Posts about: "Engine Failure (All)" [Posts: 410 Pages: 21]

Lookleft
2025-06-20T05:20:00
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Post: 11906598
Assuming there is some credence to the article, dual engine failure due to water contamination is the leading theory.
No it can't be. All discussion about power loss is to be focused on TMCA, FADEC and RATs being deployed. Anything else has already been discussed.

Last edited by Lookleft; 20th Jun 2025 at 05:57 .
Musician
2025-06-20T05:30:00
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Post: 11906603
TCMA things, imagination and evidence

Originally Posted by neila83
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.
Yes. I simplified. The point stands that the throttle needs to be pulled back, as it was in the ANA event, because that was a landing and not a take-off.

Originally Posted by user989
V. Shutdown of engines by TCMA
A parallel is drawn to the ANA incident. However, this would require not only a fault in the air/ground logic but also a sensed discrepancy between T/L position (not necessarily idle) and thrust output on both engines simultaneously.
First, you posted a good summary. I'd have added "unanticipated hardware fault" and "unanticipated software fault" as generic causes.

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.


Originally Posted by Lead Balloon
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?
It has been mentioned before that this capability existed as part of the N2 overspeed protection: the FADEC would shut down a runaway engine by cutting its fuel before it disintegrates.
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.
The thrust lever sensors are wired directly to the FADEC (and hence the TCMA). No data bus is involved with this item.

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 50 feet (?) below aerodrome elevation .
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

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Lead Balloon
2025-06-20T05:47:00
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Post: 11906608
Originally Posted by ignorantAndroid
I don't have any direct knowledge, but yes, that's my understanding based primarily on tdracer's comments. It also just makes sense. I'm pretty confident that all the necessary hardware already existed because of the need for N2 overspeed protection. A failure in one FADEC channel could drive the FMV fully open, leading to an overspeed and uncontained engine failure. For regulatory purposes, it would be unacceptable to have a single point of failure with catastrophic consequences, so it would be necessary to make the inactive FADEC channel capable of cutting off fuel in that case.

The air/ground signal would've already been present as well. It would be needed for switching between ground idle, flight idle, and approach idle. Tdracer has discussed that as well, in past threads.
I see the logic. If the N2 exceedance functionality in the FADEC already included authority to shut off fuel in some circumstances, might as well 'piggyback' TMCA output onto that part of the FADEC functionality.

Re air/ground input to the TMCA, I am more interested in whether that is hardwired from the sensors / systems involved or - as appears to be the case just based upon what I've read here - read from ARINC or other format data.

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skwdenyer
2025-06-20T06:18:00
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Post: 11906620
Originally Posted by ignorantAndroid
TCMA is simply a bit of software in the FADECs, so it has the same separation as everything else. There's no inter-engine interaction when it comes to TCMA.
There is no inter-engine interaction. But if both FADECs are running the same software then they will potentially behave identically to some bug or external input / condition. That's why software redundancy isn't always redundancy.

Originally Posted by ignorantAndroid
This is technically possible, but of course the FADECs would have the ability to shut down the engines anyway, even if TCMA didn't exist. If there's a software bug, it could involve TCMA but it could easily be somewhere else.
I agree.

Originally Posted by ignorantAndroid
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.
Thank you. So saying "TCMA would only trigger if WoW and RA have failed somehow" is incorrect? Those are simply inputs to software, which might itself fail badly for other reasons. See the FAA AD to replace FADEC software on certain P&W engines following a previous uncommanded dual engine shutdown.

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sabenaboy
2025-06-20T07:45:00
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Post: 11906669
Originally Posted by Musician

"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 a crash site 50 feet (?) below aerodrome elevation.
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.
You're absolutely right, Musician! Your text in bold print is what happened! And you and I and many other pilots know what the most probale cause for that is. What evidence do we need?
The EAFR will tell the story, but the reason for the crash will always remain a "mystery" because the B787 was not equipped with EPTPR's! ( E nhanced P ilot's T hought P rocess R ecorders)

I think AI171 will go down in history with MSR990 an MH370.
amsm01
2025-06-20T08:36:00
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Post: 11906715
Originally Posted by wheelsright
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.
There you go, that’s the tell I expect. Someone sloshing chemicals around and it was finally been noticed in the paperwork somewhere and then they start checking the accident records or vv

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

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Someone Somewhere
2025-06-20T08:41:00
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Post: 11906722
Engine failure due to water contamination is surely a different investigation to biocide contamination? I expect they're looking into both, but they're not that closely linked.

