Boeing 747 AC Main Busses boeing 747 pilot training course

WELCOME ADVANCE FLYING ACADEMY

BOEING 747 FULL PILOT TRAINING COURSE STEP BY STEP

AC Main Busses

Each IDG normally powers its related AC bus through a generator control breaker

(GCB). Each AC bus is connected to the synchronous bus by a bus tie breaker

(BTB). With the Bus Tie switch in AUTO, the related BTB controls the AC bus

connection to the synchronous bus.

If power on an AC bus is unacceptable, the related BTB opens and the bus

disconnects from the synchronous bus. However, the AC bus remains powered by

its IDG. If the IDG is not able to maintain acceptable power quality, the GCB

opens and the BTB closes to provide power from the synchronous bus.

The EICAS message ELEC BUS ISLN displays if the BTB is open. Pushing the

Bus Tie switch OFF, then AUTO, resets logic circuitry and allows the BTB to

close if the fault is corrected. The AC bus is reconnected to the synchronous bus.

The main AC busses power individual equipment items such as:

• transformer-rectifier units (TRUs)

• navigation radios and flight control computers

• flight deck lighting

• pitot and window heat

The main AC busses also power other AC busses:

• AC bus 1 powers the ground service bus and provides back-up power for

both transfer busses

• AC bus 2 powers the First Officer’s transfer bus

• Each AC bus powers a utility and galley bus

• AC bus 3 powers the Captain’s transfer bus and the main standby bus

Ground Service Bus

The ground service bus is powered on the ground and in flight whenever AC bus

1 is powered. The ground service bus powers individual equipment items such as:

• main and APU battery chargers

• fuel pumps for APU start

• upper deck doors

• flight deck flood, navigation, and service lights

• miscellaneous service outlets and equipment

• horizontal stabilizer fuel pump for defueling On the ground when AC bus 1 is not powered, the ground service bus can be connected to the same source powering the ground handling bus by pushing the Ground Service switch on the flight attendant’s panel at door 2L.

Ground Handling Bus

The ground handling bus is powered on the ground when APU generator 1 or

external power 1 is available. If both are available, external power is automatically

used. The ground handling bus powers individual equipment items such as:

• lower cargo handling equipment and compartment lights

• fueling system

• auxiliary hydraulic pump 4

Transfer Busses

Two transfer busses provide AC power to critical flight-related equipment. The

transfer busses have a common back-up power source. Transfer to the back-up

power source is automatic.

Captain’s Transfer Bus

The Captain’s transfer bus is normally powered by AC bus 3. If AC bus 3 is

unpowered, AC bus 1 powers the Captain’s transfer bus. The Captain’s transfer bus powers the APU standby bus.

The Captain’s transfer bus powers individual equipment items such as:

• center EIU

• left HF

First Officer’s Transfer Bus

The First Officer’s transfer bus is normally powered by AC bus 2. If AC bus 2 is unpowered, AC bus 1 powers the First Officer’s transfer bus. The First Officer’s

transfer bus powers individual equipment items such as:

• FMCS autothrottle servo

• lower EICAS display

• right ADC, right EFIS control, right EIU, right FMC

• right CDU, right HF, right ND, right PFD

Utility and Galley Busses

Each main AC bus powers one utility bus and one galley bus. Each utility/galley

bus is controlled by an electrical load control unit (ELCU) that protects the

electrical system from utility and galley bus faults, and provides load management through automatic load shedding. With the Left Utility Power switch ON, utility busses 1 and 2 and galley busses 1 and 2 are powered according to ELCU logic.

With the Right Utility Power switch ON, utility busses 3 and 4 and galley busses

3 and 4 are powered according to ELCU logic.

Utility busses power individual equipment items such as:

• forward main fuel pumps 2 and 3

• forward override/jettison pumps 2 and 3

• center override/jettison pump

• recirculating fans

Galley busses power the galleys located throughout the cabin.

The EICAS message ELEC UTIL BUS displays and the utility OFF light

illuminates if one or more utility or galley bus become unpowered due to a fault.

Cycling the Utility Power switch OFF, then ON restores power to the affected bus

if the fault is corrected.