Posts about: "Hydraulic Failure (All)" [Posts: 103 Pages: 6]

mechpowi
2025-06-17T15:25:00
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Post: 11904394
RAT powered systems

There’s been some debate over what electrical and hydraulic systems are powered. According to one unauthorized and Not For Flight -labeled FCOM copy that seems quite authentic:
“In-Air Rat Only Operation” mode (Standby Power) – If loss of all electrical power to captain’s and first officer’s flight instruments occurs in flight, the RAT energizes the captain’s flight instruments and other essential equipment for flight controls, navigation, and communication. The main battery provides standby power until RAT deployment. Significant loads energized by this mode include:

captain’s inboard DU
• captain’s outboard DU
• lower DU
• autoflight system
• MCP
• autopilot
• captain’s / first officer’s ACP
• captain’s / first officer’s
flight interphone
• VHF L
• TCP L
• DSP L
• MFK L
• CCD L
• IRU L, R
• AHRU L, R
• INR L, R
• ISFD
• FMC
• PFC
• CCR L
• center pitot heat
• engine/APU fire detection
• miscellaneous lightin

Note: Autothrottle, flaps, slats, stabilizer, thrust reversers, auto speedbrakes, air conditioning / pressurization, Head-Up displays, HF radios, SATCOM systems, external lighting, WIPS, and window heat are inoperative on standby power.

“In-Air Battery Only” mode (Standby Power) –
This mode energizes the same
equipment as the “In-Air RAT Operation Only” mode with the exception of center
pitot heat.
The list of inoperative systems include most/all secondary fligh controls powered by the center hydraulic system. There we can conclude that centre hydraulic system electric pumps are not available when operating on APU RAT (or battery) only electrical mode.

Last edited by Senior Pilot; 17th Jun 2025 at 21:44 . Reason: Remove false information

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rkenyon
2025-06-17T17:19:00
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Post: 11904487
Originally Posted by Magplug
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
Do you stand by your previous assertion :-

Originally Posted by Magplug
Speaking as a B787 Captain..... There is so much rubbish and stupid suggestion being written here.

- 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.

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Aerospace101
2025-06-17T19:01:00
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Post: 11904572
Originally Posted by Irishshamrock
If power was lost at the moment gear was selected \x91up\x92 it would have started the process only for it to stop, hence why the forward tilt was present - perhaps
That is one conclusion (which I now suggest is unlikely), because you are assuming:

1. The hydraulic failure happened exactly when the truck had tilted forward but the split-second before the gear doors could open. That exact timing seems too coincidental.
2. The crew selected Gear Up. We have no proof of this. I speculate the crew never got as far as "Positive Rate...Gear Up" because they were already engrossed in flying the aircraft and processing their thrust problem. If an electrical problem had developed (as evidenced by the RAT deployment) the flight instrumentation would have been flickering and a flurry of silent master caution alerts would be very distracting.

I suggest it's more likely that the truck remained in a forward tilt from the takeoff run because the hydraulic failure happened prior to rotation, therefore keeping it in this unusual position. This answer does not rely on the gear lever position either. It also insinuates that their hydraulic problems occurred probably between V1 and VR. See my earlier post .

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dragon6172
2025-06-18T02:05:00
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Post: 11904837
Originally Posted by Xeptu
It doesn't have to be both engines. The loss of one engine with the gear down will still produce the same outcome.
That still doesn't explain the bogies stowed position for retraction unless there was a total hydraulic failure, which could only occur if the aircraft was despatched with an unserviceable hydraulic system on the live engine side. Which in any case would be a no no.
The landing gear is run off of the center hydraulic system which is pressurized by electric pumps. The left and right hydraulic systems run their respective thrust reversers and some flight controls, and that is it.

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Xeptu
2025-06-18T04:28:00
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Post: 11904891
Originally Posted by dragon6172
The landing gear is run off of the center hydraulic system which is pressurized by electric pumps. The left and right hydraulic systems run their respective thrust reversers and some flight controls, and that is it.
Thankyou I didn't know that and I'm not questioning you. I guess it doesn't matter electrically driven or engine driven, they cannot share the same power supply the gear must be retracted within 12 seconds. this in my opinion improves the argument for total electrical failure. Unless of course gear up was never selected..There must also have been a substantial loss of thrust on at least one engine.
Aerospace101
2025-06-18T16:46:00
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Post: 11905393
Loss of all Hydraulics

I still consider the forward truck tilt is a massive clue to a C system Hydraulic failure prior to wheels-up, which must have been caused by loss of electrics (since C is solely electrically powered), so I am still wondering if hydraulic failure happened before or after the loss of thrust.

