OldnGrounded
2025-06-21T12:49:00
permalink Post: 11907719

Quote:

Originally Posted by The Baron

There was no sign of asymmetric thrust failure, but rather, nearly total loss of thrust just after rotation. Something has caused a catastrophic electric failure that has impacted the air /ground logic functions. There are signs that this event is unique, although there have been cases in the past where the logic has failed and the aircraft no longer knows if it's airborne or not. The crew were probably faced with something they weren't trained for and overwhelmed them. There would have been not enough time to troubleshoot this.

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 \x97 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?

Subjects: FADEC TCMA (Activation) TCMA (All)

2 users liked this post.

OldnGrounded
2025-06-21T17:05:00
permalink Post: 11907908

Quote:

Originally Posted by syseng68k

Aerospace101:

I think tdracer said upthread, that there are two rad alt sensors, and one wow, giving three. Need to verify, `but if two out of three are in agreement, that might be enough redundancy.

tdracer was detailing the inputs to TCMA on a 74 and has said repeatedly that he doesn't know the details of that function on 787s. I'm sure of that because I misread his original post and based a post of mine on the misreading. The inputs he described were multiple WoW switches and multiple RAs, with inputs from each group required to determine that the aircraft is on the ground to enable TCMA-initiated shutdown. It seems likely that the 787 is similar, but that hasn't been established here. I've been looking but haven't found the details yet.

Last edited by OldnGrounded; 21st Jun 2025 at 17:06 . Reason: Clarification

Subjects: TCMA (All)

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OldnGrounded
2025-06-22T01:35:00
permalink Post: 11908213

Quote:

Originally Posted by Lead Balloon

Perhaps my earlier post was incredible and that's what prompted the SLF's question.

Let us assume a simple, hypothetical WoW sensor arrangement: One sensor per main landing gear.

One of those sensors is indicating weight OFF wheels and the other is indicating weight ON wheels. What does the TCMA in each engine interpret that ostensibly contradictory sensor information to mean? (Note: For the time being, ignore the question whether the information is erroneous. It may be true.)

Are both engine TCMA's in the 'in the air' state, are both 'on the ground', or is one 'on the ground' and the other 'in the air'?

Given the purpose of the TCMA, I would have thought that any 'doubt' in this case would be resolved in favour of the 'on ground' state for both TCMAs.

But maybe it's the other way around. Maybe any 'doubt' would be resolved in favour of both TCMA's being in the 'in the air' state.

I have difficulty in envisaging any advantage in the TCMA system being designed such that one engine's TCMA is in the 'in the air' state and the engine's 'on the ground'.

Whichever the design and outcome, there will be benefits and there will be risks.

Yes, all true. And I didn't think your earlier post was incredible, just that it was an exercise not intended to explain everything about air/ground logic. I think that, in the real world, it's most likely that the air/ground decision will take inputs from other sensors, not just WoW. Radio altimeters seem the most likely choice and we've been told they've been used on earlier Boeings (which I think I already knew or assumed). I'd certainly feel safer with that arrangement.

Subjects: Gear Retraction TCMA (Air-ground Logic) TCMA (All)

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OldnGrounded
2025-06-22T14:04:00
permalink Post: 11908564

Quote:

Originally Posted by TryingToLearn

Since this damaged front recorder is the one thing which could solve the miracle of this crash, I would think twice before I would like to give it to a new lab as the first thing to play with.

The lab is new, not necessarily the scientists and technicians who work there. Indeed, that seems unlikely. And I rather suspect that those experts would be offended by a reference to their work in this horrific crash as "play." I know I would be.

Subjects: None

8 users liked this post.

OldnGrounded
2025-06-22T14:51:00
permalink Post: 11908593

Quote:

Originally Posted by TryingToLearn

I do SMD rework and failure analysis on automotive control devices and if there would be so much at stake, even if I would be a known expert, I would ask for something to 'play with' or in less offending words: test the equipment, process and success rate on something from the 'scrapyard'. Experience would prevent me from starting with the FDR from the flight.
Maybe they officially need a week to decide where to put it while exactly this is happening, they are doing a dry-run.

Sure. I often referred to the zillion-dollar systems I worked with as toys to play with, but only in some contexts and with some listeners. And I would likely be offended by my work in the context of a catastrophic event being referred to as playing. I think most of us might be.

Also, please note that I have no comment on whatever decisions may have been made or be pending, in this accident, about where to download and analyze the data. Well, no comment except that I see no reason to question the Indian authorities' ability to make those decisions appropriately and with adequate knowledge and expertise.

Subjects: FDR

5 users liked this post.

