B757-200
Flight Crew Air Transport Pilot (ATP)
Aircraft Equipment Problem Critical; Deviation / Discrepancy - Procedural MEL / CDL
Air Traffic Control Provided Assistance; Flight Crew Landed in Emergency Condition
Aircraft
Aircraft had a Right Generator MEL that required the APU to run for the entire flight. In cruise we received an APU FUEL VALVE message on the EICAS. The QRH directed that the APU to be shutoff and not restarted. If we shut down the APU then we are put into an emergency situation since we would be operating on a single generator. The APU was running normally at 100% RPM and steady EGT (Exhaust Gas Temperature). We elected to continue while monitoring the APU since it was providing needed electrical power. We were monitoring possible divert fields as we went in case the APU did actually fail. The APU ran steady until on the decent through FL200 when it failed. We [advised ATC] due to single generator operation and landed uneventfully.
On a B757-200 a failed APU coupled with a deferred right engine generator caused to operate in a single generator mode.