While thinking about the consequences of a total hydraulics failure around time of rotation (caused by a suspected dual engine failure), here is a new observation. I searched the previous threads to see if anyone lese had noticed it.

Observation/Question - in the accident video, the view from behind the aircraft as it pitches up seems to show inboard spoilers aft of the engines on both wings partially open - is that what I'm seeing? Pic at 99 above roof top video screen shot see here] also shows what I'm looking at... Spoilers, or markings on the wings maybe?

@
treadigraph




In the rooftop video, as it's just approaching the treeline, there are spoiler deflections visible just behind the engines on each wing.

With a total hydraulics failure, the pilots control column using direct wiring, will only control this spoiler pair and the stabiliser. The RAT does not control this spoiler pair (hydraulically), only the most inboard spoilers pair. Its an interesting observation because it means this spoiler pair were being deflected electrically, either by the battery or RAT. But if the RAT provides emergency C hydraulic power why didn't the RAT powered spoilers deflect instead? Does this mean the RAT was unable to provide emergency hydraulic at such low airspeed?

I think this also re-affirms the critical loss of power (dual engine failure), and rules out many other theories. It tragically lost all power, both engines, all hydraulics and electrics (apart from battery and RAT).

I am wondering if anyone else has noticed other flight control deflections?

(Tried posting link to youtube but unhelpfully frames entire video)

Last edited by Aerospace101; 18th Jun 2025 at 16:48 . Reason: Removed youtube video

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Roo
2025-06-19T11:19:00
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Post: 11905941
The RAT is a small turbine that operates an electrical generator and a hydraulic pump.
Well said thanks &
“A ram air turbine (RAT) pump converts mechanical input power into hydraulic power for the center system flight controls. The RAT is in the right, aft wing-to-body fairing.”

Last edited by Saab Dastard; 19th Jun 2025 at 12:11 . Reason: Quoting deleted post
JPI33600
2025-06-19T11:34:00
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Post: 11905954
The RAT is an electrical generator, not a hydraulic pump. How many times does this need to be said?
To make things clear, just check this B787-related alert service bulletin dated 25 Nov 2014 (my bold):

This service bulletin provides instructions to replace the Ram Air Turbine (RAT) Pump and Control Module
Assembly to prevent failure of the hydraulic pump at low air speed. The RAT Assembly provides an emer-
gency source of electrical and hydraulic power
for the primary flight control if the left, center and right main
hydraulic systems fail. Loss of the RAT Pump and Control Module Assembly could lead to loss of control
of the airplane when emergency power from RAT Assembly is needed. If this change is not incorporated
on the RAT Assembly and hydraulic power is lost on the left, right and center main hydraulic systems, then
the RAT Assembly may not provide sufficient hydraulic power which could result in the loss of many critical
control systems that are necessary for safe flight.


787 RAT hydraulic pump location

Last edited by Senior Pilot; 19th Jun 2025 at 11:40 . Reason: Image

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Someone Somewhere
2025-06-19T12:25:00
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Post: 11905981
RAT types vary significantly by aircraft family. The 777 and 787 types (along with most BBD aircraft) are indeed dual hydraulic-electric (lifted from the 2010 FCOM available online):


Other types are different. The A350/A380 do have an electric-only RAT with adequate electric flight controls.

The A320/A330 have a hydraulic-only RAT with a separate hydraulic-driven electrical generator. The 757/767 are similar except the generator is optional.

Some used an electric RAT to drive an electric hydraulic pump.

Be careful when attempting to transfer knowledge from one type to another.

Last edited by Saab Dastard; 19th Jun 2025 at 15:45 . Reason: Reference to deleted post removed

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Aerospace101
2025-06-19T14:11:00
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Post: 11906054
Originally Posted by PBL
It does not follow that MCAS malfunction is a software malfunction.
As far as I know, the software functioned exactly as it was specified/required to function. The problem did not lie in the quality of the software, as you suggest. It lay in the functional requirements for the function, and the hazard analysis of those requirements, and those are manufacturer tasks.
I thought this was a very good point. There are some detailed posts discussing the TCMA patent including this post . Is it possible the TCMA software functioned exactly as specified, and the issue is input data into TCMA?

In a total electrical failure, when the system switches to emergency battery power, how are input variables like rad alt and wow switches processed? (these were inputs someone mentioned on the 747-8, have the TCMA inputs been identified yet?)

I speculate the gear truck forward tilt is a symptom of a C hydraulic failure caused by a total electrical failure around the time of VR. Once they got 10 deg nose up on the rotation, with a total electrical failure, could the FADEC receive erroneous rad alt or wow inputs, and how would TCMA handle these inputs in the transition from ground to air logic?

