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BOEING 737 SYSTEMS REVIEW
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ELECTRICAL
1. GENERAL
¡ Primary electrical power is provided by two engine driven generators which supply three-phase, 115
volt 400 cycle alternating current.
¡ B737 Classics : Each generator supplies its own bus in normal operation and can also supply
essential loads of the opposite side bus system when one generator is inoperative.
¡ B737-NG : Each generator supplies its own bus in normal operation and can also supply essential
loads and non-essential loads of the opposite side bus system when one generator is inoperative.
¡ Step down transformers provide low voltage AC power for lighting, instruments and other circuits that
use alternating current at a lower voltage.
¡ Transformer rectifier (TR) units supply DC power and a battery provides backup for the AC and DC
standby system.
¡ B737 Classics : The APU operates a generator identical to the engine generators and can supply
both engine generator busses on ground or either system in flight.
¡ B737 NG : The APU operates a generator identical to the engine generators and can supply both
transfer busses both on ground and in flight.
¡ There are two basic principles of operation for the 737 electrical system :
1. There is no paralleling of the AC sources of power.
2. All generator bus sources must be manually connected through the movement of a switch. The
source of power being switched onto a bus will automatically disconnect an existing source.
¡ The electrical power system may be categorized into 3 main divisions :
- AC Power System,
- DC Power System,
- Standby Power System.
2. MAIN COMPONENTS AND SUBSYSTEMS B737 CLASSICS vs B737 NG
ELECTRICAL POWER GENERATION
B737 Classics - Engine Generators
¡ Primary power is obtained from two generators, one mounted on each engine. Each generator is
part of a generator drive unit which maintains a constant frequency throughout the normal operating
range of the engine. The generator is coupled directly to the engine and operates whenever the
engine is running.
B737 NG - Integrated Drive Generators
¡ Primary power is obtained from two engine Integrated Drive Generators (IDGs), one mounted on
each engine. The IDG is a common housing containing a generator and drive which allows the
generator to maintain a constant generator speed and thus a constant frequency throughout the
normal operating range of the engine. The IDG is coupled directly to the engine and operates
whenever the engine is running.
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APU Generator
¡ The APU generator may be used to supply primary power on ground and will serve as backup for
either engine generator in flight. The APU generator is identical to the engine generators but has no
generator drive unit since the APU itself is governed and will maintain a constant generator speed.
External Ground Power
¡ An external AC power receptacle located near the nose gear wheel well, on the lower right side of
the fuselage, allows the use of an external power source. Status lights on a panel adjacent to the
receptacle permits the ground crew to determine if the external power is being used.
¡ A GRD POWER AVAILABLE Light provides cockpit indication that ground power is plugged in.
¡ A GRD PWR Switch allows connection of external power to both generator busses. The Battery
Switch must be ON for the GRD PWR Switch to be operable. Positioning the Battery Switch to OFF
will automatically disconnect external power.
B737 Classics - Ground Service
¡ For ground servicing a solenoid held Ground Service Switch is on the forward attendant's panel.
This switch provides power from ground power directly to the AC ground service bus for utility
outlets, cabin lighting and the battery charger without powering all electrical busses.
¡ The Ground Service Switch is magnetically held in the ON position and trips OFF when the GRD
PWR Switch is positioned to ON.
B737 NG - Ground Service
¡ For ground servicing a solenoid held Ground Service P/B is on the forward attendant's panel. This
P/B provides power from ground power directly to the AC ground service busses for utility outlets,
cabin lighting and the battery chargers without powering all electrical busses.
3. AC POWER SYSTEM
¡ AC power = 4 sources (2 engines, 1 APU & 1 Ground Power)
¡ B737 Classics : Each AC power system consists of a generator bus, a main bus and a transfer bus.
Each transfer bus has an associated transfer relay which automatically selects the opposite
generator bus as a power supply if its normal generator bus fails and the Bus Transfer Switch is in
AUTO. The generator busses are powered by momentarily positioning the associated Generator
Switch to ON. This connects the generator to its associated generator bus.
¡ B737 NG : Each AC power system consists of a transfer bus, a main bus, two galley busses and a
ground service bus. Transfer bus 1 also supplies power to the AC standby bus. If the AC source
powering either transfer bus fails or is disconnected, the transfer bus can be powered by any
available source through the tie bus with the Bus Tie Breakers (BTBs).
