|Quoting 7BOEING7 (Reply 4):|
On all Boeing airplanes that have a RAT it is tested inflight. On the 777/787 it is dropped on the ground during the B-1 preflight checks by manually pushing the Ram Air Turbine switch -- in flight the generator control switches are turned off until the RAT drops and the APU autostarts confirming correct operation of the system.
On the 777, the RAT only drops automatically if both engines go sub-idle. On the 787 the RAT will drop automatically for both engines going sub-idle, and also for some combinations of generator failures. Remember that the 787 is highly dependent on electrical power for everything.
For the 757 and 767 the RAT will also drop automatically if both engines go sub-idle.
As 7BOEING7 indicates, there is also a switch on the overhead panel to manually drop the RAT. In fact, the Dual Engine Failure checklists have a step to push the RAT Deploy switch as a backup to the automation.
The 787 RAT supplies electric power only. The 777 RAT supplies both electric and hydraulic pressure. The 757 and 767 RAT supplies hydraulic pressure only.
On the 777 and 787 the APU will automatically start if both engines go sub-idle. I wasn't aware that it will automatically start on the 787 for generator failures also, as implied above. Again, for the Dual Engine Failure checklist, there is a step to manually start the APU to backup the automation.
The APU will not automatically start on the 757, 767 when both engines go sub-idle. As you might imagine, there is a step in Dual Engine Failure checklist to manually start it if both engines fail.
[Edited 2015-07-09 08:14:36]