Posts about: "Thread Moderation" [Posts: 120 Pages: 6]

I was not aware that we have granular ADS-B data from the a/c itself showing airspeed post rotation (rather than speed interpolated from GPS). Apologies if I have missed it. If it does show acceleration after takeoff I tend to agree with you.

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.

This is a new thread with a small selection of the contributions from the original Air India crash thread which is now closed. By no means comprehensive they give a grounding for further discussion here on PPRuNe.

Please remember this is a Professional Pilots forum, and speculation by new signups, SLF and others will only be accepted if proven relevant and accurate.

On a lighter note we achieved our highest number of viewers ever a couple of days ago, highlighting the interest that PPRuNe has as a source of factual information across the Forums. My sincere thanks to our regular members and to the Mod Team for their continued hard work behind the scenes 👍

To Senior Pilot , T28B and all the other nameless Mods who have been moderating this and the previous thread, may I thank you all most sincerely for your excellent work. As an ex-senior flight manager, now long retired, I know from experience how thankless these tasks can be. I am sure other readers of these two threads will join me in thanking the whole team.

Not an avionics specialist, but electronics / software engineer here, with extensive experience in hardware fault tracking, protocol monitoring and software debugging in embedded systems : mods, feel free to delete this post if I am completely out of track (and thank you for the huge amount of work you've done trying to keep this discussion clean).

After I have read the whole thread, I think most of the community agrees about a lack of engine thrust being the cause of the crash. Searching in that direction, I'm trying to "think out of the box", discarding the usual suspects (birds ingestion, TCMA, human mistake...), and to find a plausible single point of failure among the various subsystems involved. I was thinking of reversing the causality of the event, i.e. exploring a case where the engines would have behaved unexpectedly because of a major electrical failure, instead of the already explored case where both powerplants went AWOL first.

Therefore, I have a couple questions for tdracer / fdr / other informed contributors (BTW, fantastic contribution guys, please keep the good info coming):

1. From the scarce info available, is it reasonable to conclude that the engines were totally shut down? Could they have just been set to idle or reduced thrust instead?

2. In the second case, if (and that's a big IF) a major electrical failure happened first (which could have triggered RAT deployment), and considering this plane is a FBW aircraft, could there exist a case where the FADECs would command idle thrust -- or significant thrust reduction -- because they receive invalid input data from the throttle controls? Kind of a garbage in-garbage out case?

The associated scenario would be: major electrical fault (with subsequent RAT deployment) -> major protocol disturbance on ARINC/AFDX buses -> FADECs detect invalid data from the controls -> FADECS enter some kinf of safe mode and command reduced or idle thrust.

Does it make sense or is it pure fantasy?

No. The throttle position sensors (dual per engine) are part of the FADEC. The throttle position data is not transmitted through the ARINC busses of the aircraft.

Senior Pilot has written in this thread:

He has very wisely chosen his words carefully, read them literally!

A few posters have sent pleading PM's asking why their post was deleted, while others remains. Please just go with the flow posters, and re read the quote above. The moderator's actions are final, and we don't have the time to personally justify moderation actions. This post, following on to Senior Pilot's and T28's post are a reminder of very general justification of moderator actions.

This very sad event could touch on a troubling factor, it is not ruled out yet, neither authoritatively suggested. We, professional pilots, on a professional pilot website will give our colleagues the benefit of the doubt. If new facts come to light from an authoritative (certainly NOT A.I.) source, we will discuss them.

There have been pleads from some to weed out newbie speculation posts, this thread is that! For PPRuNe it's not going to be to exclude non pilots, but this thread shows our effort to put very authoritative posters in the forefront.

Moderating this thread has been a lot of work, we're doing our best. The event now has a big vacuum, waiting for the flight data recorder information to be formally released. Many of us have speculation, and it can only remain that until flight data recorder facts are publicly available to show what we really should be discussing.

The best thing maybe just to hold your speculation, until you are commenting on known facts , and I'm confident that we will, when the time is right...

Pilot DAR

​​​​​​​Agree. Ban them, mods. Maintain Prune's high standard.

I'm afraid that this reconstituted thread is more polluted worse than the original as mods try to hold back this flood of sewage. While there are multiple examples of air disasters caused solely by astonishingly egregious malfeasance by pilots, I see absolutely no point in unsupported speculation about such things. I can't ever say "no professional aircrew would ever do that" but spouting off random theories with no evidence and no practical experience or knowledge of the actual operations involved is really annoying and pointless. If you are a GA pilot, SLF with an engineering degree or a sim enthusiast then maybe, just maybe, you can manage to ask an intelligent question--preferrably one that hasn't already been asked and answered five times already. But please keep your inane hypotheses and conclusions to yourself for now. There's no prize for guessing correctly!

There\x92s been unsupported speculation about everything else up to this point. Which involves professional aspects of engineering design, maintainers, fuelers, etc. While it was phrased poorly ignoring operational factors isn\x92t right either.

B787 Skipper.... No longer able to sit on my hands!

The joker that published one or other of the fictional accident reports before the dead are even identified needs stringing up by the thumbs.... Well done mods. OK, some of these theories, just bearing in mind the the flight recorder (there is only one FDR + CVR combined) was in the intact section of the undamaged tail and has been with the authorities for almost five days......

