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

das Uber Soldat
2025-06-15T08:34:00
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Post: 11902234
Originally Posted by Icarus2001
There is no evidence of an electrical failure. What evidence? A surviving passenger thought he saw flickering lights? Give me a break.
Well, could be that. Or...

Could be that the gear is quite clearly in the forward tilt position (indicating initial gear retraction has commenced), but then the gear never goes up. How does that occur? The gear in the 787 is driven in retraction by the center hydraulic system. How is the center hydraulic system powered? Electrically, via C1 and C2 EMP's. How are those EMP's supplied with power? Engine 1 and 2 (via a bootstrap from L1/2 R1/2 gens). The rat also connects to the center hydraulic system, but importantly, it does not supply hydraulic power to the gear. Only a select number of flight control surfaces.

So, my question to you is, given you're claiming there is zero evidence of electrical failure, how did the gear move into the forward tilt position, but then not retract?

Originally Posted by Icarus2001
For the children on holiday, yes I fly transport category jets, current on two types.
Me too. Including the 787-8. Do you fly the 787-8?


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Sailvi767
2025-06-15T10:58:00
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Post: 11902355
Originally Posted by JunkAccount
This is probably a very stupid question, but what would happen if a BPCU fault (or other cause) led to VFSGs on opposite sides of the aircraft being connected to the same 230 VAC bus?

My understanding is that the left engine VFSGs are not synchronized in frequency or phase with the right engine VFSGs. Cross-connecting them, electrically, could be quite violent from both an electrical and mechanical perspective.

Is it realistically possible that the torque shock from cross-connected VFSGs could damage their associated accessory drive trains to the extent that the associated FADEC alternators would no longer make power? In this situation, there would be a loss of aircraft electrical power due to the BPCU fault, no FADEC alternator power due to damage to the accessory drive train, and, therefore, no engine thrust.

I presume each VFSG has a frangible link to protect the accessory drive train in the event the VFSG seizes up, which ought to make this loss-of-engine-thrust scenario impossible, but presumption is not knowledge, and this is a possible failure chain that doesn't involve stacking up multiple 10e-9 events.
What you suggest might be plausible. I had a tower shaft snap on a 767. The engine quits immediately. You lose fuel flow, oil pressure, generator and hydraulic pressure instantly. That could account for the gear not coming up. In a normal shutdown or flameout hydraulic pressure is maintained for a considerable period of time and windmilling will provide some pressure. I would have expected the gear to move further up in the retraction cycle. Tie this in with claimed electrical issues and the concept is at least interesting.

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old dawg
2025-06-15T11:03:00
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Post: 11902361
Retired engineer here. Following my post a while ago on the avionics electrical system I have read all the posts and also noticed mention of the hydraulics system.
Returning to my original source, which is Book 1 Introduction to B787 Avionic/Electrical, I read on p. 96 that the RAT will deploy if any of three conditions are met.

https://fliphtml5.com/quwam/qhdw/Boo...ics_Electrical

These conditions for deployment of the RAT specifically are:
Loss of both engines
Loss of power to the instrument buses
Loss of all three hydraulic systems

The latter one may be worth a close look because it would appear that problems took place when the wheels left the runway and I assume there was a change of states in various sensors. I surmise these sensors are different from the engine systems where both commands and power are needed to force a change of state in, say, fuel pumps. Is it the same for thrust control?
It says there are three hydraulic systems but is there a common reservoir? I'm not an expert in that field but google tells me that B787 has a bootstrap reservoir system which I understand to mean that a pressure of 5000 psi is maintained using a piston arrangement.

