B777 Electrical System
Transcribed by TurboScribe.ai. Go Unlimited to remove this message.
Electrical System
For normal in-flight operations, all required electrical power is provided by two enginedriven generators. A third generator, driven by the APU, is available in flight. It is equal in
size and capacity to either engine-driven generator.
Any two of these three generators meet all electrical requirements. With only one
generator available, an electrical load management system automatically shuts down
some low-priority items, such as the galleys. Let's use the electrical synoptic to show the
main electrical components.
Two main AC power buses, left and right, distribute power to all airplane electrical
systems. A bus tie and two isolation relays operate automatically to ensure both main
buses remain powered at all times. Now let's operate the system.
Let's begin with the airplane secured and external power disconnected. Controls for the
electrical system are located on the overhead panel. With all electrical power switched
off, the main battery still continues to power a few important items through the hot
battery bus.
These include engine fire extinguisher bottles, the APU fire extinguisher bottle, and the
Adiru, as long as the Adiru switch is on. Turn on the battery. The on indication in the
battery switch appears, and several flight deck annunciator lights illuminate.
Now let's connect ground power to the airplane. There are two external power switches
on the electrical panel, primary and secondary. The available light indicates primary
external power is connected to the airplane and is of usable quality.
Push the primary external power switch to connect external power to the main airplane
electrical system. The on light indicates primary external power is connected to the
airplane's main electrical system. If no other power source is available, the primary
external source powers the left and right buses.
With a single external power source, some non-essential electrical items, such as the
galleys, may be unpowered if loads are high. Connecting secondary external power to
the airplane gives full electrical load capacity for most ground situations. When
secondary external power is available and of usable quality, the available light
illuminates.
Connect secondary external power. Notice the on light illuminates and the available light
extinguishes when secondary external power is connected. When both primary and
secondary external power are available, primary powers the right main bus, and
secondary powers the left main bus.
Any two power sources support full electrical loading. In this example, the APU is
running, but not connected. Let's connect the APU generator to the system.
Push the APU generator switch to the on position. Notice that APU power replaces
secondary external power. The external power switch changes to available.
When the APU is running and primary external power is online, the APU powers the left
main bus, and the primary external power source powers the right main bus. The APU
generator switch is normally left in the on position at all times. In the on position, the
APU generator breaker opens and closes automatically.
There is no interruption of power when switching power sources, except when switching
between two external sources. Remove primary external power. Notice the primary
external power switch changes to available.
The APU now powers both buses. With only one power source, the electrical load
management system again sheds non-essential electrical loads. Re-establish primary
external power.
Re-establish secondary external power. The only indication of the change from APU
power to external power is the available enunciation changing to an on enunciation in
the external power switches. Each main engine generator and its drive unit are housed
in a single case and share a common oil supply.
This combined unit is called an integrated drive generator or IDG. Each IDG is controlled
by a generator control unit, which provides automatic control functions and system
protection. The generator control switches automatically connect the IDG to the main
buses.
The generator control switches remain in the on position for all normal operations. In this
position, the on is in view and the breakers are armed for automatic operation. The off
light illuminates when the breaker is open.
Note this is different than the APU switch, which only illuminates with a fault or when the
switch is turned off. Now let's start the right engine. The IDG begins to rotate to
operating speed.
The generator breaker closes and the right generator off and drive lights extinguish. The
right generator now powers the right main bus. At the same time, primary and
secondary external power automatically disconnect from the main buses and the right
generator also powers the left main bus.
After a few seconds with the APU running, APU power replaces right generator power on
the left main bus. When the left engine is started, the left bus tie breaker opens and the
left main bus is powered by its own generator. Each main AC bus powers one utility bus.
There are no switches for the utility buses. For normal operation, the bus tie switches are
left in auto. In auto, the bus tie breakers keep both main buses powered at all times from
any available source in a priority order.
The first priority is the associated engine generator, when available. Second is the APU
generator. And third is the opposite engine generator.
External power is the only AC power source not automatically selected. You must apply it
manually. Note that both bus tie breakers are open whenever the IDGs are powering
their respective buses.
Push the left bus tie switch to position it off. Notice the auto annunciation disappears and
the isolation light illuminates. The left bus tie breaker is locked open with the switch off.
Faults associated with the isolation lights are covered in another lesson. However, notice
that even though the right bus tie breaker is open, the right bus tie isolation light
remains extinguished. The isolation lights always remain extinguished when the bus tie
switches are in auto and the bus tie breakers are performing their normal automatic
operation.
Reset the left bus tie switch and notice that auto is again enunciated. Both engines are
now running and both engine generators are powering their associated bus. The only
indication on the electrical panel is that the generator off lights and generator drive
lights are extinguished.