¡ B737 Classics : On ground, and external power connected, momentarily positioning the Ground
Power Switch to ON trips both engine generators and connects external power to both generator
busses.
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¡ B737 NG : On ground, and both generator control switches OFF, or with both engines shut down,
selecting the GRD PWR switch to ON connects external power via the tie bus to both transfer
busses.
¡ B737 Classics : When the APU is operating, electrical power output of the APU generator can be
connected to both generator bus No. 1 and generator bus No. 2 through the respective APU
Generator Switches. Whenever ground power is on both generator busses, and APU or engine
generator power is applied to one generator bus, ground power continues to supply power to the
remaining generator bus.
¡ B737 NG : Selecting either APU GEN switch ON connects APU power via the tie bus to both transfer
busses. Whichever source is selected last powers both busses. The transfer busses can be
powered from the engine generators by momentarily positioning the related generator switch to ON.
This connects the generator to the transfer bus. Whenever external power or APU is powering both
transfer busses and engine generator power is applied to its onside transfer bus, external power or
APU continues to supply power to the remaining transfer bus.
¡ B737 Classics : In flight, each engine generator normally powers its own generator bus. If a
generator is inoperative, the APU generator can be used to power one inoperative generator bus.
Since the entire electrical system is powered from the two generator busses, all electrical
components can be powered with any two operating generators.
¡ B737 NG : In flight, each engine generator normally powers its own transfer bus. If an engine
generator is no longer supplying power, the BTBs automatically close to allow the other engine
generator to supply both transfer busses through the tie bus and BTBs. The APU can power either
or both busses via the tie bus and the BTBs. The system also incorporates an automatic generator
on-line feature in case the aircraft takes off with the APU powering both transfer busses. In flight, if
the APU is either shut down or fails, the engine generators are automatically connected to their
related transfer busses.
B737 Classics - Bus Transfer System
¡ The generator busses and main busses supply the heavy and nonessential electrical loads,
respectively. The transfer busses supply the essential loads.
¡ If a generator trips inflight due to a fault, the generator bus and main bus will be de-energized, but
the transfer bus will transfer automatically to the operating generator bus.
¡ The electrical system monitors itself for correct voltage and frequency, ground faults in the generator,
or excessive current draw from any generator.
NOTE : If generator No.1 trips off the line, the battery charger transfers to Main Bus No.2 for AC power
B737 NG - Bus Tie System
¡ Either generator or the APU can supply power to both transfer busses. If the BUS TRANS switch is
in the AUTO position and the source powering the transfer bus is disconnected or fails, the BTBs
close and both transfer busses are powered from the operative power source.
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B737 Classics - Automatic Galley Load Shedding
¡ In flight electrical load shedding provides the capability to reduce power demands automatically
when operating on one generator. A protective circuit will turn off all galley power.
¡ Additionally for 737-400: When the APU is providing electrical power on ground, and total electrical
power requirements exceed design limits, electrical galley loads will automatically be shed and the
galley switch will trip to OFF.
B737 NG - Engine Generators Automatic Load Shedding
¡ During single generator operation, the system will shed electrical load incrementally based on actual
load sensing. The galleys on transfer bus 2 are shed first; if an overload is still sensed, the galleys
on transfer bus 1 are shed; if overload still exists, main bus 1 and main bus 2 are shed.
¡ When two generators are again available, automatic load restoration of the main busses and galley
busses occurs. If automatic restoration of galley power does not occur, manual restoration can be
attempted by moving the gaily power switch to OFF, then back to ON.
B737 NG - APU Automatic Load Shedding
¡ In flight, if the APU is the only source of electrical power, all galley busses are automatically shed. If
electrical load still exceeds design limits, both main busses are also automatically shed.
¡ On ground, if an overload condition is sensed, the APU sheds galley busses first and then main
busses until the load is within limits. Manual restoration of galley power can be attempted by moving
the gaily power switch to OFF, then back to ON.
ELECTRICAL POWER CONTROL AND MONITORING
B737 Classics - Generator Drive
¡ Each engine driven generator is connected to its engine through a generator drive unit which
provides a constant generator frequency output of 400 Hz. Each generator drive is a self-contained
unit consisting of oil supply, cooler, instrumentation and disconnect device which provides for
complete isolation of the generator in the event of a malfunction.