At this point think timeline, and changes of states.
There is an operational change when the wheels leave the ground. The associated sensors would send that data to the CCS. What was sent? Maybe the CCS read Hydraulic L + Hydraulic R + Hydraulic C = incorrect or fail, which would trigger deployment of the RAT. What would the electrical and control system do then? More importantly what exactly did all the systems do on this aircraft following such an event.
Was there a problem with the fluid in the hydraulics? Does hydraulic fluid ever 'go off' in very hot conditions. Or maybe there wasn't as much fluid in there than there should have been? How would hydraulics systems be compromised if indeed that was the case.
All speculation - but forensic system analysis is a bit like that.
Finally - what was the noise the survivor heard? Was it before or after the lights flickered? It may have been a bit of the airframe hitting something and snapping.
The survivors in the doctor's hostel heard a noise too which may be jet engines running. They would know the difference between that and other noises being close to an airport. Need a timeline for everything here.
Apologies for the long post. Just my thoughts.
RIP to all who didn't survive.

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Someone Somewhere
2025-06-15T11:16:00
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Post: 11902370
Originally Posted by old dawg
Retired engineer here. Following my post a while ago on the avionics electrical system I have read all the posts and also noticed mention of the hydraulics system.
Returning to my original source, which is Book 1 Introduction to B787 Avionic/Electrical, I read on p. 96 that the RAT will deploy if any of three condions are met.
Maybe the URL will work this time

https://fliphtml5.com/quwam/qhdw/Boo...ics_Electrical
These conditions for deployment of the RAT specifically are:
Loss of both engines
Loss of power to the instrument buses
Loss of all three hydraulic systems

The latter one may be worth a close look because it would appear that problems took place when the wheels left the runway and I assume there was a change of states in various sensors. I surmise these sensors are different from the engine systems where both commands and power are needed to force a change of state in, say, fuel pumps. Is it the same for thrust control?
It says there are three hydraulic systems but is there a common reservoir? I'm not an expert in that field but google tells me that B787 has a bootstrap reservoir system which I understand to mean that a pressure of 5000 psi is maintained using a piston arrangement.

At this point think timeline, and changes of states.
There is an operational change when the wheels leave the ground. The associated sensors would send that data to the CCS. What was sent? Maybe the CCS read Hydraulic L + Hydraulic R + Hydraulic C = incorrect or fail, which would trigger deployment of the RAT. What would the electrical and control system do then? More importantly what exactly did all the systems do on this aircraft following such an event.
Was there a problem with the fluid in the hydraulics? Does hydraulic fluid ever 'go off' in very hot conditions. Or maybe there wasn't as much fluid in there than there should have been? How would hydraulics systems be compromised if indeed that was the case.
All speculation - but forensic system analysis is a bit like that
Three hydraulic systems, each with their own reservoir, pumps, and accumulators. Engine bleed air (I think; some newer aircraft have a semi-permanent nitrogen charge) keeps a few tens of PSI of positive pressure in the reservoir to prevent pump cavitation; loss of this is not an emergency.

Left and right hydraulics have an engine driven pump that will keep turning as long as the engine is turning unless explicitly disabled.

Low reservoir levels are both a maintenance check and something that will raise an EICAS warning.

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blind pew
2025-06-15T12:52:00
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Post: 11902440
Originally Posted by nachtmusak
I'm a bit bemused by all the confidence that a few days passing means anything other than operational error can be ruled out. These are a few "serious flaws" (you can quibble about the definition of "serious") involved in fatal catastrophe that I can recall:

- The DC-10 cargo door issues involved in the loss of Turkish 981

- The 747 cargo door issues involved in the United 811 accident

- The 737 rudder issues involved in the loss of United 585 and USAir 427

- The 737 MCAS issues involved in the loss of Lion Air 610 and Ethiopian 302

I would mention Lauda Air 004, but in fairness the loss of the flight data recorder hampered the investigation.

Anyway, I don't recall the case being cracked and fleets worldwide being grounded within mere days of any of these accidents. In fact several of them had fingers pointed at primarily flight crew or ground crew error before deeper investigation brought these flaws to light. You can chalk the first three up to ancient times, but the MCAS saga is recent enough to have had all the telemetry and other fancy modern tools and methods that this accident's investigators will have at their disposal. And there's also (thankfully) non-fatal flaws like with the Pratt & Whitney 4000 that ultimately led to affected 777s being grounded, but not immediately after the first fan blade failure incident in 2018 either.

