901007 |
Speaking as a B787 Captain..... There is so much rubbish and stupid suggestion being written here.
This aircraft was airborne for a grand total of 22 seconds, half of which was climbing to no more than 150' aal. - No Flaps? Due to the setup of the ECL it is physically impossible to go down the runway without some sort of take-off flap set. The T/o config warning would have been singing it's head off. Despite assertions to the contrary I have seen no video clear enough to detect a lack of flaps. - RAT out? Almost impossible, I have seen no quality footage that definitively witnesses the RAT being out. Those who think they car hear a RAT type noise might be listening to a motorcycle passing or similar. It takes a triple hydraulic failure or a double engine failure to trigger RAT deploment. They happily went through V1 without a hint of rejected take off so as they rotated the aircraft was serviceable. These are big engines, they take a long time to wind down when you shut them down. I have never tried it however engine failure detection takes 30s or for the aircraft to react and they were not even airborne that long. - Flaps up instead of gear? The B787 flaps are slow both in and out. Given that the 'Positive rate' call is not made the second the wheels leave the ground, a mis-selection of flaps up would not cause any loss of lift for at least 20 seconds, by which time they had already crashed. I believe the gear remained down not because of mis-selection but because of a major distraction on rotate. Discounting the impossible, two hypotheses remain: 1. Invalid derate set through incorrect cross-checking. Trundling down the runway takes very little power to reach Vr. It is only when you rotate that you create more drag and discover that you do not have sufficient thrust vs. drag to sustain a climb. Or.... 2. Put 200' as the altitude target in the FCU. Immediate ALT capture and all the power comes off. PF is still hand flying trying to increase pitch but is already way behind the aircraft. It could be after this that Boeing are forced to review the B787 practice of exploring the very edges of the performance envelope. Even though these are big engines with plenty of inertia, when you select engine shut off they spool down very quickly if on load. IE, The generators, two per engine and hydraulic pumps, etc, being driven by the (relatively) small mass of the N2 rotor will drag the speed down very quickly, the gennies will trip offine in seconds, the pumps will quickly reduce flow and pressure. As for what went wrong. If the engines have stopped working there has to be a common failure mode, fuel is one but as has been said, no other aircraft has had a problem, as far as we know. FOD? It would have to be something major to shut down two GeNX engines and there would be debris all over the runway, we would know by now. I have no idea if the RAT has deployed, I can't see it in the video and the noise could be something else. We shall see. There is compelling evidence that flaps are set correctly and not retracted inadvertently. I await further evidence. Edit to add. LAE 40 years, type rated on 737 to 787 with lots of others in between. 5 users liked this post. |
| Wannabe Flyer
2025-06-14T13:43:00 permalink Post: 11901499 |
Reports coming in that Indian regulator has sent out a note for inspection of all GenX engines on the 787
|
| AndyJS
2025-06-16T01:10:00 permalink Post: 11903023 |
This has just appeared in the Times (of London) which is usually one of the most reliable sources of information in the UK.
"A loss of engine power is emerging as the most likely cause of the crash of the Air India Boeing that killed at least 279 people at Ahmedabad on Thursday. The Boeing 787-8 series appeared to have suffered from lower than normal thrust from its General Electric GEnx engines as it took off and failed to climb more than 450ft before crashing, video and reports from the Indian authorities have indicated.The new information has eclipsed an early focus on the unusual configuration of the aircraft\x92s wing flaps and landing gear." https://www.thetimes.com/world/asia/...rash-vhqw6b7v3 (subscription required to read the article) 3 users liked this post. |
| OldnGrounded
2025-06-16T01:40:00 permalink Post: 11903037 |
This has just appeared in the Times (of London) which is usually one of the most reliable sources of information in the UK.
"A loss of engine power is emerging as the most likely cause of the crash of the Air India Boeing that killed at least 279 people at Ahmedabad on Thursday. The Boeing 787-8 series appeared to have suffered from lower than normal thrust from its General Electric GEnx engines as it took off and failed to climb more than 450ft before crashing, video and reports from the Indian authorities have indicated.The new information has eclipsed an early focus on the unusual configuration of the aircraft\x92s wing flaps and landing gear." https://www.thetimes.com/world/asia/...rash-vhqw6b7v3 (subscription required to read the article) And it quotes Juan Browne (Blancolirio): \x93There was something terribly wrong with this 787 jet and we need to find out really quickly what went wrong because we\x92ve got a thousand of these operating today and operators need to find out what happened.\x94 5 users liked this post. |
| Pip_Pip
2025-06-16T03:08:00 permalink Post: 11903073 |
Times (of London).
