Hello,

working on the 777 for MSFS i think the Electricals are almost done. But i have Question about the Backup Sys:

Lets assume you're cruising and all works well...

Now you get an "ELEC GEN DRIVE R" Warning, so you disconnect the right generator drive. As the APU is not running, the left IDG connects to the right AC Bus and the right Backup Gen connects to the right Transfer Bus:


Unfortunally, a fault is detected in the left IDG, causes the left Gen Breaker to trip. The right Backup Gen now connects to the left Transfer Bus too. As now both AC-Bus are unpowered, the APU starts automatically (if i remember right the RAT will also automatically deploy, but RAT is just not modelled at the Moment):


As APU power becomes available the APU is connected to both AC-Bus,the Backup Gen remains connected to the Transfer Buses:


And at this Point, here is my Question: is it correct that the APU connects only to the AC Buses, leaving the Backup Gen connected to both Transfer Buses? Or will one Transfer Bus reconnected to its AC-Bus if APU Power becomes availlable? Or, are any other Errors in this "Simulation"?

Thanks for any Hint.

Kind Regards

Sven

 

The APU autostarts and the RAT auto-deploys when you lose power to both Transfer Busses, not the AC Busses. With the loss of both AC Busses, you get some load shedding but no real loss of capability.

Any main generator (including APU generator) is capable of powering the entire system but I believe the logic is that when down to one main source, or if one AC Bus is unpowered, the Backup Generators power one or both of the Transfer Busses. I'll check the next time I'm in the simulator whether it is one or both as the manual states either. It might even be a function of the ELMS to decide depending on power requirements. The normal power source for the Transfer Busses is by their respective AC Bus. Also, in the Approach Mode for an Autoland, bus isolation takes place with the left AC Bus powering the left Transfer Bus and the Backup Generators powering the Right Transfer Bus.

 

I also have same question. My training cd shows the APU GEN repowering not only the mains but also both the transfers. Did you or anyone else find the definite answer ?

Thanks,
John B-777 Capt