Surprising that you can do nearly a minute of takeoff+climbout then fail cleanly and silently within seconds of each other.

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ignorantAndroid
2025-06-20T08:53:00
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Post: 11906736
Originally Posted by skwdenyer
So saying "TCMA would only trigger if WoW and RA have failed somehow" is incorrect? Those are simply inputs to software, which might itself fail badly for other reasons.
In general, we can classify computer errors into 3 categories:
  1. Errors in system design, specifications, or algorithms. These are cases where the computer does exactly what it was designed to do, but the design itself was flawed or inadequate, or had unforeseen consequences. This would include things like MCAS on the 737 MAX.
  2. Software errors. These are cases where the computer does exactly what the code tells it to do, but not what the designers wanted it to do. This results from mistakes in writing the actual code, and this is usually what we'd refer to as a "bug." This includes things like race conditions, loops that fail to terminate, incorrect type conversion, etc.
  3. Hardware malfunctions. These are cases where the computer does something different from what the code instructs. It can involve memory corruption or data bus corruption. It can result in a system that appears to work, but returns incorrect outputs, e.g. a calculator saying 2+2=5. It can also cause the computer to execute valid instructions, but at an inappropriate time. It can result from manufacturing defects in components, cosmic rays (SEU or SEE), radiofrequency interference (HIRF), moisture ingress, failed solder joints, and numerous other things.
As I said previously, I think we can rule out category 3 in this case. Hardware malfunctions are essentially random events. They're inherently unpredictable since there's little to no relationship between what the system was supposed to do and what it actually does. It would be astronomically improbable for the same failure to occur on both engines at the same moment.

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:
  • Airplane on ground
  • Thrust higher than commanded by thrust lever angle (TLA) for some period of time
It's reasonable to assume that the first condition is common to both engines. But that still leaves the second. To my knowledge, there's no plausible scenario that would cause that condition to be met on both engines simultaneously. Each thrust lever has 2 resolvers which are wired directly to the corresponding FADECs. That means the signals don't pass through any common component. An incorrect reading from one resolver could conceivably trigger TCMA, but I don't see any reason why that would happen to both engines simultaneously.

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 .

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martinebrangan
2025-06-20T09:07:00
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Post: 11906750
Originally Posted by Someone Somewhere
Engine failure due to water contamination is surely a different investigation to biocide contamination? I expect they're looking into both, but they're not that closely linked.

Surprising that you can do nearly a minute of takeoff+climbout then fail cleanly and silently within seconds of each other.
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?
MaybeItIs
2025-06-20T09:24:00
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Post: 11906767
Originally Posted by Musician
"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 a crash site 50 feet (?) below aerodrome elevation.
Isn't "close to rotation" a little broad? "Close to" can be before or after. If before, and with about 4,000 feet of runway remaining, why did they take off at all? How did they take off, for that matter?

Can anyone do the Momentum / Energy calculations to work out how high the plane would have travelled at the normal climb gradient purely on the momentum it had at rotation? I'll try, see how far I get. A stone, fired into the air at an upward angle, begins to slow down and curve towards the earth the moment it leaves the catapult. It appears to me that the plane climbed at approximately a steady speed until about the 200 ft mark, so I submit that it had adequate climb thrust up to "about" that point.

Which, as confirmed in the earlier thread, is about where GEARUP is typically called. I say those two events are linked, led by GEARUP, but it could be coincidence. Though I don't think so. Coincidence usually refers to unrelated events and that would be very hard to say, here.

On that point, the gear, as far as I can establish (not openly published according to Google), weighs around 8-odd to 10 tonnes. Typically, retracts in about 10 seconds. I estimate it's no more than a 2 metre lift. As far as I can work out (using 3m to make the value higher), that requires about 30kW (rough estimate, budgetary figure, not accounting for it being a curved path, so it's probably higher closer to fully up), but whether wind pressure affects it, I have no idea. Anyway, 30kW isn't a huge (additional) load on a 225kVA alternator. Less than I'd imagined.

Now I'm wondering how big (power ratings) the hydraulic pump and motor are? No doubt, they're driven by a VSD. Can anyone comment, please?

