Originally Posted by
BugBear
As I understand the potential, this is a .25 Megawatt system. The third loss of (two) generators melted the Emergency Batteries in the aft EEBay., with arcing present.
Dunno any more than that... (From PPRune 12 Dec. 2012)
In that thread the Captain had more to say. Worth a read.
Understood the batteries are in a titanium closet now, but the current and load must-have been sumthin
Regards, bb. ( Somewhat curious )
Re AI171, if the APU started, when and how long to regain flight controls and instruments? Quickly enough to power up and climb???? I don't believe that was flare....would Fadec have come in on standby power???? If the Emergency Batteries burned up here as in United/New Orleans, the APU won't start.... Is Fadec in any way APU reliant. Sorry for all the questions ....when one knows little, he has many too many questions??
If you're going to quote a post, it would be nice to have a link to the original for context.
On the face of it, it reads as... rubbish.
Generally speaking the loads on the batteries are tightly specified; the capacity of the full system is irrelevant because most loads are shed, not moved to the battery. Only the various standby/battery buses get moved to the battery, and the battery is designed to handle them. It's not a ~10kW battery suddenly being loaded to 750kW.
(edit: just because a battery is designed for a load does not mean it will never fail at that load... but it doesn't mean that a generator failure causes a battery to operate outside its certified limits)
The A320 manual claims "about 8 seconds" for emergency electrical, but that includes sequentially:
- deploying the RAT
- the RAT spinning up in airflow
- pressurising the blue hydraulics enough to open the priority valve (i.e. flight controls are satisfied)
- spinning up the hydraulic EMER GEN
- Generator controller being satisfied and putting the EMER GEN on line
During that time, batteries and static inverter power the flight computer(s?) and hydraulic pressure is probably somewhat available from a winding down or windmilling engine (on green and/or yellow) and the accumulators.
787 should be faster because the generator is directly on the RAT shaft.
I haven't seen a figure for the cutout speed on the 787 flight control PMGs - it seems possible that they're comparable to FADEC alternators and work down to 10-15%. If so, the airplane is likely controllable on windmilling alone with windmilling L/R hydraulics and PMGs, but I think you would need RAT or batteries for instruments beyond standby or any radios.