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LATAM B773 complete electrical failure?

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LATAM B773 complete electrical failure?

Old 23rd Dec 2018, 08:33
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It's been many years since I was involved in the 777 development, but IIRC, the RAT generator fed the battery bus directly - if they still had the battery bus they would have still had the RAT generator.
Looking at the FCOM, the RAT supplies the C1 & C2 TRUs, which power the capt & FO instruments and via them, the battery busses. I also see a note: “The main battery can power the standby system for a minimum of 10 minutes”, which isn’t much!

This should not have happened. Uncle Boeing is going to get more grey hair
I suppose there are two ways of looking at this: on one hand, that this shouldn’t have happened and is evidence of poor design (sorry td) but on the other, that after a once-in-a-hundred-million-flight-hours level failure(s), there was enough redundancy/backup in the system to allow the crew to make a successful approach and landing, which is indicative of a very good implementation. I think I would side with the latter...
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Old 23rd Dec 2018, 09:54
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Originally Posted by FullWings

there was enough redundancy/backup in the system to allow the crew to make a successful approach and landing, which is indicative of a very good implementation.
Hold on! They landed yes. But, there were probably some circumstances that made this possible and, others where it would not have ended so well. Those who operate similar aircraft will possibly get the full detail on this and learn something. The rest of us are probably not going to know that much. Would be good, when the truth is known, for someone in the real know to describe exactly what happened, please. In the meantime, I don't think anyone can make statements about how good/bad the systems performed here.
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Old 23rd Dec 2018, 10:20
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A lot depends on how far down the probability tails you want to proceed. In almost all FMEAs you get to conditions occur only where several highly improbables occur concurrently in a highly improbable sequence. It gets very difficult to persuade system developers to redesign systems to cope with probabilities in the 10^-15 range.
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Old 23rd Dec 2018, 12:04
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Notable incident of the 777 electrical system: United 777 on the Ground in London 2007 with a complete meltdown of a contactor
https://aviation-safety.net/database...?id=20070226-0


Also this diagram seems interesting:
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Old 23rd Dec 2018, 14:28
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Do I presume correctly that one IRU must have been powered for one attitude platform to be functional?
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Old 23rd Dec 2018, 14:37
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Originally Posted by Ian W
A lot depends on how far down the probability tails you want to proceed. In almost all FMEAs you get to conditions occur only where several highly improbables occur concurrently in a highly improbable sequence. It gets very difficult to persuade system developers to redesign systems to cope with probabilities in the 10^-15 range.
OK Ian! Yes, you are correct. However, for simple folk, like me, we work in a simple world where the faults in aircraft electrical power systems work something like this.... Short circuit in a system=CB Trip. Overload of a Busbar=Isolation or,=Auto load shedding (normally alternate systems avail on another Bus). T/R fail=Isolation (also, normally alt sys avail). Gen Fail= Isolation, (normally single Gen fail accommodated with alt source select and, no services lost). Multi Gen fails, protected by isolation of Busbars to protect partial remains of system with working generators and avoid overload. Batt fail, can be complicated but, generally systems work as long as engines/Generators working. etc...
Therefore, rather than talking mathematics, it would be great to know what failure mitigation worked or, didn't work in this case?
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Old 23rd Dec 2018, 15:55
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Still a better design than Airbus were one grenaded engine in the QANTAS A380 took out 55 systems......
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Old 23rd Dec 2018, 18:03
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LATAM B773

I not qualified on 777, but every Boeing I flew the anti-skid was AC powered.... These guys land Hot, Heavy, and no flat tires. Deflated due to heat not having the bottom worn off ;-)) Job well done.
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Old 23rd Dec 2018, 18:27
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Did you see what the debris did to the wiring looms on that A380? This is entirely different - it's one tiny fault that should never, ever, have progressed any further than an EICAS advisory.

I think there will be some concerned engineers at Boeing over Christmas.
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Old 23rd Dec 2018, 18:49
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According to the commander, interviewed on TV, power came back after the engines were shut down. Presumably from the APU, on the footage the cabin lights were visible.
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Old 23rd Dec 2018, 19:44
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Originally Posted by tdracer

DeeVee - again, a couple decades since I was involved, but based on what I remember, the only way they could have lost all main bus electrical power (with both engines running) would have been either:
latent failures in the electrical bus isolation system (there are regular maintenance checks to confirm such failures are not present),
Notable incident of the 777 electrical system: United 777 on the Ground in London 2007 with a complete meltdown of a contactor
https://aviation-safety.net/database...?id=20070226-0
Would speculate that we getting in the right track ...

AC Power is the "standard", or is prefered, for several systems in commercial air transport aircraft. Don't ask me why, as I don't know the reason ...

AC Power has however a big inconvenience. You can have 10 sources of good working AC Power available. However only 1 of those sources can supply power to an AC Bus at a time. You cannot connect them in Parallel, or in Series, like with DC Power.

If a contactor becomes stuck, for whatever reason, and doesn't "let go", to allow another source of AC Power to power the an AC Bus that is without power, then this kind of scenario can happen. Several sources of AC Power available, but none is allowed to supply power to AC Buses that are without power, because the contactor is stuck in position that allows only the Failed source to power the Bus, or it didn't shift completely to allow another source in.

