Electrical

Question 1

Sources of main AC power are

Answer 1

the left and right engine Integrated Drive Generators (IDGs), an

APU GEN, and

PRIMARY and SECONDARY EXT PWR.

Any two main AC sources can supply power for the entire airplane.

Question 2

Sources of DC power are

Answer 2

from main battery,

the APU battery,

the flight control DC electrical system and

the main DC electrical system which uses four Transformer-Rectifier Units (TRUs) to produce DC power.

The TRUs are powered by the AC transfer busses.

Question 3

With all electrical power turned OFF, the main battery can still power a few items through the hot battery bus.

Answer 3

These items are the engine fire extinguisher bottles,

the APU fire extinguisher bottle

and the Air Data Inertial Reference Unit (ADIRU).

The ADIRU remains powered as long as the ADIRU switch is ON.

Question 4

The ground handling bus can be powered, on the ground only, from

Answer 4

either the APU generator or

from the primary external power source.

It is provided for loads such as cargo handling,

fueling / defueling operations,

potable water servicing and functionality

and lavatory servicing and functionality.

Question 5

The ground handling bus is automatically powered when

Answer 5

the primary external power is connected or if APU power is available.

If the APU is powering the bus, selecting the external power switch to ON will give the external power priority.

Question 6

The ground service bus powers the

Answer 6

main battery charger,

the APU battery charger,

cabin lighting and outlets

and the left forward fuel pump.

Question 7

The Generator Control Unit is controlled from the electrical panel on the overhead by

Answer 7

the GEN CTRL switches, and represented on the Electrical system synoptic by the GENERATOR CONTROL icons.

Question 8

Electrical Load Management System (ELMS) provides

Answer 8

load management and protection to ensure power is available to critical and essential equipment.

Question 9

If the electrical loads exceed the power available (airplane or external), ELMS automatically sheds AC loads by priority.

The load shedding is

Answer 9

galleys first, then utility busses. Utility busses are followed by individual equipment items powered by the main AC busses.

Question 10

When an additional power source becomes available,

in this case secondary external power, or

the loads decrease,

ELMS restores power to shed systems

Answer 10

(in the reverse order).

Note the PRI EXT PWR powers the right main bus and the SEC EXT PWR energizes the left main bus. The right bus tie breaker is open.

Question 11

The backup electrical system automatically provides power to selected aircraft systems. If

Answer 11

both IDGs and the APU generator are inoperative, a backup generator powers essential aircraft equipment.

Question 12

The essential aircraft equipment is powered by

Answer 12

the L and R XFR busses.

Question 13

In the event of a series of failures causing a main bus to become unpowered, a backup generator will

Answer 13

automatically power the affected transfer bus to maintain essential power.

Question 14

If both main AC busses fault, one backup generator will power

Answer 14

both left and right transfer busses.

If there is only one main generator available, one backup power generator will energize one or both transfer busses.

Only one of the two backup generators will be used to power the transfer busses at anytime. Either one could perform this task.

Question 15

Backup power energizes a transfer bus when APP mode is selected for autoland operations

Answer 15

to enhance system integrity.

Question 16

The DC electrical system includes

Answer 16

the main DC electrical system and the flight control DC electrical system.

Question 17

The main DC electrical system uses four Transformer–Rectifier Units (TRUs) to produce DC power. The TRUs are powered by

Answer 17

the AC transfer busses.

Question 18

The left TRU powers

Answer 18

the left main DC bus,

which provides a second DC power source for:

left flight control Power Supply Assembly (PSA); right main DC bus.

Question 19

The right TRU powers

Answer 19

the right main DC bus, which provides a second DC power source for:

right flight control PSA; left main DC bus.

Question 20

The C1 TRU powers

Answer 20

the Captain’s flight instrument bus and the battery bus.

The Captain’s flight instrument bus provides a second DC power source for:

center flight control PSA; First Officer’s flight instrument bus.

Question 21

The C2 TRU powers

Answer 21

the First Officer’s flight instrument bus, which provides a second DC power source for the Captain’s instrument bus.

Question 22

The main battery is connected directly to t

Answer 22

the hot battery bus and provides standby power to other busses as described in the standby electrical system.

Question 23

The main battery charger normally powers

Answer 23

the hot battery bus and maintains the main battery fully charged.

Question 24

The APU battery is connected directly to

Answer 24

the APU battery bus

and provides dedicated power to the APU electric starter,

which is used when sufficient bleed air duct pressure is unavailable for the APU

air turbine starter.

Question 25

The APU battery charger normally powers

Answer 25

the APU battery bus and maintains the APU battery fully charged.

Question 26

The standby electrical system can supply

Answer 26

AC and DC power to selected instruments,

communications and navigation systems,

and the flight control system,

in the event of a loss of all main and backup generators.

Question 27

Critical DC loads are

Answer 27

he Captain’s and First Officer’s flight instruments and displays.

