Electrical Flashcards
Sources of main AC power are
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.
Sources of DC power are
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.
With all electrical power turned OFF, the main battery can still power a few items through the hot battery bus.
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.
The ground handling bus can be powered, on the ground only, from
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.
The ground handling bus is automatically powered when
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.
The ground service bus powers the
main battery charger,
the APU battery charger,
cabin lighting and outlets
and the left forward fuel pump.
The Generator Control Unit is controlled from the electrical panel on the overhead by
the GEN CTRL switches, and represented on the Electrical system synoptic by the GENERATOR CONTROL icons.
Electrical Load Management System (ELMS) provides
load management and protection to ensure power is available to critical and essential equipment.
If the electrical loads exceed the power available (airplane or external), ELMS automatically sheds AC loads by priority.
The load shedding is
galleys first, then utility busses. Utility busses are followed by individual equipment items powered by the main AC busses.
When an additional power source becomes available,
in this case secondary external power, or
the loads decrease,
ELMS restores power to shed systems
(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.
The backup electrical system automatically provides power to selected aircraft systems. If
both IDGs and the APU generator are inoperative, a backup generator powers essential aircraft equipment.
The essential aircraft equipment is powered by
the L and R XFR busses.
In the event of a series of failures causing a main bus to become unpowered, a backup generator will
automatically power the affected transfer bus to maintain essential power.
If both main AC busses fault, one backup generator will power
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.
Backup power energizes a transfer bus when APP mode is selected for autoland operations
to enhance system integrity.
The DC electrical system includes
the main DC electrical system and the flight control DC electrical system.
The main DC electrical system uses four Transformer–Rectifier Units (TRUs) to produce DC power. The TRUs are powered by
the AC transfer busses.
The left TRU powers
the left main DC bus,
which provides a second DC power source for:
left flight control Power Supply Assembly (PSA); right main DC bus.
The right TRU powers
the right main DC bus, which provides a second DC power source for:
right flight control PSA; left main DC bus.
The C1 TRU powers
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.
The C2 TRU powers
the First Officer’s flight instrument bus, which provides a second DC power source for the Captain’s instrument bus.
The main battery is connected directly to t
the hot battery bus and provides standby power to other busses as described in the standby electrical system.
The main battery charger normally powers
the hot battery bus and maintains the main battery fully charged.
The APU battery is connected directly to
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.
The APU battery charger normally powers
the APU battery bus and maintains the APU battery fully charged.
The standby electrical system can supply
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.
Critical DC loads are
he Captain’s and First Officer’s flight instruments and displays.
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,
the RAT will deploy automatically providing standby power.
The RAT will deploy automatically for reasons other than electrical faults.
The Rat will also deploy for:
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
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
the battery bus through a standby inverter.
During operation of the RAT
Hydraulic demands take priority over electrical demands.
If the RAT sheds electrical loads, the main battery powers the
standby power system.
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,
only the Captain’s instrument bus is available.
The IFE / PASS SEATS power switch controls power to the IFE and passenger seats.
Pushing the switch OFF removes power from the following:
IFE (all components); passenger seats (including seat motor power, personal computer power outlets and telephones).
The CABIN / UTILITY power switch controls power to various cabin and utility systems.
Pushing the switch OFF removes power from items such as:
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.
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
inhibited in flight.
The AVAIL indication in the EXT PWR switches means that external power is connected to the airplane and is of
usable quality.
When PRIMARY EXT PWR is selected on it powers
both main AC busses.
The L and R BUS TIE breakers are closed, supplying the AC power from one source.
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
43,100 ft.
Now we start the APU.
The main airplane battery has to be ON to open the APU inlet door,
operate the fuel valve and
fire detection system.
With APU power available, the APU has priority over the
SECONDARY EXT PWR.
ELMS makes the change smoothly.
There is no power interruption when switching power sources except when switching between two external sources.
The APU GENERATOR switch is left in the ON position for all
normal operations.
In this position the APU GENERATOR breaker
opens and closes automatically.
Normal Start
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.
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.
When the generator capacity increases or the loads decrease, ELMS restores power to the shed systems.
Two power sources keep busses powered. One generator powers
all essential flight loads.
During flight, if there is a fault in the left generator, the protection circuits
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.
The electrical control panel has the L DRIVE light illuminated. This is an indication of
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.
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.
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
still an option, from the electrical standpoint, because autoland is possible with the backup generator system only.