What is baffling is the simultaneous nature of the suspected dual engine shutdown. There is no obvious asymmetry, with the flight path or rudder movements. If the engine fuel control switches had been manually cut one at a time, there should have been some visible flightpath change or flight control response. Something happened to both engines at exactly the same time.

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user989
2025-06-19T23:26:00
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Post: 11906480
Summary of main theories

DISCLAIMER: Poster (a) is one of the (apparently quite numerous) lawyers following this thread; (b) a long-time forum lurker and aviation enthusiast who loves studying FCOMs for fun (to each his own, I guess); (c) has followed and read this thread from the start.

What I cannot do is add new theories or uncover any new facts the actual experts have not already thought of. However, since summarizing and structuring information is one thing lawyers tend to regularly do (and sometimes even do well), here is my attempt at a useful contribution to this thread: an attempt to summarize the main theories discussed here since day one (which I think hasn't been done for quite some time) in the hope that a birds-eye view will be helpful to those who have not read everything since the beginning or might even trigger some new flash of inspiration for someone more knowledgable than me. I have focused on the cons since there does not seem to be enough evidence to come to any positive conclusion.

I shall try to be concise and to refrain from personal evaluations of my own. Of course, no disrespect whatsoever is intended towards all those who have contributed to this thread and to the individual theories, one or combinations of which may turn out to have led to this tragic outcome. That arguments can be made against every single theory that has been propagated seems to be the result of the highly improbable and unusual nature of this deplorable event and certainly not due to any lack of knowledge or reasoning skills in this forum.

DEAR MODS: If I have distorted anything or if, meaning well, should have achieved the opposite \x96 I guess you know where the delete button is\x85

Anyway, here goes:

A. Misconfiguration or wrong takeoff data
Widely refuted, since
  • rotation, takeoff and initial climb seem normal;
  • likely extreme errors would have been required to have such tragic effect (the fuel tanks should have been only about half full, so not close to MTOW);
  • there is strong evidence that at least some flaps were extended for takeoff (post-crash photo, perhaps also visible in video from behind)
B. Flaps retracted post-takeoff instead of gear
Still brought up from time to time. However, widely disregarded due to
  • the fact that with two working engines an inadvertent flap retraction should easily be recoverable, even with gear down;
  • strong indications that hydraulic and electric power were lost (audible/visible indications of RAT extension, survivor statement, lack of engine noise, position of MLG bogies).
For a while, the forward tilt of the bogies as first part of the retraction cycle was seen as additional evidence that the gear had been selected up. However, it has been pointed out that the forward tilt and the opening of the gear doors occur almost simultaneously so that it seems unlikely that hydraulic power was lost in the split second between bogie tilt and gear door actuation. It is now assumed the forward tilt of the bogies was merely a consequence of the hydraulic power loss.
It should be pointed out that the question of "RAT in or out" was for a while the most contentious in this thread.

C. Low-altitude capture
Still argued, even if refuted by many since
  • inconsistent with apparent loss of hydraulic/electric power;
  • PF would have been flying manually (however, A/T reaction would have been unexpected for the PF);
  • should have been recoverable (unless one assumes that the crew (a) remained unaware of the changed FMA annunciations although alerted by the unexpected FD commands; and (b) was so startled that an A/T thrust reduction was not noticed and corrected, even though the PF was apparently sufficiently alert not to follow the FD commands).
D. Loss of both engines at or shortly after rotation
Various possible reasons for this have been discussed:

I. Bird strike/FOD
  • Would have to have occurred simultaneously due to lack of rudder/aileron input indicating symmetric thrust.
  • No remains/traces on runway, no visual indications (flocks of birds, flames, structural engine damage).
II. Fuel-related
1. Loss of electric fuel pumps
Suction feed would have provided sufficient fuel pressure.

2. Fuel contamination
No other aircraft affected, no measures taken at airport. Simultaneous flameout due to contaminated fuel very unlikely.

3. Vapour lock
Unlikely to occur in this scenario. Even if (momentarily) no sufficient fuel pressure from the center tank, the engines would have been fed by the wing tanks.
III. Improper maintenance
Unclear which maintenance measures could possibly have been performed that would have resulted in simultaneous loss of both engines. No apparent relationships between malfunctions reported by previous passengers and essential systems.

IV. Large-scale electrical fault (e.g. due to water in E&E bay)
The engines will continue to run if electrical power is lost. FADECs are powered independently.

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.

