B767-300 and 300 ER
Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 100; Flight Crew Total 20000; Flight Crew Type 10500
Aircraft Equipment Problem Critical
Flight Crew Returned To Gate; General Maintenance Action
Aircraft
We pushed back normally. Our plan was to perform a one engine taxi (right engine) to the runway and do a cross-bleed start of the left engine on the way. After start everything was normal until the APU shut down after its one minute cooldown. At that time we received the L BUS OFF EICAS along with all the other messages that entails. One factor we had to consider was an equipment cooling message. In light of the seriousness of our problem we elected to perform a return to the gate without consulting Dispatch and Maintenance Control. We called the gateway and told them we were taxiing back in. We started the APU again and taxied in using only the right engine. As the APU came online the L Bus was again powered but the equipment cooling problem remained along with several other messages. After block-in we described the problem to Dispatch and Maintenance Control and wrote it up in the logbook. I am concerned that had we started both engines or had we taxied with one engine and left the APU on we would not have known that the right generator would not pick up the left AC bus through the bus tie system. I wonder if other 767's might have the same issue.
Following a single engine taxi out a B767-300 flight crew returned to the gate when the right engine generator failed to pick up the left AC buss when the APU was shut down; indicating a failure of the bus tie breaker.