Power for the Autopilot/Flight Director System is provided by the engine or APU driven generators via the left and right AC buses. The left and right transformer rectifier units (TRUs) provide DC power to the left and right DC buses respectively. During normal operation, airplane cruise or takeoff, the center AC and DC buses are powered by the left buses. During autoland operation, the center AC and DC buses are powered by the static inverter and hot battery bus. Center bus switching is provided by the FCCs internal isolation request and system relay logic. The switching provides each FCC with an independent power source during autoland operation.