The idea that a computer or power failure could shutdown both engines
might be possible on some aircraft, but not this one.
Once lit, a normally operating Trent (or GE for that matter) will keep
running until fuel is stopped or a signal is sent to the
engine mounted FADEC box (EEC) on the 777.
Fuel:
Both the spar fuel cutoff and HMU main fuel valves are controlled in an
ELMS cabinet via relays which are hardwired to the Run/Cutoff switch.
The EEU (ELMS computer) does not command the relays nor does
any other digital devil.
EEC Shutdown signal:
The EEC can initiate a commanded shutdown only after both channels
A and B are given the reset signal, once again from
the relays controlled directly by the Run/Cutoff switches. The
channels are connected to separate relays.
Power loss will not affect the spar/HMU valve commanded positions
as these valves require power to both open and close. In addition,
the relays used for Run/Cutoff control are latching types, meaning that
a loss of power will not change relay state.
The autothrottles have been covered already. AT commands are right
in front of you - servos physically move the levers.
The 777 has no digital control link between the autothrust
computer and the engine control computers.
Throttle control inputs are powered by the EECs themselves and are
analog inputs. Each channel (A & B) has its own resolver.
So there you have it. Hardwired controls. Dual channel EECS with
dual power sources and inputs for each engine. It's a mystery.
It had been asked earlier:
An AA engineer tells me that the Trent 800 series
HMUs and EECs are manufactured by Goodrich.