Again I'm not saying that this accident definitely falls one way or the other but that's my point really - closing your mind to the possibility of anything other than operational error because "we'd know by now" seems a bit premature. If anything it seems to me like a design/procedural/mechanical flaw would take more time to surface than simple pilot error.
World wide grounding of the DC10 fleet after Chicago where maintenance had modified engine removal procedures which led to a wing engine loss taking out hydraulic systems that allowed the slats to retract ..aircraft stalled assymetrically and rolled inverted. Many other aircraft had damaged mounts/bolts not all were reported.
nachtmusak
2025-06-15T13:16:00
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Post: 11902458
Originally Posted by blind pew
World wide grounding of the DC10 fleet after Chicago where maintenance had modified engine removal procedures which led to a wing engine loss taking out hydraulic systems that allowed the slats to retract ..aircraft stalled assymetrically and rolled inverted. Many other aircraft had damaged mounts/bolts not all were reported.
I'm not quite sure that one counts as a design flaw? If we were to try to list all the times that cocked-up maintenance/repair led to an accident we'd probably run afoul of the character limit...I tried to keep the list down to things in the design that actively increased the probability of or exacerbated the outcome of an accident (like the DC-10 cargo door loss being able to collapse the cabin floor). In any case the grounding came two weeks after the accident, no? Pretty quickly, but not exactly three days.

This accident too could well be down to a maintenance error, but given that the investigators are not magicians, I don't see how they'd be able to say so with confidence after just a few days. Surely they'd need to review logs, do some lab analysis, etc for that? Perhaps worth noting that an inspection of Air India's 787 fleet has been ordered, but I've seen it dismissed as a CYA move. Might not be...

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OldnGrounded
2025-06-15T13:29:00
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Post: 11902475
Originally Posted by blind pew
World wide grounding of the DC10 fleet after Chicago where maintenance had modified engine removal procedures which led to a wing engine loss taking out hydraulic systems that allowed the slats to retract ..aircraft stalled asymetrically and rolled inverted. Many other aircraft had damaged mounts/bolts not all were reported.
AA191. The NTSB report with probable cause was published within less than a year, IIRC. But it wasn't all that difficult to determine once they knew that forklifts were being used to R&R engines. In the 737 rudder hardovers, I think eight years passed between the first crash and a probable cause finding (in the second crash) that implicated the PCU servo. Of course, as someone has already posted, much more monitoring, data recording and transmission is available now in 21st century aircraft, although the accident aircraft in this case may not have transmitted much in the time available.

Last edited by OldnGrounded; 15th Jun 2025 at 13:37 . Reason: Typo. to not yo
First_Principal
2025-06-16T00:57:00
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Post: 11903017
Originally Posted by grumpyoldgeek
Not trying to be a smart ass, but how do you "ground test" a RAT?

From the detail in this video:

" Just a short video of the Boeing 787 RAT being driven by an attached hydraulic motor. This test is performed to check the RAT's hydraulic pump and electrical generator are functioning correctly. The motor that is bolted onto the back of the RAT is driven using an external hydraulic rig that is feeding the motor with hydraulic fluid at 4200psi at 40GPM ."

FP.

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Magplug
2025-06-13T22:13:00
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Post: 11903713
Speaking as a B787 Captain..... There is so much rubbish and stupid suggestion being written here.

This aircraft was airborne for a grand total of 22 seconds, half of which was climbing to no more than 150' aal.

- No Flaps? Due to the setup of the ECL it is physically impossible to go down the runway without some sort of take-off flap set. The T/o config warning would have been singing it's head off. Despite assertions to the contrary I have seen no video clear enough to detect a lack of flaps.

- RAT out? Almost impossible, I have seen no quality footage that definitively witnesses the RAT being out. Those who think they car hear a RAT type noise might be listening to a motorcycle passing or similar. It takes a triple hydraulic failure or a double engine failure to trigger RAT deploment. They happily went through V1 without a hint of rejected take off so as they rotated the aircraft was serviceable. These are big engines, they take a long time to wind down when you shut them down. I have never tried it however engine failure detection takes 30s or for the aircraft to react and they were not even airborne that long.
- Flaps up instead of gear? The B787 flaps are slow both in and out. Given that the 'Positive rate' call is not made the second the wheels leave the ground, a mis-selection of flaps up would not cause any loss of lift for at least 20 seconds, by which time they had already crashed. I believe the gear remained down not because of mis-selection but because of a major distraction on rotate.