"A loss of engine power is emerging as the most likely cause of the crash of the Air India Boeing that killed at least 279 people at Ahmedabad on Thursday. The Boeing 787-8 series appeared to have suffered from lower than normal thrust from its General Electric GEnx engines as it took off and failed to climb more than 450ft before crashing, video and reports from the Indian authorities have indicated."
The story also says, "No cause has yet been identified for what would be an extremely rare power loss from both engines, but on Sunday the Indian civil air authority (DGCA) began urgent pre-flight inspections of fuel systems, electronic engine controls and other systems on Indian Boeing 787s."
And it quotes Juan Browne (Blancolirio): \x93There was something terribly wrong with this 787 jet and we need to find out really quickly what went wrong because we\x92ve got a thousand of these operating today and operators need to find out what happened.\x94 |
| lighttwin2
2025-06-16T08:51:00 permalink Post: 11903270 |
If TCMA cut fuel flow while still on the runway the aircraft would have been decelerating from the moment it lifted off, which is not what the ADS-B data indicates. The kinetic energy in the rotating parts of the engine wouldn't add much speed to the aircraft as the engines run down with no more energy being added via fuel.
In no particular order, here are some more thoughts on TCMA having caught up on the thread: If you cut the fuel from two big engines at take-off power, there must be some delay before n2 decays below the threshold for generation (below idle n2), the generators disconnect and RAT deploys. GEnx have relatively long spool up/down times as the fan is so large (and would be exposed to 170+kts of ram air). Perhaps someone has a view on how long this would be, but I imagine it could easily be 10s or more between fuel cut off and RAT deployment. On AI171 the RAT appears to be already deployed at the beginning of the bystander video. That starts c. 13s before impact and around 17s after rotation. This does not prove anything except that the supposed shut down must have happened very close to rotation and could have happened just before rotation while the a/c was on the ground. As a thought experiment, imagine if ANA985 in 2019 had decided to go around. The a/c rotates and is ~50 ft above the runway, suddenly both engines spooling down, very little runway left to land on and no reverse thrust available. I am struck by how similar this scenario is to AI171. This theory would require there to have been unexpected thrust lever movement in the moments before rotation - but plausibly one pilot moving to reject, followed by an overrule or change of heart - or even a simple human error such as the recent BA incident at LGW - could achieve this. This is perhaps more likely that any sensor fault that you would expect to only impact a single engine given the redundancy of systems. Tdracer writes that a key requirement of TCMA is to identify an engine runaway in the event of an RTO, in order to allow the a/c to stop on the runway. This will have been tested extensively - it is a big leap to imagine a false activation could be triggered. It did happen on ANA985 but through a very unusual set of inputs including application of reverse (albeit this latter point may not be relevant if TCMA logic does not distinguish between the reverser being deployed or not). Incidentally there is an assumption the TCMA software version in place on the ANA flight had already been patched and fixed on AI171. That probably is the case but I am not sure it is a known fact. In summary I remain baffled by this tragic accident. I have not yet read anything that explicitly rules out TCMA activation and it remains a possibility due to the vanishingly small number of factors that could shut down two engines at apparently the exact same moment when they have fully redundant systems. Fuel contamination, for example, has typically impacted each engine a few minutes (at least) apart. I am also cautious (as others have pointed out) of a form of confirmation bias about Boeing software systems with four-letter acronyms. In my mind the cause could equally well be something completely different to anything suggested on this thread, that will only become clear with more evidence. All of the above also incorporates a number of theories, i.e. that there was an engine shutdown - that are not conclusively known. Thank you to the mods for an excellent job. 3 users liked this post. |
| TURIN
2025-06-13T23:49:00 permalink Post: 11903714 |
Speaking as a B787 Captain..... There is so much rubbish and stupid suggestion being written here.
