Electrics - System Operation Show Schematic only


When the Battery Switch is pushed ON and the battery is the only source of power on the airplane, all electrical system indicator lights become operative except the Battery Switch OFF Light and Utility Bus OFF Lights SCHEMATIC.

With the Standby Power switch selected ON the Main and APU batteries provide power for the HOT BAT, BAT and STBY buses.

When the APU is started or External Power is provided, both Main AC buses are powered, together with ALL the other aircraft buses. Also battery charging begins.

Engine Start

The Generator Control Switches are normally left ON. Therefore if the right engine is started first, the generator breaker closes automatically after the right Bus Tie Breaker opens. The engine then powers the right buses and the APU Generator or External Power supplies the left buses SCHEMATIC.

When the left engine is started, the Left generator Breaker closes after the left Bus Tie Breaker opens. If external power was in use the external power contactor trips open automatically. Each engine then powers its respective bus SCHEMATIC.

Flight Instrument Transfer Bus

Normally the Captain's flight instruments are powered by the Left main AC Bus. However certain Captain's instruments are on the Flight Instrument Transfer Bus and have an alternative source of power (Bus Equipment List).

If the Left Bus Tie Breaker Switch is in AUTO and the Left AC Bus is lost, power from the Right main AC Bus supplies these instruments automatically.

Similarly, if power to the Right main AC Bus is lost with the Right Bus Tie Breaker Switch in AUTO, certain First Officer's instruments are powered by the Left main AC Bus automatically (Bus Equipment List).

Landing - Center Buses

The Center buses power the Center autopilot. Normally these buses are powered from the left main AC and DC buses.

With at least two generators operating and three autopilots armed for approach, the Center Buses transfer from the left main buses to the Standby Inverter and Hot Battery Bus SCHEMATIC. This provides three independent power sources for the three autopilots:

  • The left main system powers the Left autopilot.
  • The right main system powers the Right autopilot.
  • The Standby/battery system powers the Center autopilot.

Between arming of the three autopilots and 200 ft, transitioning to single generator operation results in the electrical system reverting to normal operation SCHEMATIC. The Center Buses switch back to the left main AC and DC, and both Bus Tie Breakers close to keep both AC buses powered. At the same time, NO LAND 3 appears on the Autoland Status Annunciator.

The electrical system does not revert to normal if a transition to single generator operation occurs below 200 feet. The Center Autopilot remains powered from the Standby/battery system and the Bus Tie Breakers prevent the remaining generator from powering both main AC buses. The remaining generator powers only one main AC bus and the associated autopilot. The other main AC bus and associated autopilot are not powered. The automatic approach, landing and roll-out continue using the remaining two autopilots.

The electrical system reverts to normal non-isolated operation when the autopilots are disengaged or an autopilot go-around is performed.


After landing, when the APU is started, the APU generator breaker closes automatically. When the first engine is shutdown, the associated engine and generator breaker opens and Bus Tie Breaker closes. The APU Generator then powers the associated bus SCHEMATIC.

When the second engine is shutdown, the second generator breaker opens, and the second Bus Tie Breaker closes allowing the APU generator to power of all the buses on the airplane SCHEMATIC.

External Power can be used instead of the APU Generator during engine shutdown. External power can be plugged in will be engine generators are still operating. Pushing the External Power Switch opens both engine generator breakers and closes the External Power Contactor and both Bus Tie Breakers. All buses on the airplane are then supplied by external power SCHEMATIC.

Cruise - Extended Range Operations

A Hydraulic Driven Generator (HDG), supplied from the Left Hydraulic System, is installed on some airplanes as a fourth AC generating source. It operates automatically, after a short delay, providing a backup power source with the loss of both left and right AC buses in flight. Because the DC voltage from an operating HDG may be less than that from a fully charged Battery, the Battery discharge lights may illuminate for a period time when the HDG first begins to operate. The HDG powers the:


The First Officer's Flight Instrument Transfer Bus is not powered by the Hydraulic Driven Generator SCHEMATIC.

A test switch located on the Accessory Panel permits system checkout. When the switch is moved to the HYD GEN position, the left hydraulic system pressurizes to provide hydraulic power for the motor. Status messages are displayed to indicate proper system operation.

A list of equipment supplied by the Left and Right AC Transfer Buses is provided in the Bus Equipment List.

Bus Equipment