Discounting the impossible, two hypotheses remain:

1. Invalid derate set through incorrect cross-checking. Trundling down the runway takes very little power to reach Vr. It is only when you rotate that you create more drag and discover that you do not have sufficient thrust vs. drag to sustain a climb. Or....
2. Put 200' as the altitude target in the FCU. Immediate ALT capture and all the power comes off. PF is still hand flying trying to increase pitch but is already way behind the aircraft.

It could be after this that Boeing are forced to review the B787 practice of exploring the very edges of the performance envelope.

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TURIN
2025-06-13T23:49:00
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Post: 11903714
Originally Posted by Magplug
Speaking as a B787 Captain..... There is so much rubbish and stupid suggestion being written here.

This aircraft was airborne for a grand total of 22 seconds, half of which was climbing to no more than 150' aal.

- No Flaps? Due to the setup of the ECL it is physically impossible to go down the runway without some sort of take-off flap set. The T/o config warning would have been singing it's head off. Despite assertions to the contrary I have seen no video clear enough to detect a lack of flaps.

- RAT out? Almost impossible, I have seen no quality footage that definitively witnesses the RAT being out. Those who think they car hear a RAT type noise might be listening to a motorcycle passing or similar. It takes a triple hydraulic failure or a double engine failure to trigger RAT deploment. They happily went through V1 without a hint of rejected take off so as they rotated the aircraft was serviceable. These are big engines, they take a long time to wind down when you shut them down. I have never tried it however engine failure detection takes 30s or for the aircraft to react and they were not even airborne that long.
- Flaps up instead of gear? The B787 flaps are slow both in and out. Given that the 'Positive rate' call is not made the second the wheels leave the ground, a mis-selection of flaps up would not cause any loss of lift for at least 20 seconds, by which time they had already crashed. I believe the gear remained down not because of mis-selection but because of a major distraction on rotate.

Discounting the impossible, two hypotheses remain:

1. Invalid derate set through incorrect cross-checking. Trundling down the runway takes very little power to reach Vr. It is only when you rotate that you create more drag and discover that you do not have sufficient thrust vs. drag to sustain a climb. Or....
2. Put 200' as the altitude target in the FCU. Immediate ALT capture and all the power comes off. PF is still hand flying trying to increase pitch but is already way behind the aircraft.

It could be after this that Boeing are forced to review the B787 practice of exploring the very edges of the performance envelope.
I have to agree with everything here except your assertion about engine shutdown.
Even though these are big engines with plenty of inertia, when you select engine shut off they spool down very quickly if on load. IE, The generators, two per engine and hydraulic pumps, etc, being driven by the (relatively) small mass of the N2 rotor will drag the speed down very quickly, the gennies will trip offine in seconds, the pumps will quickly reduce flow and pressure.
As for what went wrong.
If the engines have stopped working there has to be a common failure mode, fuel is one but as has been said, no other aircraft has had a problem, as far as we know. FOD? It would have to be something major to shut down two GeNX engines and there would be debris all over the runway, we would know by now.
I have no idea if the RAT has deployed, I can't see it in the video and the noise could be something else.
We shall see.
There is compelling evidence that flaps are set correctly and not retracted inadvertently.
I await further evidence.
Edit to add. LAE 40 years, type rated on 737 to 787 with lots of others in between.

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First_Principal
2025-06-16T00:57:00
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Post: 11903734
Originally Posted by grumpyoldgeek
Not trying to be a smart ass, but how do you "ground test" a RAT?

From the detail in this video:

" Just a short video of the Boeing 787 RAT being driven by an attached hydraulic motor. This test is performed to check the RAT's hydraulic pump and electrical generator are functioning correctly. The motor that is bolted onto the back of the RAT is driven using an external hydraulic rig that is feeding the motor with hydraulic fluid at 4200psi at 40GPM ."