¡ Operating conditions of the generator drive can be observed on the Generator Drive Oil Temperature
Indicator. Oil temperature is measured as it enters and leaves the generator drive. Temperature of
oil entering the generator drive is indicated on the IN scale. Temperature differential between outlet
and inlet is indicated as RISE. A RISE reading above 20°C indicate excessive generator load or
poor condition of the drive.
¡ Two amber caution lights indicate generator drive excessive oil temperature in the internal oil tank or
low oil pressure. When the generator drive has been disconnected, the LOW OIL PRESSURE Light
will be on. The HIGH OIL TEMPERATURE Light remains on until the oil is cooled.
¡ A Generator Drive Disconnect Switch is installed. This switch disconnects the generator from the
engine in the event of a generator drive malfunction. Reactivating the generator can be
accomplished only on ground by maintenance personnel.
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B737 NG - Generator Drive
¡ The IDGs contain the generator and drive in a common housing and are lubricated and cooled by a
self-contained oil system. An integral electromechanical disconnect device provides for complete
mechanical isolation of the IDG.
¡ The DRIVE amber caution light comes on when :
- Low oil pressure is sensed in the IDG
- The IDG is disconnected automatically due to high oil temperature
- The IDG is disconnected manually with the drive disconnect switch
- The engine is not running.
¡ A generator drive disconnect switch is installed. This switch disconnects the generator from the
engine in the event of a generator drive malfunction. Reactivation of the generator may be
accomplished only on the ground by maintenance personnel.
AC & DC METERS
AC Voltmeter and Frequency Meter
¡ AC voltage and frequency can be read. on the AC Voltmeter and Frequency Meter for standby
power, ground power, generator No.1, APU generator, generator No.2 and static inverter. The
frequency of the generator is dependent on the speed of the generator drive. Frequency will be
indicated only when the generator is electrically excited.
¡ The voltage regulator automatically controls the generator output voltage.
¡ Current readings for the two engine generators and the APU generator can be read only on the
associated Ammeters on the overhead panel.
¡ The TEST position is used by maintenance.
DC Voltmeter and Ammeter
¡ DC voltage and amperage may be read on the DC Voltmeter and Ammeter for the battery and each
of the three transformer-rectifiers. The standby power and battery bus will display only DC voltage.
¡ The TEST position is used by maintenance.
NOTE : TR1 and TR2 voltage is measured at their respective DC busses.
4. DC POWER SYSTEM
¡ DC power = 3 sources (3 Transformer Rectifiers -TR - & external DC power on B737 Classics only)
¡ B737 Classics : Three transformer-rectifier (TR) units are the primary source of 28V DC power. The
battery provides 28V DC as a backup source of DC power for essential equipment.
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¡ B737 NG : Three Transformer - Rectifier (TR) units are the primary source of 28V DC power. The
batteries provide 28V DC as a backup source of DC power for essential equipment. On ground, an
amber ELEC light comes on to indicate that DC system or standby system equipment has failed.
The ELEC light is inhibited in flight.
Transformer-Rectifier Units
¡ The TR units convert 115-volt AC to 28-volt DC, and are identified as TR 1, 2 and 3.
¡ B737 Classics : TR units 1 & 2 receive AC power from the transfer busses and power the DC busses
1 and 2. With the Bus Transfer Switch in the AUTO position, TR 1 & TR 2 are operated in parallel.
TR 3, supplied by main AC bus No. 2, normally powers the battery bus and backs up TR units 1 & 2.
TR1 :
- Normal power source is Transfer bus No.1 & supplies DC Bus 1
- Backs up TR2 if TR3 disconnect relay is closed.
- Blocking diode prevents it from backing up TR3 to supply Battery bus.
TR2 :
- Normal power source is Transfer bus No.2 & supplies DC Bus 2
- Backs up TR1 if TR3 disconnect relay is closed.
- Blocking diode prevents it from backing up TR3 to supply Battery bus.
TR3 :
- Normal power source is Main bus No.2 & primary power for battery bus & back-up for DC No.1
and/or DC No.2 if TR3 disconnect relay is closed.