VI. (Inadvertent) shutdown by flight crew
1. Spontaneous execution of memory items (fuel control switches OFF, then ON; deploy RAT) due to assumed engine malfunction
In contrast to mistakenly shutting down the wrong engine after having correctly diagnosed the problem as per SOP, this would require not only a simple error in execution but a counter-intuitive unilateral action immediately after takeoff against basic principles of SOP or CRM.

2. No indications whatsoever of an intentional shutdown for nefarious reasons
(Would also be inconsistent with the content of the alleged mayday call.)

VII. Malfunction/mishandling of the fuel cutoff switches (most recent)
1. Wear or improper operation of the switches, so that they do not lock but can shift back into the OFF position.
Argued to be impossible due to robust switch design, preventing switch release in any other than a locked position.
Actuation of the switches by an item placed before them which was pushed onto the switches by retarding thrust levers seems equally unlikely due to force required to pull the switches out of the locked position.

2. Spilled drink leading to short in the wiring
Hardly conceivable that before takeoff open liquid containers would be placed anywhere where they could spill onto the pedestal.


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Someone Somewhere
2025-06-20T01:02:00
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Post: 11906517
Originally Posted by StudentInDebt
No, it doesn’t “only” happen in these cases!
It is very, very, very close:

Both engines failed: yup, both engines have failed.
Triple hydraulic pressure low: either you've been hit by a SAM/uncontained engine failure causing massive fluid leaks, or both engine driven pumps have failed (likely because the engines have failed) and all four electric pumps have failed (because the engines have failed).
Loss of all electric power to flight instruments both sides: total AC electric loss, and I think battery/static inverter too? Given four generators and four buses, either massive electrical failure (swimming pool in E&E bay) or engines have failed. Note failure of an individual contactor that can tie two buses together should not cause a quad-bus outage.
Loss of all four electric motor pumps: total AC failure, see above.

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StudentInDebt
2025-06-20T01:13:00
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Post: 11906520
Originally Posted by Someone Somewhere
It is very, very, very close:

Both engines failed: yup, both engines have failed.
Triple hydraulic pressure low: either you've been hit by a SAM/uncontained engine failure causing massive fluid leaks, or both engine driven pumps have failed (likely because the engines have failed) and all four electric pumps have failed (because the engines have failed).
Loss of all electric power to flight instruments both sides: total AC electric loss, and I think battery/static inverter too? Given four generators and four buses, either massive electrical failure (swimming pool in E&E bay) or engines have failed. Note failure of an individual contactor that can tie two buses together should not cause a quad-bus outage.
Loss of all four electric motor pumps: total AC failure, see above.
thanks for posting the conditions, while likely it deployed as a result of a simultaneous loss of both engines, not the only circumstances then.
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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Someone Somewhere
2025-06-20T09:48:00
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Post: 11906783
Originally Posted by MaybeItIs
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?
Listed here as 37GPM at 4750PSI each for the EMPs, and 27GPM for the EDPs - most unusual to have beefier EMPs than EDPs. Source looks like it could be a wealth of other information.

37GPM at 4750PSI is ~76kW before considering pump, motor, and converter losses. Ouch.

Very surprised they kept the demand pumps for left/right systems the same size given they only do flight control and perhaps reverser loads - and reverser operation off an EMP is presumably rare as it implies the EDP failed (or was MELed) without the corresponding engine.

777 centre EMPs were apparently only 6GPM 3000PSI with gear/flaps using the air-driven demand pumps.

(we may be re-approaching 'hamster wheel' territory)

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TURIN
2025-06-20T10:26:00
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Post: 11906808
Originally Posted by MaybeItIs
... 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?
The fixed doors on the landing gear are set at an angle that will ensure they assist gravity deployment in an emergency. During retraction that means the effort to stow the gear is increased due to these air loads. This is is standard on all the large aircraft I have worked on.
As for hydraulic pumps, they are limited to how much flow they can produce. The pressure drops significantly during large control movements and the landing gear actuators in particular need a large flow to keep them moving.
When all pumps are operating, engine driven or otherwise that pressure drop is limited, when down to just one small RAT driven pump there's only so much it can do and the design ensures that control of the aircraft can be maintained on just RAT power.
There won't be enough power from the RAT generator to power emergency aircraft systems and large hydraulic pumps. This is why it has its own small hydraulic pump.
Aerospace101
2025-06-21T00:41:00
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Post: 11907411
Originally Posted by MaybeItIs
Anyway, FWIW, not everyone agrees with RAT Deployment - see recent post by shep69. Would love to know why he doesn't go with RAT deployment...
For those postulating the RAT was not deployed, what counter explanations do you have for the following clues?
  • Distinctive RAT sound in the rooftop video, audio analysis here .
  • RAT visible in rooftop video, example in this image .
  • APU door open suggesting auto APU start, suggestive of a full electrics failure (one of the criteria for auto RAT deployment)
  • Loss of ADSB data suggestive of a full electrics failure (one of the criteria for auto RAT deployment)
  • Unusual gear forward tilt position, suggestive of hydraulic failure and/or full electrics failure (one of the criteria for auto RAT deployment).
  • Loss of all thrust, ie dual engine failure (one of the criteria for auto RAT deployment)