FP.

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Feathers McGraw
2025-06-16T22:33:00
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Post: 11903844
I'd like to mention something that, while unrelated, does shed a bit of light on how computerised systems can misinterpret input data and take the wrong action. I spent 40 odd years as an electronics engineer involving complex systems, it can be surprising just how careful one must be in systems that sample data and then process it for decision making in software.

On August 9th 2019, there was a significant grid failure in the UK leading to load shedding (removing supply to many consumers, including Newcastle Airport) which started when a series of several lightning strikes in Hertfordshire caused a trip out of generators at Little Barford combined-cycle gas turbine generation plant. This was followed by the shut down of the power concentrator and grid connector from the Hornsea1 off-shore wind farm, significant changes in the grid frequency away from the acceptable limits which is what triggered further load shedding.

The reason I mention it is that Hornsea1 going off line was due to the software that operated the concentrator/connector sensing large voltage transients due to the lightning strikes 120 miles away, however these transients were only of the order of 10us length spikes with nominal 20ms cycles at 50Hz on the grid. In old reliable grid equipment that had been in use for decades such spikes would have been ignored because the large rotating machine inertia would make them irrelevant. Other systems went into various states of shut down for protection reasons, some of the Siemens Class 700 trains had to be reset by the train crew, others required a Siemens engineer to drive to each train and reload its firmware. The train protection mode occurred because the grid frequency on the 25kV AC supply went below 49.8Hz, this was a programmed default and it turned out to have been a very conservative one, the trains could have continued to operate normally at even lower frequencies but someone decided to write the programs without actually testing what a sensible limit was. The whole very widespread problems this caused could have been avoided by not acting instantly on microsecond transients in a 50Hz system.

Is it possible that the monitoring systems on a Boeing 787 also sample the electrical system voltages and currents at a relatively high frequency, and thus in the event of a transient of some type perhaps over-react to this event by taking precipitate action instead of waiting a short time before re-sampling again. I have seen a suggestion that an alternative explanation for the "bang" heard by the survivor in seat 11A might have been the sound of a Bus Tie Contactor closing, which in itself suggests something quite important relating to the electrical system. The 787 is unusual in that it uses variable frequency AC generators whose outputs are rectified and then inverted to other AC voltages and also quite high DC voltages, some in the 250-300V region.

I hope that some hard information is going to come out from the investigators soon, but given that the flap mis-selection idea is effectively debunked and we know that the main undercarriage either started its retraction cycle with bogies tilting forwards or that falling hydraulic pressure caused the same thing to happen, then the only thing that fits the observed flight path is loss of thrust on both engines which could have either preceded or followed an electrical failure. We also know that the RAT deployed and in the relatively undamaged tail cone the APU inlet was open or opening indicating a likely auto-start of the APU due to the parameters to trigger that occurring.

I would like to know how many tests of the electrical/computer interactions in 787 development involved arcing/shorting voltage/current transient testing. Is this the sort of thing that is done at all? The EECs (FADECs) in the engines are self-powered via magnetos and self-controlling in many circumstances, but perhaps something caused them to think that the thrust levers had been retarded, and such a thing might have been down to the effect of electrical transients on the various signals received by the EECs.

I have read the original 65+ pages of the thread, but I have not seen any discussion of this particular idea. Maybe that is because the 787 is quite a significant departure from Boeing's previous design practices with totally different electrical systems, higher pressure hydraulics and no doubt other aspects as well.

What do you all think?

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Aerospace101
2025-06-16T22:38:00
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Post: 11903849
Truck forward tilt discussion

I previously speculated the forward truck tilt was proof the gear had been selected UP and the retraction sequence was interrupted.

I’m not so sure now and believe there is a different conclusion from this non-normal gear position.

In normal retraction sequence the gear doors open almost instantaneously after the forward truck tilt. It does seem coincidental the tilt was completed while no indication of the doors opening is visible on the rooftop video, which would suggest hydraulic failure at that exact moment; this precise timing of interruption in the retraction sequence feels unlikely. So is there a more likely answer for the forward truck tilt that does not involve movement of the gear lever?