The electrical panel is now configured for flight. External power is disconnected. Notice in
this normal flight configuration all electrical panel lights are extinguished.
The APU may be shut down. The system uses two power sources to keep the buses
powered unless only one power source is available. Any one generator has sufficient
capacity to supply all essential flight loads.
If only one power source is available, the electrical load management system load sheds
non-essential loads. First, galleys and galley chiller loads are shed, and then left and
right utility buses. The utility buses power passenger entertainment systems and
recirculating fans.
Now look at the automatic switching that takes place during the normal shutdown
procedure. You have arrived at the ramp. The APU is running and primary external power
is available.
Let's shut down the left engine. When the fuel control is moved to cut off, the IDG
synchronizes with the bus tie. The left bus tie breaker closes, the engine generator
breaker opens, and the APU powers the left bus.
The right bus is still powered by the right engine generator. When the right engine is
shut down, the right bus tie breaker closes, and the APU automatically powers the right
main bus. To keep the main buses powered, select external power prior to APU
shutdown.
Notice that primary external power now powers the right main bus, and the APU
continues to power the left main bus. Now turn the APU off. If the APU is not available,
select primary external power prior to engine shutdown.
Select primary external power. This causes both generator breakers to open, and
primary external power now powers both main buses. Since this is not a through-flight,
the airplane is secured and external power is disconnected.
The main buses are now off. The only remaining source of power is the main battery. In
addition to the previously mentioned systems, the battery powers the standby power
system, the captain's displays and CDU, and the left airplane information management
system, AIMS.
Now complete the power down. Turn off the battery. All electrical power is now off
except those items continuously powered by the hot battery bus.
The airplane has been parked overnight. You are preparing for the first flight of the day.
Establish full electrical power.
Source & Operation
The airplane just landed and is taxiing to the gate. Prepare for engine shutdown. The
engines are shut down and external power is plugged in.
Establish full electrical external power. Set the electrical panel to secure the airplane,
but keep external power connected for maintenance personnel. Now turn the APU off.
Power Distribution
Two buses provide power for servicing the airplane, the ground handling bus and ground
service bus. There are no controls for the ground handling bus. The bus is automatically
powered when external power is plugged in.
The APU can also power the ground handling bus. If external power is not available and
the APU is running, the bus is automatically powered. If the APU is running and external
power is plugged in, external power has priority.
The ground handling bus powers the cargo doors, cargo compartment lights and
maintenance access lights. The bus is not powered in flight Now let's look at the ground
service bus. The ground service bus is for cabin service personnel.
The ground service bus powers loads needed both on the ground and in the air. It powers
cabin lights and outlets, battery chargers for the main and APU batteries and the left
forward fuel pump which supplies fuel to the APU. When external power is the only
source of power and the main buses are not powered, a ground service switch at the
attendance panel at door 1 left can be used to power the ground service bus.
Whenever the right main AC bus is powered, the ground service bus is powered. If the
right main AC bus is not powered and the APU is running, the ground service bus is
powered by the APU generator through the same ground service switch. DC electrical
power is used to operate important items such as the primary flight displays and the
navigation displays, Global Positioning System or GPS and the Air Data Inertial Reference
Unit or ADARU.
Now let's look at the sources of DC power. The 777 has two batteries. The APU battery
powers the APU starter motor and APU control circuits.
The main battery powers critical airplane systems when no other power sources are
available. Battery functions are monitored by the Electrical Load Management System or
ELMS and displayed at the bottom of the electrical synoptic. Also, battery messages are
displayed on ICAS.
The primary source of DC power is four transformer rectifier units or TRUs. The TRUs
convert AC power into DC power. They are powered from the two transfer buses.
The TRUs in turn supply four DC buses, the left main DC bus, the right main DC bus, the
captain's flight instrument bus and the first officer's flight instrument bus. If a TRU fails,
DC tie relays connect the left and right main DC buses and the captain's and first
officer's flight instrument buses. This allows all buses to remain powered.
There are no controls for the DC system as it is fully automatic. The flight controls on this
airplane are actuated hydraulically but controlled electrically. This is known as fly-bywire.
There are two backup generators, one on each engine. Each backup generator is a nontime limited backup source. We'll discuss the backup generators in more detail in nonnormal operations.
Two permanent magnet generators or PMGs are housed within each backup generator.
These PMGs are the power source for the flight controls. The PMGs provide dedicated
power to left, center and right power supply assemblies and their respective flight
control DC buses.
