Originally Posted by
BugBear
What powers (turns) the alternator?? If the generator(s) go off line, and the emergency batteries (Thales) are inop, how does the APU start? The batteries were destroyed when two generators failed in the United incident. Panel? EICAS??
Isn't the RAT intended to support descent and safe landing??
Not save a TO?
How resilient is the RAT to overload when in flow with arcing and shorting in the Main Generation system??
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Thanks GF
TDRACER May have to answer these,I’m just a pilot, but here goes.
The PMA is driven off the engine’s accessory gearbox. During engine start, there’s little rotation, so it’s powered by A/C or a dedicated battery. Once the started, it shifts to the gearbox driven PMA. Yes, it’s unpowered IF the gearbox fails but the engine won’t run for other reasons like no oil pressure, no fuel pressure, high or low.
i can’t say for the 787, but the APU usually has a dedicated start battery near the APU to power the APU FADEC during start, and the APU starter.
The RAT is for anytime it’s needed by the emergency conditions causing its deployment. As long as the airspeed is sufficient, it matters not if it TO, climbing, descending or landing. They’re certfied to provide the needed services. Usually when the speed decays, the electrical drops off giving priority to hydraulics to power the flight controls. That’s all specific to the design. There’s lots of configurations for the RAT, just hydraulics, hydraulics and a generator, just a generator that is large enough to drive an aircraft pump for flight controls. If you have serious arcing on the buses powered by the RAT, well it just isn’t your day. Usually, the RAT only powers the essential AC & DC buses and is separated from the Main load buses.
As I said somewhere in this dreary vale of tears, loss of all generators and loss of all engines will look much the same. Especially in a dark simulator. The first thing to look for is what is the engine indications and feel like. Loss of all engines means loss of all generators, but not vice versa.
If a RAT-equipped plane lost all electrics but will hand thrust (or one inop) on lift-off the RAT is required to enable a return. Admittedly, the crew will have a job of work, need to be smooth as the flight controls are not as responsive esp OEI, but they’ll be flight controls, avionics on the captain’s side, comms.