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EDLB
2025-06-18T19:15:00 permalink Post: 11905486 |
@syseng68k
Consensus here is, that both engines where stopped by a closing fuel cut off valve, wich yields a fast loss of N2. The generators then shut down very quick as does the thrust in a few seconds. This is supported by the quick RAT extension which allowed the crew to control the flight. The APU did autostart too. A thrust changed with the thrust leaver to idle is much slower and would not result in the dramatic change in performance. Thrust set to idle will not engage the RAT since the electric generators would still work. So a thrust leaver changed to idle or any intervention by Autothrust (AT) would not yield to the RAT extension. Something or someone activated a fuel cut off. How and why that happened is the big question, the investigators have to answer. 25 users liked this post. |
Nick H.
2025-06-18T21:30:00 permalink Post: 11905563 |
@syseng68k
Consensus here is, that both engines where stopped by a closing fuel cut off valve, wich yields a fast loss of N2. The generators then shut down very quick as does the thrust in a few seconds. This is supported by the quick RAT extension which allowed the crew to control the flight. The APU did autostart too. A thrust changed with the thrust leaver to idle is much slower and would not result in the dramatic change in performance. Thrust set to idle will not engage the RAT since the electric generators would still work. So a thrust leaver changed to idle or any intervention by Autothrust (AT) would not yield to the RAT extension. Something or someone activated a fuel cut off. How and why that happened is the big question, the investigators have to answer. ![]() |
777ret
2025-06-18T22:38:00 permalink Post: 11905597 |
@syseng68k
Consensus here is, that both engines where stopped by a closing fuel cut off valve, wich yields a fast loss of N2. The generators then shut down very quick as does the thrust in a few seconds. This is supported by the quick RAT extension which allowed the crew to control the flight. The APU did autostart too. A thrust changed with the thrust leaver to idle is much slower and would not result in the dramatic change in performance. Thrust set to idle will not engage the RAT since the electric generators would still work. So a thrust leaver changed to idle or any intervention by Autothrust (AT) would not yield to the RAT extension. Something or someone activated a fuel cut off. How and why that happened is the big question, the investigators have to answer. 3 users liked this post. |
BuzzBox
2025-06-19T00:08:00 permalink Post: 11905631 |
On the B777, each EEC is powered by a dedicated control alternator whenever the engines are running. The control alternators are mounted on the forward side of the main gearbox of each engine.
The flight controls power supply assemblies (PSA) have several sources of power, including PMGs located within the backup generators. The EEC control alternators do NOT perform double duty by powering the flight control system PSAs. I can't imagine the B787 being much different. ![]() Last edited by BuzzBox; 19th Jun 2025 at 01:36 . Reason: Updated drawing to show backup generator location, minor wording 3 users liked this post. |
Someone Somewhere
2025-06-19T10:54:00 permalink Post: 11905921 |
I still think that the small black area is the back of the engines visible through the small gap of the extended flaps.
Furthermore: The small hydraulik pump of the RAT only powers some of the flight controls that are powered by the center hydraulic system. The ones powered by the engine driven pumps will not work once the engine(s) failed. This doesn't apply if the pumps are depressurised by a fire handle, or to allow easier engine relight. |
Roo
2025-06-19T11:19:00 permalink Post: 11905941 |
The RAT is a small turbine that operates an electrical generator and a hydraulic pump.
“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 permalink Post: 11905954 |
The RAT is an electrical generator, not a hydraulic pump. How many times does this need to be said?
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 6 users liked this post. |
Someone Somewhere
2025-06-19T12:25:00 permalink 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 10 users liked this post. |
rigoschris
2025-06-19T16:54:00 permalink Post: 11906193 |
SLF Engineer(electrical -not aerospace) so no specialised knowledge of any kind.