AC Power ... we owe that to Tesla

Last edited by zerograv; 24th Dec 2018 at 15:26. Reason: not applicable info
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Old 23rd Dec 2018, 20:24
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zerograv,
AC power generation and systems on aircraft are used for reasons of efficiency and weight. DC is needed for systems that require it and, is generally derived from Transformer rectifiers. Of course, the DC battery is usually used as a power source for starting a fully shut-down aircraft and, linked into the DC system, it can act as a limited power source in malfunctions. To back this up, there are usually Inverters to provide AC for certain equipment where/when it is not available from the normal AC Generators. AC power supplies on large aircraft can usually be paralleled and the AC system will normally be operated in parallel.
You are correct that "contactors" can have switching or latching failures. In fact, all components can fail. Where faults occur, it may be that a generator cannot be paralleled. These systems on large aircraft are quite complex but offer large resources of failure mitigation, which is why this apparently serious failure is important. Cheers
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Old 23rd Dec 2018, 21:34
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These systems on large aircraft are quite complex but offer large resources of failure mitigation, which is why this apparently serious failure is important. Cheers
Thinking about this some more, I bet when the full story is out inaction or inappropriate action by the crew contributed to the severity of this situation, like for instance the LOT 767 gear up. The 777 is a very mature design so I find it hard to believe that there is a fail mode that can kill everything.

There does appear to be a trend to minimizing the importance of system knowledge in modern training systems.

in any case however the crew got to a dark cockpit, they still did a very good job of getting the airplane on the ground with negligible damage.
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Old 23rd Dec 2018, 23:38
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Onceapilot

Thanks for that !

To improve my concept of "paralleling", 2 generators can supply power to an AC BUS at same time ?
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Old 24th Dec 2018, 00:16
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AC generators can most definitely share a load, and co-exist on the same "bus" or transmission medium.
The only requirement is that they are in sync, phase and frequency wise.

Pretty much exactly the same as how your home Solar Inverter feeds its energy back into the grid.

I would hope aircraft generators ran in sync, otherwise switching at an inopportune moment (like when one failed) might lead to smoke and noise......
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Old 24th Dec 2018, 00:18
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Sorry for what may be a stupid question. I am a mechanical engineer so my EE skills went by eons ago in college and I have only worked with DC circuits on mobile equipment but the electrical grid has tens of thousands of generators powering it, all synced to the same frequency. In the old days they used primitive means to sync but putting a generator on line out of phase can negate the power so they must be synced. Is this not possible on an airplane?
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Old 24th Dec 2018, 00:51
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The 777 can sync the AC buses when needed. For example, when you shutdown an engine, there is a light delay (250 ms if memory serves) before the fuel shutoff command goes to the engine. ELMS uses this delay to sync the AC bus before the engine spools down and the IDG drops off line (known as a 'breakless transfer'). So if everything is working correctly, on the 777 you don't get that annoying ~50 ms power interrupt when a generator comes on or drops off line that causes the displays to flicker on the 747-400/757/767/
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Old 24th Dec 2018, 02:23
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Phase lights

My memory is hazy but we synced the generators on the 727 using phase lights. If you rushed the sync you could get a little shudder in the A/C.
As part of my expansion of CV during the Arab Oil Emargo, where no flying gig was available, I did a stint in a nuclear/fossil power plant. They told a story of a many ton generator being twisted off mounts when somehow put online out of phase. Off topic sorry.
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Old 24th Dec 2018, 02:24
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Originally Posted by tdracer
The 777 can sync the AC buses when needed.
The 777 does parallel generators. But only for the duration of a power transfer (shutting down or spinning up a generator). The IDGs (and APU controller) are equipped with a speed control input that bring two sources into sync by throttling one up or down. Once synced, the new source is paralleled with the system by closing the proper breaker. Once the new source is on line, a tie breaker is opened to isolate the two sources and their respective loads. Buses do not operate continuously tied together.

On the various 747 models, the generators did remain in parallel continuously, each feeding into the four main buses tied together. 747-400 buses will revert to isolated operation for certain autoland configurations requiring isolated and redundant autopilot channels. On 757 and 767, the electrical systems remain isolated. All transfers are break before make, resulting in a short but annoying power interruption.

During normal operation, the 747 is the only model where an electrical fault can be 'seen' momentarily by all buses. Until the protection systems act to isolate the faulted section. Because of this parallel operation and the need to identify the location of and isolate a fault, the 747 has perhaps the most complex protection system. The heart of this system is something called differential protection, which can identify where in the system a fault has occurred and open the correct breakers to isolate it. All other models have much simpler (if any) differential protection systems and may trip more breakers than needed upon sensing fault currents passing through.

Due to the isolated (at most times) operation of the 777 electrical system, no single fault should affect more than the half of the system (left or right) where it occurs. So this particular incident is troubling. One possible cause is that (for some reason) one of the generator control units failed to transfer load to its generator upon engine start. Leaving the left and right main buses powered from one generator through a bus tie breaker. A subsequent fault, or even an overload condition due to one generator available resulted in the good generator tripping off line. Leaving only standby (battery and RAT) sources to power critical loads.
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Old 24th Dec 2018, 05:05
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Yes, on large alternators that sort of thing can and has happened.
On large diesel sets the most common failure mode from trying to sync out of phase was either to shear the alternator to flywheel coupling bolts or to snap the crankshaft.

Fortunately that is now a thing of the past since auto-sync became common.
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