- All you guys who are rushing down the TCMA rabbit hole: If it was established that a software error drove both engines to idle without warning, after rotate.... Don't you think the worldwide fleet of B787s would have been grounded by now? Such a glaring failure would be absolutely inescapable on the FDR.

Whatever was wrong with this aircraft was present at rotate, unbeknown to the crew. The fact that no ADs or notices to operators have been issued usually means that the cause is known and the aircraft was serviceable. The statement from a prominent Indian Captain about the skill and tenacity of the crew, right up to the last minute is absolutely laudable. However, the cynic in me says that the way is being paved for some bad news and by that I mean news that will do Air India reputational damage. Expect more management of expectation in the coming days.

I'm still going with
a) Incorrect derate + low Vspeeds or
b) Low altitude capture

SLF/attorney here, and staying in (what I hope will be seen as) very narrow limits for posting here.

Disclosures of information by unofficial sources during on-going Annex 13 investigations would not seem to be unusual - not to excuse the occurence. If I'm reading OldnGrounded's post (at 10:04) correctly, it isn't really unexpected.

I went back and reread the AvH report from which gear lever's post (at 4:27; AvH link at 4:43) was taken. Yes, AvH misidentifies an ELT. But consider: the content of the unofficially-sourced statement about the CVR-type portion of the forward recorder does not appear to have indicia of unreliability. To the contrary, as other posters have pointed out, by now the CVR-type portion of the recorder would have been read in all likelihood. Further, the rest of the June 17th unofficially-sourced statement aligns with the absence of any emergency ADs, or at least most all of that statement aligns that way. As for how the unofficial source was described, well of course that description would be vague or ambiguous - for one thing, the AvH wants to keep its source inside the flow of events and information.

As long as this post already is risking intrusion on the serious aviation-community discussion going on, I'll just add that the mods are to be greatly appreciated for many more forum hours into the future, for their incessant attention to the threads. Thank you!

Do you stand by your previous assertion :-

a) 🤷‍♂️
b) Wouldn\x92t it capture the Alt rather than descending thru it?

Maybe you should have kept sitting on your hands, theory a) in particular is about as risable as the last one you posted. Do you still think engine failure takes 30 seconds for the plane to react?! Thanks to rkenyon for alerting us not to take your post seriously. You still don't believe the RAT was out despite the in depth audio analysis done by an expert in the field. You still think its a motorbike?!

Given they took off at a perfectly normal point, at a perfectly normal speed, I don't see how you come to incorrect derate and low v speeds, or how on earth that would lead to a normal takeoff followed by sinking?

Without incurring the wrath of the moderators, can I add another improbable and maybe with current understanding impossible possibility.

Electro magnetic interference. Intended or accidential.

I apologise that it might be another rabbit hole, but at present, and to quote the esteemed tdracer, 'never say never' and 'this is perplexing'.

I looked at the possibility of a Directed Energy Weapon (DEW) being used to disrupt the onboard systems; India's Defence Research and Development Organisation (DRDO) has been developing several types of DEW to counter drones ( ADITYA and KALI projects). However this seems to be a very, very unlikely cause due the extent of shielding on the 787 electronics.

Yes, there's another snag in the million dollar Q for the explanation. If the MN4 microprocessor can run amok, so could perhaps the TCMA not exactly act as intended. But I take in both your pertinent comments and will (as per the mods advice) from now on sit on my hands and only read until the prelim report hopefully soon!

BR, D Bru

I agree with this. For all the SLF reading and not posting ( good work!) there is a delay between the end of the takeoff run and the gear being selected up. I fly 737’s so any 78 folk feel free to correct me but it looks like this;
PM: “ V1 ….Rotate”
The PF then begins to rotate the aircraft up to a pre determined attitude which is normally between 13 and 15 degrees. They do this at a rate of between 2 and 3 degrees per second so about 5 or 6 seconds later the aircraft is at its climb out attitude. The PM is then looking at their instruments to confirm that the aircraft has a positive rate of climb, this takes a moment, maybe 1 to 3 seconds then;
PM “ positive rate”
PF: “ gear up”.
So minimum 8 seconds but probably longer between the PM calling “rotate” and the gear being selected up.
The relevance of all that is to say that if you suspect that the gear up cycle has been interrupted by a dual engine failure, then the engines may well have been producing thrust up to an altitude of 50-100ft or so, which ties in nicely with the max height reached, distance travelled etc.
Mods this is clearly not a theory, just info for those who don’t fly airliners to aid understanding.

I don't agree. There are lots of subtopics and I think the Mods are doing a commendable job in keeping the thread readable and (for me at least) informative.

I don't think everyone here realises what a lot of work it is to keep a forum like this up and running and the interactions appropriate. It's huge. And not everyone manages it. I was recently on the forums of a professional engineering society to which I belong. I had to leave. They couldn't identify or control the bots; they certainly couldn't moderate absurd or inappropriate comments -- and this with a professional moderator.

Thank god someone has pointed out the absolute cobblers some people who claim to have some experience of this type (and other completely unrelated types) have been spouting. And if anyone else wants to erroneously compare the 757/767 low level EPR ALT CAP scenario , that keeps the TO thrust on, it doesn\x92t reduce it! This is truely one of the worst of these accidents threads I\x92ve read in a long time, I pity the mods.