This aircraft was airborne for a grand total of 22 seconds, half of which was climbing to no more than 150' aal. - No Flaps? Due to the setup of the ECL it is physically impossible to go down the runway without some sort of take-off flap set. The T/o config warning would have been singing it's head off. Despite assertions to the contrary I have seen no video clear enough to detect a lack of flaps. - RAT out? Almost impossible, I have seen no quality footage that definitively witnesses the RAT being out. Those who think they car hear a RAT type noise might be listening to a motorcycle passing or similar. It takes a triple hydraulic failure or a double engine failure to trigger RAT deploment. They happily went through V1 without a hint of rejected take off so as they rotated the aircraft was serviceable. These are big engines, they take a long time to wind down when you shut them down. I have never tried it however engine failure detection takes 30s or for the aircraft to react and they were not even airborne that long. - Flaps up instead of gear? The B787 flaps are slow both in and out. Given that the 'Positive rate' call is not made the second the wheels leave the ground, a mis-selection of flaps up would not cause any loss of lift for at least 20 seconds, by which time they had already crashed. I believe the gear remained down not because of mis-selection but because of a major distraction on rotate. Discounting the impossible, two hypotheses remain: 1. Invalid derate set through incorrect cross-checking. Trundling down the runway takes very little power to reach Vr. It is only when you rotate that you create more drag and discover that you do not have sufficient thrust vs. drag to sustain a climb. Or.... 2. Put 200' as the altitude target in the FCU. Immediate ALT capture and all the power comes off. PF is still hand flying trying to increase pitch but is already way behind the aircraft. It could be after this that Boeing are forced to review the B787 practice of exploring the very edges of the performance envelope. Even though these are big engines with plenty of inertia, when you select engine shut off they spool down very quickly if on load. IE, The generators, two per engine and hydraulic pumps, etc, being driven by the (relatively) small mass of the N2 rotor will drag the speed down very quickly, the gennies will trip offine in seconds, the pumps will quickly reduce flow and pressure. As for what went wrong. If the engines have stopped working there has to be a common failure mode, fuel is one but as has been said, no other aircraft has had a problem, as far as we know. FOD? It would have to be something major to shut down two GeNX engines and there would be debris all over the runway, we would know by now. I have no idea if the RAT has deployed, I can't see it in the video and the noise could be something else. We shall see. There is compelling evidence that flaps are set correctly and not retracted inadvertently. I await further evidence. Edit to add. LAE 40 years, type rated on 737 to 787 with lots of others in between. 2 users liked this post. |
| AndyJS
2025-06-16T01:10:00 permalink Post: 11903736 |
This has just appeared in the Times (of London) which is usually one of the most reliable sources of information in the UK.
"A loss of engine power is emerging as the most likely cause of the crash of the Air India Boeing that killed at least 279 people at Ahmedabad on Thursday. The Boeing 787-8 series appeared to have suffered from lower than normal thrust from its General Electric GEnx engines as it took off and failed to climb more than 450ft before crashing, video and reports from the Indian authorities have indicated.The new information has eclipsed an early focus on the unusual configuration of the aircraft\x92s wing flaps and landing gear." https://www.thetimes.com/world/asia/...rash-vhqw6b7v3 (subscription required to read the article) |
| lighttwin2
2025-06-16T08:51:00 permalink Post: 11903752 |
If TCMA cut fuel flow while still on the runway the aircraft would have been decelerating from the moment it lifted off, which is not what the ADS-B data indicates. The kinetic energy in the rotating parts of the engine wouldn't add much speed to the aircraft as the engines run down with no more energy being added via fuel.
