Posts about: "Generators/Alternators" [Posts: 161 Pages: 9]

@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.

A naive glider pilot question: if fuel cut off was (inconceivably) selected, would both fuel control levers have been flipped downwards from Run to Cutoff? And if they were then immediately flipped back to the Run position, how much time would have been needed to achieve enough thrust to maintain altitude?

The simplest answer: Lo Lvl Alt Cap; Thrust to Idle; Startle Factor; Inappropriate Memory Items : ( RAT deployed; insufficient time for Eng relight.

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.

Engines generally deliver hydraulics while running down, as they operate down to a much lower speed than the generators do. Useful flight control power might be available from windmilling alone (hence why the 747 didn't have a RAT until the -8). That's subject to similar speed issues to the RAT but the engines have much more inertia. The Virgin Atlantic 024 report (A340 partial-gear-up landing at Heathrow) notes an expected 25-30 seconds of useful hydraulics after engine shutdown.

This doesn't apply if the pumps are depressurised by a fire handle, or to allow easier engine relight.

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.”

To make things clear, just check this B787-related alert service bulletin dated 25 Nov 2014 (my bold):



787 RAT hydraulic pump location

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.

The TCMA runs on the FADECs (Full Authority Digital Engine Control). There is one per engine, located inside the nacelle. Each one has a dual channel design inside, to compare computations, inputs etc. and generally provide redundancy (but I don\x92t know further details on the design/logic).

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.

None of the flight controls can be powered by the battery. Both the stabilizer motors and the electrically-actuated spoilers require high-voltage DC which is only available with at least one engine-driven generator or APU generator working. They cannot be powered by the RAT generator either.

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.

There is. And the disconnection is tested after engine start up. A failed disconnection will give an EICAS "No dispatch" message.

A good round-up of dominant themes, including this:

You may be at risk of assuming that the air/ground control logic is in some way hard-wired, as opposed to being a function of software. I don't believe we (yet) know this to be true.

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.

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.

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?

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.

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)

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.

Not quite. It was software for generator controllers. That bug, glitch or whatever it was accurately described to be was not the cause of any engine failures (so far as I am aware).

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.