If one or more PMGs fail in flight, the main DC buses or the captain's flight instrument
bus power the flight control DC buses. If the main DC buses become unpowered, the hot
battery bus provides power to the left and center power supply assemblies. Should a
transfer of power from one source to another occur, one minute batteries are provided
on each power supply assembly to ensure no interruption of power during the transfer.
To allow automatic landings with the lowest possible minimums, the electrical system
configures so that each of the three autopilots is powered by a separate power source.
During approach, power to the right transfer bus switches from the right main AC bus to
the backup generators. This transfer provides independent sources to each of the three
autopilots.
The electrical system returns to normal when an autopilot go-around is performed or
when the autopilots are disengaged. The 777 has a backup power system consisting of
two backup generators. These ensure electrical power is available for essential loads.
Essential loads are powered by the left and right transfer buses. Essential loads include
instrument and panel lighting, captain's and first officer's navigation and communication
radios, and center pitot and engine probe heat. The transfer buses are normally powered
by the left and right main AC buses.
Non Normal
The transfer buses connect to one of the backup generators when their AC buses are
unpowered. They are normally left on. The backup generator switch off lights remain
illuminated until their associated engine is running.
With the engines running and the backup generator switches on, automatic switching is
armed. Notice that the backup generators are running. Push the left backup generator
switch to the off position.
With the switch off, power to the transfer buses is not available from that generator,
even though the generator is operating normally. Now reset the switch. The backup
power system provides uninterrupted transfer of power to essential loads in the event of
loss of one main AC bus, the loss of both main AC buses when only one main generator
is available and during autoland.
In the event of loss of all main and backup generators, a standby system powers critical
AC and DC equipment. Critical AC loads include center ILS and radio altimeter, marker
beacon left, ATC transponder left, VOR left and engine igniters. Critical DC loads include
the captain's and first officer's flight instruments and displays.
The standby system consists of the battery buses, the captain's and first officer's flight
instrument buses and an AC standby bus. Normal power for these buses is from the left
and right AC transfer buses, either directly or through TRUs. Should the transfer buses
become unpowered, a ram air turbine or RAT provides DC power through the TRUs and
the standby AC bus is powered from the battery bus through a standby inverter.
The ram air turbine or RAT can be deployed in flight to provide electrical and hydraulic
power simultaneously. It is the primary source of standby electrical power if all normal
sources are lost. It is located in the aft right fairing and is a non-time limited source of
power.
However, its output is small, providing less than one-tenth the power of the other
generators. The RAT is deployed automatically if both AC transfer buses are unpowered,
if all three hydraulic system pressures are low or with dual engine failure and the center
hydraulic system pressure low. If the RAT fails to deploy automatically, the crew can
deploy it manually.
The ram air turbine switch is located on the hydraulic panel. Deploy the RAT. The ram air
turbine unlocked light illuminates when the RAT is not in the fully up and locked position.
The pressure light illuminates when the RAT is producing normal hydraulic pressure to
the flight controls. If the RAT is unable to maintain hydraulic pressure, the electrical load
is shed until hydraulic pressure is satisfactory. When RAT electrical loads are shed or
during periods prior to RAT deployment, the main battery powers the standby power
system.
When the RAT provides electrical power, both the captain's and first officer's flight
instrument buses are powered. When the battery provides standby power, only the
captain's flight instrument bus is powered. Now let's look at some non-normal
indications.
Shortly after takeoff, the ICAS advisory message, electric generator off left, displays and
the off light in the generator control switch on the overhead panel illuminates. A
generator fault caused the protection circuits to trip the field and the generator breaker
to open. Pushing the left generator control switch off and on resets the system.
Reset the left generator. The fault was temporary. The ICAS message is removed and the
generator operates normally after resetting.
During climb, the ICAS advisory message, electric generator drive left, displays and the
left generator drive light illuminates. The message indicates there is low oil pressure in
the generator drive. Hold the guarded switch for one second to disconnect the generator
drive from the engine.
The generator drive is now disconnected. It can only be reconnected by maintenance
action. High generator drive temperature automatically disconnects the generator drive,
giving these same indications.
With the generator drive disconnected, the message, electric generator off left, also
displays and a backup generator is immediately connected to the left transfer bus. The
associated engine generator is lost for the duration of the flight. The right engine
generator powers the left main AC bus.
Power to the left transfer bus is supplied by the left backup generator. The APU can be
started to provide a second generator for the system. Start the APU.
The ICAS messages may be cancelled. Cancel the messages. After the left engine
generator fails, the right engine generator fails.
The APU now powers both main buses. With two generators lost, the right backup
generator powers the transfer buses. Only one backup generator at a time can power the
transfer buses.
This file is longer than 30 minutes.
Go Unlimited at TurboScribe.ai to transcribe files up to 10 hours long.