In no particular order, here are some more thoughts on TCMA having caught up on the thread: If you cut the fuel from two big engines at take-off power, there must be some delay before n2 decays below the threshold for generation (below idle n2), the generators disconnect and RAT deploys. GEnx have relatively long spool up/down times as the fan is so large (and would be exposed to 170+kts of ram air). Perhaps someone has a view on how long this would be, but I imagine it could easily be 10s or more between fuel cut off and RAT deployment. On AI171 the RAT appears to be already deployed at the beginning of the bystander video. That starts c. 13s before impact and around 17s after rotation. This does not prove anything except that the supposed shut down must have happened very close to rotation and could have happened just before rotation while the a/c was on the ground. As a thought experiment, imagine if ANA985 in 2019 had decided to go around. The a/c rotates and is ~50 ft above the runway, suddenly both engines spooling down, very little runway left to land on and no reverse thrust available. I am struck by how similar this scenario is to AI171. This theory would require there to have been unexpected thrust lever movement in the moments before rotation - but plausibly one pilot moving to reject, followed by an overrule or change of heart - or even a simple human error such as the recent BA incident at LGW - could achieve this. This is perhaps more likely that any sensor fault that you would expect to only impact a single engine given the redundancy of systems. Tdracer writes that a key requirement of TCMA is to identify an engine runaway in the event of an RTO, in order to allow the a/c to stop on the runway. This will have been tested extensively - it is a big leap to imagine a false activation could be triggered. It did happen on ANA985 but through a very unusual set of inputs including application of reverse (albeit this latter point may not be relevant if TCMA logic does not distinguish between the reverser being deployed or not). Incidentally there is an assumption the TCMA software version in place on the ANA flight had already been patched and fixed on AI171. That probably is the case but I am not sure it is a known fact. In summary I remain baffled by this tragic accident. I have not yet read anything that explicitly rules out TCMA activation and it remains a possibility due to the vanishingly small number of factors that could shut down two engines at apparently the exact same moment when they have fully redundant systems. Fuel contamination, for example, has typically impacted each engine a few minutes (at least) apart. I am also cautious (as others have pointed out) of a form of confirmation bias about Boeing software systems with four-letter acronyms. In my mind the cause could equally well be something completely different to anything suggested on this thread, that will only become clear with more evidence. All of the above also incorporates a number of theories, i.e. that there was an engine shutdown - that are not conclusively known. Thank you to the mods for an excellent job. 1 user liked this post. |
| tdracer
2025-06-17T21:41:00 permalink Post: 11904694 |
I was referring to CVR/FDRs in general being specialist equipment requiring specialist facilities to process. In any case, I would be very interested to find out where those from this accident are read. It appears from a post upthread there are new facilities in New Dehli that could be used. Having said that, for the EK 521 accident in Dubai the recorders were sent to the UK for analysis, despite a "flight data recorder centre" in Abu Dhabi being opened (again with much fanfare) by the UAE GCAA five years before the occurrence.
Usually when I hear of data recorders going back to the US NTSB or the recorder manufacturer, it's because the crash damage is such that specialized equipment is needed to download the data. The recorder in the tail would likely have little damage. While the AAIB may have held off on downloading the recorders until all the major players are present, it's been several days - I'd expect everyone who matters is already there. So I think it is reasonable to believe that the investigators have done a download and have had at least a preliminary look at the data. If there is a smoking gun, they probably already know (and the longer we don't hear something regarding the rest of the 787 fleet, or at least the GEnx powered fleet, the less likely it is that they suspect a systemic problem with the aircraft and/or engine). However the proviso that I posted earlier about potential data loss/corruption due to a sudden shutdown still applies - so maybe the data simply isn't on the recorder. As has already been posted, EMI is highly unlikely - the current cert requirements for HIRF are quite high, and due to the composite airframe construction of the 787, the lighting requirements are much higher than for conventional aluminum aircraft (the higher resistance of the composite airframe results is higher lightning induced currents). FDR has suggested a large slug of water hitting critical aircraft electronics at rotation - it is possible that resultant electrical short circuits could falsely signal the engines that the switches are in cutoff. Highly unlikely that it would do that to both engines, but possible. Then again, all the other plausible explanations are highly unlikely, so... BTW, I do have a life outside PPRuNe - and I'm going to be traveling the next several days, with limited to non-existent internet access. So don't be surprised if I'm not responding posts or PMs. 16 users liked this post. |
| D Bru
2025-06-17T22:37:00 permalink Post: 11904725 |
EEC MN4 - TMCA
Without a doubt one is looking at a very, very rare event, most likely the result of an unlikely/unlucky combination of issues, the grim gravity realised by the F/C (mayday) immediately after T/O. As a very, very FF/retired EU bureaucrat/economist/lawyer with an as ever staunch penchant for data/facts, I'm therefore wondering whether VT-ANB engines had already their EEC MN4 microprocessors replaced as mandated within 11000 cycles or 12 years per AD 2021-21-05
https://www.govinfo.gov/content/pkg/...2021-25491.pdf
. This AD was prompted by in-service occurrences of loss of GEnX engines thrust control resulting in uncommanded high thrust. Uncommanded high trust on (at least one of) the engines during the TO-roll, in particular past V1, resulting in a discrepancy with the actual (likely derated)thrust settings, could have triggered TMCA on or just before lift-off.