OldnGrounded 2025-06-21T12:49:00 permalink Post: 11907719	Quote: Originally Posted by The Baron There was no sign of asymmetric thrust failure, but rather, nearly total loss of thrust just after rotation. Something has caused a catastrophic electric failure that has impacted the air /ground logic functions. There are signs that this event is unique, although there have been cases in the past where the logic has failed and the aircraft no longer knows if it's airborne or not. The crew were probably faced with something they weren't trained for and overwhelmed them. There would have been not enough time to troubleshoot this. 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 \x97 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? Subjects: FADEC TCMA (Activation) TCMA (All) 2 users liked this post.
OldnGrounded 2025-06-21T17:05:00 permalink Post: 11907908	Quote: Originally Posted by syseng68k Aerospace101: I think tdracer said upthread, that there are two rad alt sensors, and one wow, giving three. Need to verify, `but if two out of three are in agreement, that might be enough redundancy. tdracer was detailing the inputs to TCMA on a 74 and has said repeatedly that he doesn't know the details of that function on 787s. I'm sure of that because I misread his original post and based a post of mine on the misreading. The inputs he described were multiple WoW switches and multiple RAs, with inputs from each group required to determine that the aircraft is on the ground to enable TCMA-initiated shutdown. It seems likely that the 787 is similar, but that hasn't been established here. I've been looking but haven't found the details yet. Last edited by OldnGrounded; 21st Jun 2025 at 17:06 . Reason: Clarification Subjects: TCMA (All) 3 users liked this post.
OldnGrounded 2025-06-22T01:35:00 permalink Post: 11908213	Quote: Originally Posted by Lead Balloon Perhaps my earlier post was incredible and that's what prompted the SLF's question. Let us assume a simple, hypothetical WoW sensor arrangement: One sensor per main landing gear. One of those sensors is indicating weight OFF wheels and the other is indicating weight ON wheels. What does the TCMA in each engine interpret that ostensibly contradictory sensor information to mean? (Note: For the time being, ignore the question whether the information is erroneous. It may be true.) Are both engine TCMA's in the 'in the air' state, are both 'on the ground', or is one 'on the ground' and the other 'in the air'? Given the purpose of the TCMA, I would have thought that any 'doubt' in this case would be resolved in favour of the 'on ground' state for both TCMAs. But maybe it's the other way around. Maybe any 'doubt' would be resolved in favour of both TCMA's being in the 'in the air' state. I have difficulty in envisaging any advantage in the TCMA system being designed such that one engine's TCMA is in the 'in the air' state and the engine's 'on the ground'. Whichever the design and outcome, there will be benefits and there will be risks. Yes, all true. And I didn't think your earlier post was incredible, just that it was an exercise not intended to explain everything about air/ground logic. I think that, in the real world, it's most likely that the air/ground decision will take inputs from other sensors, not just WoW. Radio altimeters seem the most likely choice and we've been told they've been used on earlier Boeings (which I think I already knew or assumed). I'd certainly feel safer with that arrangement. Subjects: Gear Retraction TCMA (Air-ground Logic) TCMA (All) 2 users liked this post.
OldnGrounded 2025-06-22T14:04:00 permalink Post: 11908564	Quote: Originally Posted by TryingToLearn Since this damaged front recorder is the one thing which could solve the miracle of this crash, I would think twice before I would like to give it to a new lab as the first thing to play with. The lab is new, not necessarily the scientists and technicians who work there. Indeed, that seems unlikely. And I rather suspect that those experts would be offended by a reference to their work in this horrific crash as "play." I know I would be. Subjects: None 8 users liked this post.
OldnGrounded 2025-06-22T14:51:00 permalink Post: 11908593	Quote: Originally Posted by TryingToLearn I do SMD rework and failure analysis on automotive control devices and if there would be so much at stake, even if I would be a known expert, I would ask for something to 'play with' or in less offending words: test the equipment, process and success rate on something from the 'scrapyard'. Experience would prevent me from starting with the FDR from the flight. Maybe they officially need a week to decide where to put it while exactly this is happening, they are doing a dry-run. Sure. I often referred to the zillion-dollar systems I worked with as toys to play with, but only in some contexts and with some listeners. And I would likely be offended by my work in the context of a catastrophic event being referred to as playing. I think most of us might be. Also, please note that I have no comment on whatever decisions may have been made or be pending, in this accident, about where to download and analyze the data. Well, no comment except that I see no reason to question the Indian authorities' ability to make those decisions appropriately and with adequate knowledge and expertise. Subjects: FDR 5 users liked this post.

Posts by user "OldnGrounded" [Posts: 45 Total up-votes: 95 Pages: 3]