United Airlines Flight 266 - Accident

Accident

Two minutes into the flight, the pilots reported a fire warning for the No. 1 engine and that they were shutting it down. That action also shut down the generator on the No. 1 engine. Since the plane "...had been operating for 42 flight hours prior to the accident with the No. 3 generator inoperative, as allowed by the Minimum Equipment list," that left the No. 2 engine generator as the sole source for all electrical power on the aircraft. "Shortly after shutdown of the No. 1 engine, electrical power from the remaining generator (No. 2) was lost." All of the available evidence produced during the course of the investigation, did not enable the NTSB to determine why the number two generator also failed, after it became the sole source of power for the plane, nor why the "standby electrical system either was not activated or failed to function."

With the loss of all power to the lights and flight attitude instruments, while flying at night in instrument conditions, the pilots quickly became spatially disorientated and utterly helpless to know what inputs to the flight controls were necessary to keep the plane flying in the normal, upright attitude. Consequently they lost complete control of the aircraft and it crashed while in an abnormal attitude, killing all 38 aboard.

At the time, a battery powered back-up source for critical flight instruments was not required on commercial aircraft. The accident prompted the Federal Aviation Administration to require all transport category aircraft to have new backup instrumentation installed, and powered by a source independent of the generators.