3 users liked this post. |
| Someone Somewhere
2025-06-18T13:08:00 permalink Post: 11905228 |
I (and I think everyone else here) have been assuming that the FADEC does in fact have a dedicated permanent-magnet alternator, as is the case on the Airbuses (confirmed by FCOM) and surely the 737.
I have been told elsewhere that this is not the case. A read of the draft FCOM available online for the 777 & 787 makes no mention of a FADEC generator, but then neither does the 737 manuals. Is this simply a case of "Boeing thinks you don't need to know"? It has been proposed that the primary source of power for the FADECs is actually the flight control PMGs, mounted on the engine gearbox, but that this power goes to the avionics bay, has failover switching gear, and comes back to the EEC. Can anyone shed concrete light on this (e.g. a source that clearly states there is both an EEC alternator and a flight control PMG on the accessory gearbox)? Alternator and generator seem to be used interchangeably in this context. I don't think you'll find an actual wire list for it (or it won't be useful) as apparently most/all of the data is via an ARINC bus. I attempted to PM this but your inbox is full. [SLF with an electrical background and some exposure to ground-side critical facilities power] Last edited by Someone Somewhere; 18th Jun 2025 at 13:32 . 1 user liked this post. |
| 212man
2025-06-18T13:22:00 permalink Post: 11905242 |
I (and I think everyone else here) have been assuming that the FADEC does in fact have a dedicated permanent-magnet alternator, as is the case on the Airbuses (confirmed by FCOM) and surely the 737.
I have been told elsewhere that this is not the case. A read of the draft FCOM available online for the 777 & 787 makes no mention of a FADEC generator, but then neither does the 737 manuals. Is this simply a case of "Boeing thinks you don't need to know"? It has been proposed that the primary source of power for the FADECs is actually the flight control PMGs, mounted on the engine gearbox, but that this power goes to the avionics bay, has failover switching gear, and comes back to the EEC. Can anyone shed concrete light on this (e.g. a source that clearly states there is both an EEC alternator and a flight control PMG on the accessory gearbox)? Alternator and generator seem to be used interchangeably in this context. It's not quite that, but there is a list of received channels for a GEnx 787 in the FDR report into one of the original battery fires . I don't think you'll find an actual wire list for it (or it won't be useful) as apparently most/all of the data is via an ARINC bus. I attempted to PM this but your inbox is full. [SLF with an electrical background and some exposure to ground-side critical facilities power]
(h) Aircraft Supplied Electrical Power.
(1) The Engine Control System must be designed so that the loss or interruption of electrical power supplied from the aircraft to the Engine Control System will not - (i) Result in a Hazardous Engine Effect, (ii) Cause the unacceptable transmission of erroneous data. The effect of the loss or interruption of aircraft supplied electrical power must be taken into account in complying with CS-E 50(c)(1). |
| EDML
2025-06-18T14:00:00 permalink Post: 11905273 |
Yes. That is what I have read here, that is my assumption, that is how Airbus does it,
that is what makes sense
.
But I am being told elsewhere by someone with an A&P badge that that is not quite the whole story, and that the FADEC PMGs do double-duty as the flight control PMGs. I am hoping for some documentation to confirm/refute that. 3 users liked this post. |
| T28B
2025-06-18T14:17:00 permalink Post: 11905287 |
Well,
tdracer
worked on engine/airframe integration for Boeing. He did that for the GEnx on the B747-8. I would guess he knows more about those systems than some guy with an A&P badge.
They get to look into all of the various write-ups from the pilots when the aircraft and its systems don't work quite as they are expected to. Let's not dismiss that kind of input without due consideration for what it looks like from the point of view of someone whose task it is to keep things working. Pilots run into similar oddities - well, it's supposed to do this, per the manual, but it's doing something differently - now and again, right? 2 users liked this post. |
| lancs
2025-06-19T17:47:00 permalink Post: 11906225 |
I appreciate the link to Musician's posts. I had read them before and have now reread them, and yes, they add valuable info although for me they did not discount the potential for a problem.