Question 28

If the three main generators and the two backup generators have faults, or for some other reason the left and right transfer busses are not powered,

Answer 28

the RAT will deploy automatically providing standby power.

The RAT will deploy automatically for reasons other than electrical faults.

Question 29

The Rat will also deploy for:

Answer 29

All 3 hydraulic system pressures are low

dual engine failure and center hydraulic low

If it fails to deploy automatically

it can be manually deployed with the switch on the hydraulic panel

Question 30

The Standby system is usually powered by the transfer busses.

When required the RAT provides DC power through the TRUs,

and the standby AC bus is powered from

Answer 30

the battery bus through a standby inverter.

Question 31

During operation of the RAT

Hydraulic demands take priority over electrical demands.

If the RAT sheds electrical loads, the main battery powers the

Answer 31

standby power system.

Question 32

When the RAT provides electrical power, both the Captain’s and the First Officer’s instrument busses are powered.

When the battery is supplying the power,

Answer 32

only the Captain’s instrument bus is available.

Question 33

The IFE / PASS SEATS power switch controls power to the IFE and passenger seats.

Pushing the switch OFF removes power from the following:

Answer 33

IFE (all components); passenger seats (including seat motor power, personal computer power outlets and telephones).

Question 34

The CABIN / UTILITY power switch controls power to various cabin and utility systems.

Pushing the switch OFF removes power from items such as:

Answer 34

ground service bus (except main and APU battery chargers,

and left forward fuel pump); utility busses;

galleys; gasper fan; fluorescent cabin lighting; beacon, logo, and wing lights.

The following cabin lights are turned ON:

night and supplemental night lights; reading lights; attendant work lights; some galley / crew rest lights.

Question 35

When the BATTERY switch is selected ON, the ON indication is visible in the BATTERY switch, and a number of flight deck annunciator lights illuminate. Turning the battery power OFF is

Answer 35

inhibited in flight.

Question 36

The AVAIL indication in the EXT PWR switches means that external power is connected to the airplane and is of

Answer 36

usable quality.

Question 37

When PRIMARY EXT PWR is selected on it powers

Answer 37

both main AC busses.

The L and R BUS TIE breakers are closed, supplying the AC power from one source.

Question 38

The APU is located in the tail cone of the airplane. The air inlet is on the right side of the aircraft and the exhaust is on the left. The maximum altitude for start is

Answer 38

43,100 ft.

Question 39

Now we start the APU.

The main airplane battery has to be ON to open the APU inlet door,

operate the fuel valve and

Answer 39

fire detection system.

Question 40

With APU power available, the APU has priority over the

Answer 40

SECONDARY EXT PWR.

ELMS makes the change smoothly.

There is no power interruption when switching power sources except when switching between two external sources.

Question 41

The APU GENERATOR switch is left in the ON position for all

Answer 41

normal operations.

In this position the APU GENERATOR breaker

opens and closes automatically.

Question 42

Normal Start

Answer 42

The DRIVE DISC DRIVE light will extinguish when the fuel control switches are selected to run and the engines begin to spool up.

The BACKUP GEN ON light is illuminated to indicate the arming of the backup generator.

The GEN CONTROL ON light is illuminated to indicate the arming of the generator breaker.

This breaker will close automatically when generator power is available.

Question 43

ELMS controls bus loading.

If load shed is required, the ELMS automatically sheds

galleys, recirculation fans, the C2 electric pump,

and other non-essential systems.

Answer 43

When the generator capacity increases or the loads decrease, ELMS restores power to the shed systems.

Question 45

Two power sources keep busses powered. One generator powers

Answer 45

all essential flight loads.

Question 46

During flight, if there is a fault in the left generator, the protection circuits

Answer 46

trip the generator breaker open.

The L GEN CTRL OFF light illuminates,

and an EICAS message is displayed.

As this is only an advisory message there is no master caution annunciation.

Note: both bus ties close.

The right main generator powers both main AC busses. As only the right main generator is operating; a backup generator powers the left transfer bus.

Question 47

The electrical control panel has the L DRIVE light illuminated. This is an indication of

Answer 47

low oil pressure in the generator drive unit.

The electrical synoptic shows that the IDG is still operating but has the advisory in amber.

In the event of high IDG temperature, this action would be done automatically, the EICAS message and synoptic display would be identical.

Question 48

With two generators lost,

the APU powers the both main AC busses

and a backup generator powers both transfer busses.

A frequency converter converts the variable frequency power of the backup generator to usable 400 Hz power.

Only one generator can power the converter at a time.

Answer 48

Question 49

The electrical synoptic illustrates the condition. The L and R MAIN AC busses are not powered. All services powered by the L and R MAIN AC busses are unavailable. The backup power system continues to power the transfer busses. Essential power is available.

Despite the degree of faults in this example, autoland is

Answer 49

still an option, from the electrical standpoint, because autoland is possible with the backup generator system only.