The TCMA discussions on here seem clear that in normal operation the TCMA functions should not operate in air mode and the TCMA has multiple inputs from various sensors (Rad alts, WOW sensors). My query is to how these signals are routed to the engines, are they compared/aggregated in or routed through the EE bays. If they are present in the EE bays then fdr's idea of a water leak at rotate into the EE bays could generate shorts from the HV buses to the TCMA units. Are the TCMA units designed to cope with 400V AC inputs where a low voltage input is expected. I appreciate that the TCMA has to physically power a valve closed but applying high voltage to low voltage logic circuitry might have unexpected consequences. The sensors and actuators related to that engine (pressure, temperature sensors, various valves etc.) are most likely directly connected to it. The two throttle position sensors per engine are also directly cabled to it according to tdracer. No idea how other inputs like WoW and RADALT are connected. The FADECs don\x92t necessarily need much else, as apparently Autopilot etc. always move the actual thrust lever. There must also be a communication channel back to the flight recorders. No idea if it gets thrown into a comms bus or there is direct wiring. In terms of power, each FADEC has its own alternator driven by the engine. But there\x92s a failover connection to the AC bus of the plane. Not sure if there\x92s a physical relay keeping it disconnected in normal operations. 1 user liked this post. |
ignorantAndroid
2025-06-19T21:51:00 permalink Post: 11906415 |
No, very unlikely the pilots lost all control. In a total power failure (hydraulics and electrics) the pilots can control roll (by a specific spoiler pair) and pitch through the stabiliser. All electrically powered through wiring direct to control column.
It's possible the RAT was not providing correct power in the last moments, remember RAT provides Hydraulic (C system flight controls only) and Electrics. The evidence is the spoiler pair deflection behind the engines prior to impact. That specific spoiler pairing is only controlled by electrical power (directly connected to pilot control column). We should see other flight control deflections if the RAT was powering hydraulics. In the final moments I would speculate the flight controls had only emergency electric power from the battery. The direct wiring you're referring to is intended to provide minimal control in case of a complete failure of all ACEs. It allows for control signals but does not provide power. 1 user liked this post. |
EDML
2025-06-19T22:31:00 permalink Post: 11906447 |
1 user liked this post. |
skwdenyer
2025-06-20T00:36:00 permalink Post: 11906509 |
A good round-up of dominant themes, including this:
We know there has been a bug in the Generator Control Unit software (an overflowing counter) that could lead to simultaneous shut down of all generators and a total loss of all AC power (the 248 days bug). In the interests of completeness, we should perhaps also consider the possibility of some other previously-unknown software issue capable of creating an uncommanded dual engine shutdown. TCMS is the most likely candidate due to the deliberate separation of other systems from being able to achieve this outcome. The question then isn't whether there's some odd combination of input faults that would confuse TCMS into believing it were on the ground, but rather whether there's any way in which the software side could crash in such a way as to create an anomalous state within the system leading to engine failure. For instance, another overlooked software counter with an unwelcome failure mode. Or even just a "dirty power supply" (cf all the reports of dodgy passenger-side electrics on this a/c) leading to spurious inputs and unexpected consequences. Whatever is the cause will likely turn out to be have been a very low-probability event. But unless we have a TCMS expert who can state canonically that (say) the WoW sensor electrically disables TCMS when airborne (as opposed to merely being an input to the TCMS logic) then we cannot say with certainty that multiple inputs would have to have failed / been corrupted in order to reach the end state of this flight. 4 users liked this post. |
Someone Somewhere
2025-06-20T01:02:00 permalink Post: 11906517 |
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. 3 users liked this post. |
StudentInDebt
2025-06-20T01:13:00 permalink Post: 11906520 |
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. |
MaybeItIs
2025-06-20T09:24:00 permalink Post: 11906767 |
"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.
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 permalink Post: 11906781 |
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?
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. 9 users liked this post. |
Someone Somewhere
2025-06-20T09:48:00 permalink Post: 11906783 |
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? 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) 1 user liked this post. |
TURIN
2025-06-20T10:26:00 permalink Post: 11906808 |
... 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? 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. |
Lead Balloon
2025-06-20T11:17:00 permalink Post: 11906849 |
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. ... |
BFM
2025-06-20T12:02:00 permalink Post: 11906897 |
Yet. There is also agreement that this is probably (not beyond all reasonable doubt, but certainly more likely than not) a Black Swan event - and something unlikely and possibly unprecedented happened to cause it.
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