As I understand it, the fuel temperature is measured in the tanks, not just before the centrifugal pump and it's not inconceivable that the nacelles are warmer. I also understand that the Fuel Synoptic Display only turns to Amber if the temperature is too high. As it's not in red it might thought to be one of those "it will be OK" alerts, especially if it's not unusual in Indian summers.. 1.3 Fuel Inlet Temperature (C): At engine fuel pump inlet: GEnx‐1B Engine Series Temperatures \xb0C Minimum ‐ 53.8 Maximum 65.5 6 users liked this post. |
| soarbum
2025-06-20T10:01:00 permalink Post: 11906794 |
Engineer not a pilot. Experience in analog front ends, A2D and R2D conversion and embedded systems generally but no specific knowledge of the 787 or GEnx.
I like everyone else have no evidence that TMCA played a role but given that it is one of the few systems with the ability to cut fuel to the engines, here are some thoughts on how signal processing could have extended the window of when TMCA could bite. In particular, I'm looking at the time immediately after the nose lifts up when something may have physically shifted onboard. I'll phrase it as a number of questions but realise that the few people who can answer may not be able to for now. Thanks to tdracer's explanation on TMCA (albeit 747 not 787), we know that TMCA is a logic block within the FADEC whose only external inputs are a logic signal fron the aircraft that indicates whether it is on the ground or not and throttle position as determined by two independent resolvers per throttle side. The logic would seem to be something of the form If (G AND (N2>A OR N2>B)) Then CutOffFuel() where G is true when the aircraft is on the ground, A is an envelope defined by throttle resolver channel A and B is an envelope defined by throttle resolver channel B Q1: Am I correct in that assumption that when on the ground, overspeed with respect to EITHER resolver A OR resolver B can trigger TMCA? We have been told that the logic (ie true or false) signal G is determined from the Weight-on-wheels sensors and the RadALT. It is reasonable to suppose that the designers still wanted TMCA to function after a hard landing where some landing gear components had failed. Q2: When the nosewheel lifts off but the MLG is still on the ground and RadALT is close to ground, will G still be true? Next, it is common when data fusing multiple inputs that there is a desire to clean up a signal before it is sampled digitally. This can remove effects such as switch bounce. The inclusion of low pass filters or hysteresis will generally add a propogation delay. Q3: Is there a slow filter (Tc>=1s) in the ground/air logic which could have caused a slight delay before G became false after takeoff further extending the opportunity of TMCA to activate? Q4: Does TMCA act almost instantly or does it wait for the fault condition to stay asserted for a period of time before acting? At that point, the total energy of the system would have comprised of the kinetic energy of the aircraft travelling at Vr, the rotational inertia of the engines and the potential energy of whatever fuel is beyond the cutoff valves. Q5: Would this total energy have been sufficient to get the aircraft 100ft into the air? It would still need a mechanism for at least one throttle input to each FADEC to misbehave at the same time. Resolvers are fed with an excitation signal to the rotor and take back two orthogonal signals (Cos and Sin) from stator windings. Usually, the excitation comes directly from the resolver-to-digital (R2D) circuit but sometimes an external signal source is used. I would hope that in an aircraft system, each channel would be kept independent of everything else. Q6: Does the excitation signal for the 4 throttle resolvers (2 per side) come from 4 independent (internal) sources? My last thought for a single point of failure between both throttles would be a short between two wires or connection points carrying resolver signals, one from each side. Whether this could be caused by swarf wearing within a wiring loom, a foreign object moving about, crushed wires or even stretching of adjacent wires, I have absolutely no idea. Q7: Do resolver signals from left or right, either channel A or B, run next to each other in a loom at any point? 4 users liked this post. |
| Innaflap
2025-06-20T11:02:00 permalink Post: 11906835 |
Engineer not a pilot. Experience in analog front ends, A2D and R2D conversion and embedded systems generally but no specific knowledge of the 787 or GEnx.