Last edited by MaybeItIs; 20th Jun 2025 at 09:37 . Reason: Oops, numbers mixed up...
FullWings
2025-06-20T09:43:00
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Post: 11906781
Originally Posted by lederhosen
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?
Under most circumstances you would apply the benefit of the doubt but as the RAT deployed (something there is a lot of evidence for) it strongly suggests this. Yes, the RAT can deploy for other reasons but that would imply an even greater level of coincidence than two engines failing in a short period (3 hydraulic systems, 4 generators, etc.). The distance they went until ground contact also ties in with a loss of pretty much all thrust, as does the audio recording of idling/windmilling engines. There is also the fact, which may turn out to be an assumption, that any failure of other aircraft systems should not affect engine operation as a) the engines are effectively self-powered in flight and b) the engine controls on the flight deck are part of an isolated system powered by the FADECs.

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.

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BFM
2025-06-20T10:19:00
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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

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Lead Balloon
2025-06-20T11:17:00
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Post: 11906849
Originally Posted by BFM
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. ...
Not quite. It was software for generator controllers. That bug, glitch or whatever it was accurately described to be was not the cause of any engine failures (so far as I am aware).
MaybeItIs
2025-06-20T11:21:00
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Post: 11906857
Originally Posted by Musician
It would definitely be nice to narrow that down; I can't do it, and your method is not convincing me.
I'd agree, without expert knowledge there would have to be big assumptions, but I expect that Aircraft Flight Engineers could do it. Probably already have.

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.
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.

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.
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.)
Okay, didn't know that, I guess suggests means it's uncertain? Can you tell me from what height to what height it suggests this?

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.
Haha! Even a stone (the right shape) can do that, and I'm not disputing that kinetic energy can be converted to altitude. Wings are useful for that... Just curious to get an idea of how much in this case.

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]

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Luc Lion
2025-06-20T11:51:00
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Post: 11906889
I perfectly understand that there is much talking about TCMA here.
There is no direct evidence of what caused the crash but several indirect evidences point towards a near simultaneous shutdown of both engines without any visual clue of a catastrophic mechanical mishap. This leads to suspecting near simultaneous fuel starvation of both engines.
As the purpose of TCMA is shutting down the High Pressure Shut-Off Valve (HPSOV) and thus the fuel feed of an engine, it's normal to collect information on TCMA, on how it works, and on what data feeds it.

However, I hardly understand why there is no similar discussion about the spar valves and the systems that control their opening and closure.

I understand that the B787 spar valves are located in the MLG well, or at least are maintained from within that well.
If the engine shutdown happened when the gear retraction was commanded, that's a location commonality (although it's very unlikely that a mechanical problem happened in both wells at the same time).
Also I understand that there are several systems that command the opening or closing of the spar valves:
- opening: "Engine control panel switch" set to "START", or "Fuel control switch" set to "RUN"
- closing: "Engine fire handle" pulled out. (I wonder if "Fuel control switch" set to "CUTOFF" also closes the spar valve).
Are there direct wires running from these controls to the valves or is there a pair of control units receiving these signals and controlling the valve actuators?
If the latter is true, where are these control units? I guess that the likely location is the aft EE bay. Are they beside each other?

Last edited by Luc Lion; 20th Jun 2025 at 12:57 .

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Musician
2025-06-20T12:13:00
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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.
Originally Posted by MaybeItIs
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.
Right. ADS-B is transmitted via radio, so reception can be patchy, or obstructed by someone else transmitting on the same frequency (e.g. other aircraft), so not every datagram that the aircraft sends gets received. When that happens, the live display of FR24 assumes the aircraft kept doing what it did, and when another datagram eventually comes in, it corrects the position. It also connects the locations of these datagrams, regardless of whether the aircraft actually went there. For example, in the AI171 there's a 4-minute gap between a datagram sent on the taxiway, and the next datagram sent when the aircraft was off the ground towards the departure end of the taxiway. FR24 then connected these points via the shortest route; but we know that the aircraft actually used the intervening 4 minutes to taxi to the approach end of the runway, where it then started its take-off run. So that was false. (Another source of errors is when different FR24 receivers don't have synchronised clocks, so a mixture of data from these can have weird artifacts as a result.)
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.
Have you ever seen a parabolic trajectory from "the short end"?
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.
Yes.

(The ADS-B data suggests the speed diminished 7% for ~50 ft of climb.)
Okay, didn't know that, I guess suggests means it's uncertain? Can you tell me from what height to what height it suggests this?
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., but that's MSL and not adjusted for the weather: low air pressure makes that number higher than the actual altitude. FR24 adjusted this to 21ft climbing to 71 ft, but it could've been 30 to 60 or maybe 10 to 80, as it's rounded. I think it's fairly close to 50 feet of climb, though.