I suspect it’s more likely that C hydraulics lost power prior to rotation, as a consequence the truck could not tilt rearward during rotation as it normally should. Therefore it’s probable it always stayed in a neutral or forward tilt position from the take off run until we see it in the rooftop video. If the gear was behaving normally, and the crew had omitted to retract, it should be hanging rearwards. Watch any 787-8 takeoff video and you can see at rotation all 4 main wheels stay on the runway as the aircraft rotates. Just after wheels up they tilt rearwards. It’s a very subtle position change.

If the gear was always in a neutral or forward truck tilt position then this undermines the theory that retraction sequence was interrupted. It insinuates the C hydraulic and electrical failure happened prior to main wheels lift off.

For this reason I believe we cannot assume that gear UP was selected nor that retraction was interrupted. I’m seeing lots of social media posts which suggest the forward tilt means gear was in retraction and I don’t believe it was now.

I think the truck tilt position is key to understanding the timeline of system failures and whether the automatic RAT deployment was triggered by power failures or engine(s) failure. The question remains, did loss of center hydraulics happen before or after loss of thrust?

Last edited by T28B; 16th Jun 2025 at 23:35 . Reason: white space is your friend, and is reader friendly

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dragon6172
2025-06-17T01:30:00
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Post: 11903903
Originally Posted by Aerospace101
I previously speculated the forward truck tilt was proof the gear had been selected UP and the retraction sequence was interrupted.

I\x92m not so sure now and believe there is a different conclusion from this non-normal gear position.

In normal retraction sequence the gear doors open almost instantaneously after the forward truck tilt. It does seem coincidental the tilt was completed while no indication of the doors opening is visible on the rooftop video, which would suggest hydraulic failure at that exact moment; this precise timing of interruption in the retraction sequence feels unlikely. So is there a more likely answer for the forward truck tilt that does not involve movement of the gear lever?

I suspect it\x92s more likely that C hydraulics lost power prior to rotation, as a consequence the truck could not tilt rearward during rotation as it normally should. Therefore it\x92s probable it always stayed in a neutral or forward tilt position from the take off run until we see it in the rooftop video. If the gear was behaving normally, and the crew had omitted to retract, it should be hanging rearwards. Watch any 787-8 takeoff video and you can see at rotation all 4 main wheels stay on the runway as the aircraft rotates. Just after wheels up they tilt rearwards. It\x92s a very subtle position change.

If the gear was always in a neutral or forward truck tilt position then this undermines the theory that retraction sequence was interrupted. It insinuates the C hydraulic and electrical failure happened prior to main wheels lift off.

For this reason I believe we cannot assume that gear UP was selected nor that retraction was interrupted. I\x92m seeing lots of social media posts which suggest the forward tilt means gear was in retraction and I don\x92t believe it was now.

I think the truck tilt position is key to understanding the timeline of system failures and whether the automatic RAT deployment was triggered by power failures or engine(s) failure. The question remains, did loss of center hydraulics happen before or after loss of thrust?
I originally was onboard with the truck tilt seen in the one video being proof that the gear handle was selected rather than the flap handle. But after watching several videos and an "endless" web search for info, I also have started to think that there was a problem before the gear handle was moved.

In a normal retraction, the main gear doors begin to open before the truck tilt is complete (roughly when the gear trucks are "level", seen around 8-10 second marks of this video ). The nose gear doors open at the same time the main gear tilt starts (seen in Jetstar video linked below). It's too blurry in the video/stills of the accident aircraft to definitively say the nose doors are not open, but I'd say no. And it's pretty much a certainty that the main gear doors are not open. And finally I think it is also pretty clear that the main gear trucks are tilted down to the retract position.