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MaybeItIs
2025-06-21T01:21:00
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Post: 11907432
Originally Posted by Aerospace101
For those postulating the RAT was not deployed, what counter explanations do you have for the following clues?
  • Distinctive RAT sound in the rooftop video, audio analysis here .
  • RAT visible in rooftop video, example in this image .
  • APU door open suggesting auto APU start, suggestive of a full electrics failure (one of the criteria for auto RAT deployment)
  • Loss of ADSB data suggestive of a full electrics failure (one of the criteria for auto RAT deployment)
  • Unusual gear forward tilt position, suggestive of hydraulic failure and/or full electrics failure (one of the criteria for auto RAT deployment).
  • Loss of all thrust, ie dual engine failure (one of the criteria for auto RAT deployment)
Great summary. I've already mentioned the first below, but I'd add another:
  • The existence (and timing) of the flyby video by a young lad who apparently lived where the footage was shot from. With planes flying past every few minutes, why would he choose to film this one, before he could even see it? The video starts with the plane still approaching, out of view, and his position suggests it was unplanned, before he could move to a better vantage point. I say he already knew it was extraordinary - from the sound.
  • Eye witness account from the mother of the lad who filmed the flyby, apparently said that the plane was "shaking". I'll assume she didn't know how to describe it properly, and that maybe it sounded like it was shaking, from hearing the noise from the RAT. Or it's a translation issue of a word/s with multiple meanings or used colloquially.
One question - are there two exterior doors to the APU compartment, one on top, one below, presumably inlet and outlet of cooling airflow? I've seen photos showing two open doors, but the lower one could be something else, and busted open during the crash.

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lighttwin2
2025-06-21T15:46:00
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Post: 11907858
Originally Posted by OldnGrounded
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 — 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?
Let me try and summarise one possible scenario and then link in some of the better posts provide evidence relating to it:
  • In error, PF reduces power to idle and/or reverse at a speed after V1 (either deciding to reject, or for some unexplained reason e.g. the recent BA incident at LGW)
  • Decision is changed to continue take-off, thrust levers moved to TOGA
  • Let's say the thrust inputs are similar to NM985 and TCMA is triggered; and engines shut down around the time of rotation
  • A/C rotates achieving a maximum speed in the region of 184kts
Relevant "ruling out" questions, with links to posts that add new information:

Q: Would the a/c have enough kinetic energy a 184kts to climb to 100-150ft agl and then reach its final position if the engines had failed at, or just, before rotation?
A: Theoretically possible - see calculation here . NB, the a/c actually flew 1.5km from the end of the runway and 2.3km from the likely point of rotation.

Q: Doesn't the forward position of the gear mean that power failed after the pilots had selected gear up?
A: Inconclusive - had hydraulic power had been lost prior to rotation, the gear could also be in this position - explanation here

Q: If the throttle levers were brought to idle during take-off, would the A/C have applied autobrake, reversers and speedbrake?
A: Yes, although there is a built in delay before reverser and speedbrake actually deploy - see here .

Q: Is the ADS-B data consistent with this scenario?
A: Yes, e.g. the Flightradar data shows the aircraft decelerating rapidly (12 knots in 4.2 seconds) from close to rotation. However, it's not clear how accurate this data is. For one, the altitude data is +/- 25 feet, second, while I was under the impression FR would have received airspeed data from the a/c sensors, this post suggests maybe not.

Q: Does TCMA activation require the thrust levers to be at idle or does it function when the thrust levels are above idle, but where the actual thrust is above that commanded?
A: No, the latter is true (i.e. idle is not required) - confirmed here - there are of course many protections against false activation

Q. Did AI171 have the same software version / logic paths as NH-985
A. Unknown. That a/c had Trent 1000s so to some extent the software is different, but we understand the TCMA logic is broadly the same regardless of engine. I have not seen a post clarifying whether the TCMA software has been updated /changed via SB since 2019 to account for this incident.

Be grateful if posters could refrain from speculative responses "e.g. I think this is unlikely because I feel x". I am not opining on how likely this sequence of events is, simply trying to summarise whether or not this theory has been ruled in or out.

I also recommend this post for a summary to read before posting. .

Last edited by lighttwin2; 21st Jun 2025 at 16:13 .