Electrical - Power distribution

Question 1

How many busses provide power for airplane servicing? Which busses?

Answer 1

2 busses.

The ground handling bus & the ground service bus

Question 2

Where are the controls located for the ground handling bus?

Answer 2

There are no controls for the ground handling bus.

Question 3

How is the Ground Handling bus powered?

Answer 3

Automatically when external power is plugged in or, via the APU (automatically if it’s running).

Question 4

Which power source has priority (for the Ground Handling Bus)?

Answer 4

External power has priority over the APU

Question 5

The Ground Handling Bus powers ?

Answer 5

1. Cargo Doors
2. Cargo Compartment lights
3. Maintenance access lights

Question 6

Is the Ground Handling Bus powered in flight?

Answer 6

No

Question 7

Who needs the Ground Service Bus?

Answer 7

Cabin Service Personnel

Question 8

What does the Ground Service Bus power?

Answer 8

1. Cabin lighting & outlets
2. Main & APU battery chargers
3. The Left fwd fuel pump (which supplies fuel to the APU)

Question 9

When is the Ground Service Bus available?

Answer 9

It powers loads needed both on the ground & in the air.

Question 10

When external power is the only source of power, & the main busses are not powered, how can you power the Ground Service Bus?

Answer 10

Selecting the GROUND SERVICE switch at the attendant panel at door 1L

Question 11

Whenever the right Main AC bus is powered, the Ground Service Bus is ?

Answer 11

Powered

Question 12

Ground Service Bus Power Sources

Answer 12

1. External Power
2. Right Main bus
3. APU generator

Question 13

If the right Main AC is not powered, the APU is running, the Ground Service bus is powered by ?

Answer 13

The APU generator through the GROUND SERVICE switch

Question 14

With the CABIN/UTILITY power switch ON what is powered normally?

Answer 14

1. GND SERVICE BUS
2. Utility Bus
3. Galleys (via Main DC Bus)

Question 15

If you select the CABIN UTILITY power switch OFF what do you lose?

Answer 15

Cabin lighting & outlets

Question 16

When the CABIN/UTILITY switch is selected OFF, the following cabin lights are turned on:

Answer 16

Night lights
Supplemental night lights
Reading lights
Attendant work lights

Question 17

DC electrical power operates which important items?

Answer 17

1. PFD’s & ND’s
2. GPS
3. ADIRU (Air Data Inertial Reference Unit)

Question 18

How many batteries does the 777 have?

Answer 18

2

Question 19

What are the 2 batteries on the 777

Answer 19

1. APU battery
2. Main battery

Question 20

What does the APU battery power?

Answer 20

APU starter motor & APU control circuits

Question 21

What does the Main a/c battery power?

Answer 21

Critical airplane systems when no other power sources are available

Question 22

What monitors the battery functions?

Answer 22

ELMS (Electrical Load Management system)
Displayed on ELEC synoptic

Question 23

What is the primary source of DC power?

Answer 23

4 Transformer rectifier units (TRUs)

Question 24

What do the TRUs do & what powers them?

Answer 24

Convert AC into DC

Powered from the 2 XFER busses

Question 25

The TRUs supply 4 DC busses, they are ?

Answer 25

1. Left Main DC bus 2. Right Main DC bus 3. Captain’s flight instrument bus 4. First Officer’s flight instrument bus

Question 26

If a TRU fails……

Answer 26

DC Tie relays connect the L & R Main DC busses & the Captain’s & First Officer’s flight instrument busses

Question 27

Are there any controls for the DC system?

Answer 27

No, it’s fully automatic

Question 28

The 777 is a Fly-by-Wire airplane, what does this mean?

Answer 28

Flight controls are actuated hydraulically, but controlled electrically

Question 29

How many backup generators are there?

Answer 29

2, 1 on each engine

Question 30

2 Permanent Magnet Generators (PMGs), are housed within each backup generator. What are their purpose?

Answer 30

They are the power source for the flight controls

Question 31

The PMGs provide dedicated power to ?

Answer 31

L, C & R power supply assemblies & their respective flight control DC busses (L FCDC BUS, C FCDC BUS, R FCDC - Flight Control DC Busses)

Question 32

What happens if 1 or more PMGs fail in flight?

Answer 32

The Main DC busses or the Captain’s flight instrument bus power the flight control DC busses

Question 33

If the Main DC busses become unpowered, what provides power to the L & C power supply assemblies?

Answer 33

The Hot Battery Bus

Question 34

Should a transfer of power occur from one source to another (in the Flight Controls DC System), what insures no interruption of power during the transfer?

Answer 34

The 1 minute batteries provided on each power supply assembly

Question 35

To allow automatic landings with the lowest possible minimums, the electrical system configured to that ?

Answer 35

Each of the 3 autopilots is powered by a seperate power source

Question 36

Autoland Electrical Config - During approach, power to the R XFER bus switches from the R Main AC bus to ?

Answer 36

The backup generators (same for the L etc). This transfer provides indeed EBT sources to each of the 3 autopilots Elec system returns to normal when a GA is performed or APs are disengaged.

Question 37

Which bus operates only on the ground?

Answer 37

The Ground Handling Bus

Question 38

What provides DC power on this airplane?

Answer 38

TRUs

Question 39

What is the electrical configuration during Autoland approaches?

Answer 39

The right transfer bus isolates from the right main bus

Question 40

What is the secondary source of power for fly-by-wire if one or more PMGs are unpowered?

Answer 40

DC Busses