I have no confirmation of this, but I read (or heard someone say in a video) that the truck tilt actuator was a "single acting" actuator, meaning hydraulic pressure held it in the toe up position and it was spring loaded to the retract position. Which means if hydraulic pressure was lost due to loss of power to the electric driven center hydraulic system, then the main gear trucks would tilt forward on their own without moving the gear handle. The Jetstar burst tire video somewhat backs this up, in that the blown tire caused a leak to the center hydraulic system and an alternate extension was required (thus the reason the main doors are down during the landing). No hydraulic pressure means the main gear trucks remained pointing nose down during the landing. Video here

Last edited by dragon6172; 18th Jun 2025 at 03:01 . Reason: Edit video links

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dragon6172
2025-06-17T03:57:00
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Post: 11903950
Originally Posted by bbofh
Most pundits have identified the gear-tilt as evidence that only the centre electrically-driven pump can do the gear-tilt if the engines' other two hyd systems are suddenly both in QUIT mode (which accords also with the instant RAT deployment and loud noise heard by the sole survivor) - and an ensuing transition from climb-out to a deadly sinking and commensurate attitude change for speed maint.
One comment here, and maybe I am mis-understanding your comment, but the landing gear only operates via the center hydraulics. It does not matter whether the Left/Right engine driven hydraulic systems are operative or not. The RAT will only pressurize the primary flight control portion of the center hydraulics.
Originally Posted by OldnGrounded
Thanks. Do we know that these are monitored by TCMA for air/ground state and if so, do we know the logic used to make a determination based on those inputs? Alternatively, do you know where we should be looking for those answers?
No idea. I only got that info from the Master MEL on the FAA website. According to the MMEL the aircraft can be dispatched as long as there is one of each type sensor working on each main gear. (AIs MEL could be more restrictive)
DaveReidUK
2025-06-17T07:09:00
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Post: 11904017
Originally Posted by dragon6172
One comment here, and maybe I am mis-understanding your comment, but the landing gear only operates via the center hydraulics. It does not matter whether the Left/Right engine driven hydraulic systems are operative or not. The RAT will only pressurize the primary flight control portion of the center hydraulics.
Are you saying that the RAT cannot power retraction of the gear, neither via its own hydraulic pump nor via its generator providing power to the centre system pumps?

Or could it be that it's capable of delivering enough power (via either route) to move the tilt actuator but not a humungous retraction jack?

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Aerospace101
2025-06-17T09:28:00
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Post: 11904137
Originally Posted by DaveReidUK
Are you saying that the RAT cannot power retraction of the gear, neither via its own hydraulic pump nor via its generator providing power to the centre system pumps?

Or could it be that it's capable of delivering enough power (via either route) to move the tilt actuator but not a humungous retraction jack?
The RAT provides hydraulic power only to the flight control portion of the C hydraulics. Wing and tail flight controls only. Non return valves prevent power to other C hydraulic powered systems like the gear.

The RAT provides electrical power only to critical flight instrumentation (mostly Captains), navigation and communication. The same critical equipment that the Battery will provide. If all electrics was lost, the main Battery would provides standby power until RAT is fully deployed. The RAT electrical power would not be able to power C hydraulic electric pumps.

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Icarus2001
2025-06-17T12:43:00
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Post: 11904270
The RAT provides hydraulic power only to the flight control portion of the C hydraulics
Should we talk about the RAT being an electrical generator? No? Okay then, carry on.
Aerospace101
2025-06-17T13:06:00
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Post: 11904288
Originally Posted by Icarus2001
Should we talk about the RAT being an electrical generator? No? Okay then, carry on.
Did you not read my full post?

Originally Posted by Aerospace101
The RAT provides hydraulic power only to the flight control portion of the C hydraulics. Wing and tail flight controls only. Non return valves prevent power to other C hydraulic powered systems like the gear.

The RAT provides electrical power only to critical flight instrumentation (mostly Captains), navigation and communication. The same critical equipment that the Battery will provide. If all electrics was lost, the main Battery would provides standby power until RAT is fully deployed. The RAT electrical power would not be able to power C hydraulic electric pumps.

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IFMU
2025-06-17T14:56:00
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Post: 11904373
Originally Posted by Icarus2001
Simply that the RAT is an electrical generator, that is all.
The 787 RAT has both a generator and a hydraulic pump.

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