I like everyone else have no evidence that TMCA played a role but given that it is one of the few systems with the ability to cut fuel to the engines, here are some thoughts on how signal processing could have extended the window of when TMCA could bite. In particular, I'm looking at the time immediately after the nose lifts up when something may have physically shifted onboard. I'll phrase it as a number of questions but realise that the few people who can answer may not be able to for now. Thanks to tdracer's explanation on TMCA (albeit 747 not 787), we know that TMCA is a logic block within the FADEC whose only external inputs are a logic signal fron the aircraft that indicates whether it is on the ground or not and throttle position as determined by two independent resolvers per throttle side. The logic would seem to be something of the form If (G AND (N2>A OR N2>B)) Then CutOffFuel() where G is true when the aircraft is on the ground, A is an envelope defined by throttle resolver channel A and B is an envelope defined by throttle resolver channel B Q1: Am I correct in that assumption that when on the ground, overspeed with respect to EITHER resolver A OR resolver B can trigger TMCA? We have been told that the logic (ie true or false) signal G is determined from the Weight-on-wheels sensors and the RadALT. It is reasonable to suppose that the designers still wanted TMCA to function after a hard landing where some landing gear components had failed. Q2: When the nosewheel lifts off but the MLG is still on the ground and RadALT is close to ground, will G still be true? Next, it is common when data fusing multiple inputs that there is a desire to clean up a signal before it is sampled digitally. This can remove effects such as switch bounce. The inclusion of low pass filters or hysteresis will generally add a propogation delay. Q3: Is there a slow filter (Tc>=1s) in the ground/air logic which could have caused a slight delay before G became false after takeoff further extending the opportunity of TMCA to activate? Q4: Does TMCA act almost instantly or does it wait for the fault condition to stay asserted for a period of time before acting? At that point, the total energy of the system would have comprised of the kinetic energy of the aircraft travelling at Vr, the rotational inertia of the engines and the potential energy of whatever fuel is beyond the cutoff valves. Q5: Would this total energy have been sufficient to get the aircraft 100ft into the air? It would still need a mechanism for at least one throttle input to each FADEC to misbehave at the same time. Resolvers are fed with an excitation signal to the rotor and take back two orthogonal signals (Cos and Sin) from stator windings. Usually, the excitation comes directly from the resolver-to-digital (R2D) circuit but sometimes an external signal source is used. I would hope that in an aircraft system, each channel would be kept independent of everything else. Q6: Does the excitation signal for the 4 throttle resolvers (2 per side) come from 4 independent (internal) sources? My last thought for a single point of failure between both throttles would be a short between two wires or connection points carrying resolver signals, one from each side. Whether this could be caused by swarf wearing within a wiring loom, a foreign object moving about, crushed wires or even stretching of adjacent wires, I have absolutely no idea. Q7: Do resolver signals from left or right, either channel A or B, run next to each other in a loom at any point? What happens when the 2 disparate processes that form TCMA disagree? |
| Raffael with FF
2025-06-20T11:04:00 permalink Post: 11906838 |
Let me try to answer the questions about which I have some knowledge, as an aerospace engineer:
(I am not sufficiently informed to answer Q4,6 and 7, at the moment)
Q1: Am I correct in that assumption that when on the ground, overspeed with respect to EITHER resolver A OR resolver B can trigger TMCA?
We have been told that the logic (ie true or false) signal G is determined from the Weight-on-wheels sensors and the RadALT. It is reasonable to suppose that the designers still wanted TMCA to function after a hard landing where some landing gear components had failed.
At that point, the total energy of the system would have comprised of the kinetic energy of the aircraft travelling at Vr, the rotational inertia of the engines and the potential energy of whatever fuel is beyond the cutoff valves.
Q5: Would this total energy have been sufficient to get the aircraft 100ft into the air? Kinetic energy with a weight of 200,000kg, at Vr = 150kn = 77m/s: E_kin = 600MJ Rotational energy of a GEnX engine is hard to calculate as I don't find reliable values for the rotary inertia. I found some for a GE90 and could roughly estimate 100MJ of rotational energy for each engine. However, I seriously doubt that this energy could be effectively used to gain thrust, as the thrust will drop very quicjkly after the fuel is cut off. the required potential energy for a 100ft climb of a 200,000kg 787 is around 70MJ. This ignores aerodynamic drag, still, 100 ft of climb remains energetically feasible. However, it as been pointed out several times that the actual climb was higher than 100ft. Already for 200ft I would doubt the validity of my statement above. 2 users liked this post. |