2025-06-22T11:01:00
permalink Post: 11908441
Always possible, however since a pilot made a radio call there was some
emergency leve
l power available, which suggests the EAFR would be powered.
The Jeju recorders were okay if I recall correctly, they just had no input, was that the case?
Somoeone made a good point above about the German Wings FDR/CVR being available the next day after the aircraft was aimed at the ground like a missile. These things are built tough, as you know, this may be type specific but….
The Jeju recorders were okay if I recall correctly, they just had no input, was that the case?
Somoeone made a good point above about the German Wings FDR/CVR being available the next day after the aircraft was aimed at the ground like a missile. These things are built tough, as you know, this may be type specific but….
(from the online 2010 FCOM)
(from the maintenance training )
The 787 battery fire report says the two recorders are on the left and right 28VDC buses. I don't think those get powered on RAT by the looks of it. I would wager you get whatever is on the 235VAC 'backup bus', plus the captain's and F/O's instrument buses via C1/C2 TRUs. You won't get all of that (like the F/O's screens) because the 787 energises/de-energises specific bits of equipment, not just whole buses.
Losing recorder power looks entirely expected.
SLF Engineer (electrical - not aerospace) so no special knowledge
Perceived wisdom may be applicable in normal circumstances but not when all the holes line up.
For example I've seen it quoted many times that the engine FADECs are self powered
by the engines, the TCMAs-whether part of the FADEC or a separate unit, similarly self contained
within the engine. The perceived wisdom seems to be that there is no common single fault
which can take out both engines.
And yet we're also told that the TCMA function can only function in ground mode and receives ground-air
signals from a combination of inputs from Rad Alts and WOW sensors.
There is therefore a connection from the central EE bay to the engine.
Yes I'm sure the Rad/Alt and WOW sensor processing will use different sensors for each side and powered from different
low voltage buses.
However as an analogy, in your house your toaster in the kitchen may be on a separate circuit from the water heater in
the bathroom, each protected by a fuse at the main switchboard. In normal operation a fault in one cannot affect the other.
However a lightning strike outside the house can send much higher voltages than normal operation throughout the entire
system and trash every electrical appliance not physically disconnected at the time.
Now I'm not suggesting the aircraft was hit by lightning but FDR has proposed a single event, buildup from a water leak entering
one of the EE bays at rotate. It would be possible for one or more of the HV electrical buses to short so that all the low voltage
buses go high voltage. I have no knowledge of how the FADEC / TCMA systems connect to or process the Ground-Air signals but
there is a single fault mechanism whereby high voltage could be simultaneously and inappropriately applied to both engine control systems.
It would be unfortunate if this failure mechanism did cause power to be applied to drive the fuel shut off valve closed.
Since the likelihood is that we're looking at a low probability event then perceived wisdom about normal operations and fault modes
might not be applicable.
Perceived wisdom may be applicable in normal circumstances but not when all the holes line up.
For example I've seen it quoted many times that the engine FADECs are self powered
by the engines, the TCMAs-whether part of the FADEC or a separate unit, similarly self contained
within the engine. The perceived wisdom seems to be that there is no common single fault
which can take out both engines.
And yet we're also told that the TCMA function can only function in ground mode and receives ground-air
signals from a combination of inputs from Rad Alts and WOW sensors.
There is therefore a connection from the central EE bay to the engine.
Yes I'm sure the Rad/Alt and WOW sensor processing will use different sensors for each side and powered from different
low voltage buses.
However as an analogy, in your house your toaster in the kitchen may be on a separate circuit from the water heater in
the bathroom, each protected by a fuse at the main switchboard. In normal operation a fault in one cannot affect the other.
However a lightning strike outside the house can send much higher voltages than normal operation throughout the entire
system and trash every electrical appliance not physically disconnected at the time.
Now I'm not suggesting the aircraft was hit by lightning but FDR has proposed a single event, buildup from a water leak entering
one of the EE bays at rotate. It would be possible for one or more of the HV electrical buses to short so that all the low voltage
buses go high voltage. I have no knowledge of how the FADEC / TCMA systems connect to or process the Ground-Air signals but
there is a single fault mechanism whereby high voltage could be simultaneously and inappropriately applied to both engine control systems.
It would be unfortunate if this failure mechanism did cause power to be applied to drive the fuel shut off valve closed.
Since the likelihood is that we're looking at a low probability event then perceived wisdom about normal operations and fault modes
might not be applicable.
Weight on wheels appears to go into data concentrators that go into the common core system (i.e. data network).
Presumably there is a set of comms buses between the FADECs and the CCS to allow all the pretty indicators and EICAS alerts in the cockpit to work. The WoW sensors might flow back via that, or via dedicated digital inputs from whatever the reverse of a data concentrator is called (surely they have need for field actuators other than big motors?). Either way, left and right engine data should come from completely different computers, that are in the fwd e/e bay (or concentrators/repeaters in the wings, maybe) rather than in with the big power stuff in the aft e/e bay.
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2025-06-28T13:08:00
permalink Post: 11912484
I suspect both recorders will contain the same data. Given the radio transmission after the loss of thrust the aircraft still had at least the emergency electrical bus powered. This should have kept both recorders online. It is however possible given the 10 minute battery backup that Boeing chose to put the the recorders on another bus but that\x92s not the norm.
This shows the centre TRUs can only power the instrument buses not the L/R DC buses, the RAT can't really power the right TRU without powering both R1/R2 buses, and powering the left TRU would require powering the left 235/115 ATU which would probably be a lot of magnetising current even if not much actual load. The contactor naming supports that.
My money is on the L/R DC buses being unpowered in RAT operation; only the CA/FO instrument buses and the 235VAC backup bus.
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2025-06-28T17:04:00
permalink Post: 11912578
I am not certain on that. Remember the 737 didn't have them on the standby bus (Jeju). The NTSB doc states they're powered from the L/R 28VDC buses on the 787.
This shows the centre TRUs can only power the instrument buses not the L/R DC buses, the RAT can't really power the right TRU without powering both R1/R2 buses, and powering the left TRU would require powering the left 235/115 ATU which would probably be a lot of magnetising current even if not much actual load. The contactor naming supports that.
My money is on the L/R DC buses being unpowered in RAT operation; only the CA/FO instrument buses and the 235VAC backup bus.
This shows the centre TRUs can only power the instrument buses not the L/R DC buses, the RAT can't really power the right TRU without powering both R1/R2 buses, and powering the left TRU would require powering the left 235/115 ATU which would probably be a lot of magnetising current even if not much actual load. The contactor naming supports that.
My money is on the L/R DC buses being unpowered in RAT operation; only the CA/FO instrument buses and the 235VAC backup bus.
Last edited by D Bru; 28th Jun 2025 at 17:06 . Reason: deleting a repeat image of the elec system
5 users liked this post.