And why all the wrong figures for the height attained, quoted in previous thread? Can't all be the atmospheric conditions.
1) people taking the MSL altitude literally (625 ft)
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.
The survivor likened the sound to a car engine revving up. If you've listened to a good version of the phone video, you'll have noticed the "vroom" sound at the start that some likened to a motorcycle. That sound is the RAT in action, and you can imagine what that would sound like when it rapidly spins up: like a driver stepping on the throttle with their car engine in neutral.
If that's the case then what does that do to the possibility or otherwise that the TMCA system caused a dual engine shutdown?
The RAT deploying is a consequence of a dual engine shutdown. It says nothing about whether the TMCA was involved.

[Now I just hope your post is still there as I post this. ]

Last edited by Musician; 20th Jun 2025 at 12:26 .

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Sailvi767
2025-06-20T13:30:00
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Post: 11906974
Originally Posted by sSquares
Shutting down the wrong engine is not extremely rare:
  1. GoAir320 at Delhi
  2. Transasia AT72 at Taipei
  3. Alitalia A332 at Seoul
  4. SA Airlink JS41 at Durban

Not saying it happened here!
Shutting down the wrong engine below 400 AGL is extremely rare. So rare in fact that I believe it has not happened in a jet transport class aircraft.

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Sailvi767
2025-06-20T13:38:00
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Post: 11906979
Originally Posted by BraceBrace
During a flameout it takes a while before the fan has slowed down sufficiently. As long as it rotates, thrust is generated.

With full reverse thrust you know that when de-selecting reverser from idle reverse, you have to give the engines plenty of time to slow down otherwise you would get the little "kick in the back" as during deceleration you would generate instant forward thrust when the reversers are stowed.
The fan never stops rotating in a normal engine loss. Having been through a catastrophic engine failure in a 767 I can tell you that trust stops almost instantly. Certainly no more than 2 seconds. It also needs to be understood that thrust is not linear to engine speed in a jet. Very little thrust is generated below 70% RPM and thrust increases rapidly above 85%. The fan simple becomes a big source of drag almost immediately if the turbine section shuts down or fails.

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MaybeItIs
2025-06-20T13:47:00
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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!]

Originally Posted by Musician
When that happens, the live display of FR24 assumes the aircraft kept doing what it did, ... FR24 then connected these points via the shortest route;
Aha! Very misleading by FR24! Maybe they can devise a way to show where data is missing.

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.
Sorry, I got confused about this ^^^, when you wrote this:

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.,
but I'm assuming reliable within the 25ft granularity that the 787 creates, which mostly is neither here nor there, but in this case, not so helpful!

The RAT deploying is a consequence of a dual engine shutdown. It says nothing about whether the TMCA was involved.
Yes, got it, but wasn't what I was meaning to ask, sorry. My bad. I'm about over this TMCA stuff because it sounds like it's Trade Secret so a lot of guesswork, but the question I was intending was:

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. ]
Thankfully, Yes. Hope this Reply gets through too.

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oyaji-fr
2025-06-20T13:49:00
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Post: 11906988
Hi

Originally Posted by BraceBrace
During a flameout it takes a while before the fan has slowed down sufficiently. As long as it rotates, thrust is generated.
This got me thinking, looking at the engine mounted shutoff valve (HPSOV) in ATA 76, I see that its operated by three sources
  • FADEC, as discussed in a number of posts above
  • RUN / CUTOFF switch
  • engine fire control panel
I have a hard time believing the pilots would have touched the engine fire controls under such conditions (obviously they are highly trained for engine failure before Vr), but am I correct when I say that the behavior of the plane systems closely resembles what would happen if the engine fire signal was triggered?
  • engine fuel spar and high-pressure valves would be cutoff (obviously)
  • hydraulics pump would be depressurized and shutoff from the circuit
  • electrical generators would be disconnected
We wouldn't be talking about this flight had this occured on a single engine, so I believe this should had happened on both at about the same time.

Looking at ATA26 the engine fire control panel is energized by the hot battery bus (HOT BB). Is it credible that a failure of the hot battery bus (for example due to damage or liquid ingress in the P300 panel ) could lead to this situation?

Last edited by oyaji-fr; 20th Jun 2025 at 14:07 .