¡ B737 NG : TR 1 receives AC power from transfer bus 1. TR 2 receives AC power from transfer bus
2. TR 3 normally receives AC power from transfer bus 2 and has a backup from transfer bus 1. Any
two TRs are capable of supplying the total connected load. Under normal conditions, DC bus 1, DC
bus 2 and the DC standby bus are connected via the cross bus tie relay. In this condition, TR 1 and
TR 2 are each powering DC bus 1, DC bus 2 and the DC standby bus. TR 3 powers the battery bus
and serves as a backup power source for TR 1 and TR 2.
TR1 :
- Normal power source is Transfer bus No.1 & supplies DC Bus 1
- Backs up TR2
- Blocking diode prevents it from backing up TR3 to supply Battery bus.
TR2 :
- Normal power source is Transfer bus No.2 & supplies DC Bus 2
- Backs up TR1 if cross bus tie relay is closed.
- Blocking diode prevents it from backing up TR3 to supply Battery bus.
TR3 :
- Normal power source is Transfer bus No.2 & primary power for battery bus
- Back-up power source for TR3 is transfer bus 1
- Back-up for DC bus 2 & if cross bus tie relay is closed, for DC bus 1 & DC Standby bus
- If TR3 fails the battery charger or the battery powers the battery bus.
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¡ B737 Classics : The TR 3 disconnect relay automatically opens, isolating DC bus 1 from DC bus 2
under the following conditions :
- At glide slope capture during a flight director or autopilot [LS approach. This isolates the DC
busses during approach to prevent a single failure from affecting both navigation receivers and
flight control computers.
- BUS TRANS switch positioned to OFF.
¡ B737 NG : The cross bus tie relay automatically opens, isolating DC bus 1 from DC bus 2 under the
following conditions :
- At glide slope capture during a flight director or autopilot ILS approach. This isolates the DC
busses during approach to prevent a single failure from affecting both navigation receivers and
flight control computers.
- Bus transfer switch positioned to OFF.
In flight, an amber TR UNIT light illuminates if TR 1, or TR 2 and TR 3 has failed.
On ground, any TR fault causes the light to illuminate.
Battery Power
¡ B737 Classics : A 28 volt battery is located in the electronics compartment. The battery can supply
part of the DC system. Battery charging is automatically controlled. A fully charged battery has
sufficient capacity to provide power for a minimum of 30 minutes.
¡ B737 NG : Two 28 volt batteries, main and auxiliary, are located in the electronics compartment.
The batteries can supply part of the DC system. The auxiliary battery operates in parallel with the
main battery when the battery is powering the standby system. At all other times, the auxiliary
battery is isolated from the power distribution system. Battery charging is automatically controlled.
Two fully charged batteries have sufficient capacity to provide standby power for a minimum of 60
minutes.
¡ DC busses that are powered from the battery following a loss of both generators are :
- battery bus
- DC standby bus
- hot battery bus
- switched hot battery bus.
¡ The switched hot battery bus is powered whenever the Battery Switch is ON. The hot battery bus is
always connected to the battery, there is no switch in this circuit. The battery must be above
minimum voltage to operate units supplied by this bus. (On B737 NG, an amber BAT DISCHARGE
light comes on when excessive battery discharge is detected).
B737 Classics - Battery Charger Transformer/Rectifier
¡ The purpose of the battery charger is to restore and maintain the battery at full electrical power. The
normal source of power for the battery charger is the AC ground service with provisions for automatic
switching to the No. 2 Main Bus.
¡ The battery charger will maintain the charge in the battery at all times with AC power on the aircraft.
With the ground cart plugged in, the ground service bus powered, the Battery Switch ON, and the DC
Meters Selector in the BAT position, the voltage will read 28 volts, indicating the hot battery bus is
being powered from the battery charger. The battery charger converts 11 5V AC to 28V DC.
¡ Following completion of the primary charge cycle, the battery charger reverts to a constant voltage
TR mode. In the TR mode it powers loads connected to the hot battery bus and the switched hot
battery bus. The battery charger TR also powers the battery bus if TR 3 fails.
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B737 NG - Battery Charger Transformer/Rectifier
¡ The purpose of the battery chargers is to restore and maintain the batteries at full electrical power.
The main battery charger is powered through AC ground service bus 2.
¡ The auxiliary battery charger is powered through AC ground service bus 1. Following completion of
the primary charge cycle, the main battery charger reverts to a constant voltage TR mode. In the TR
mode, it powers loads connected to the hot battery bus and the switched hot battery bus.
¡ The main battery charger TR also powers the battery bus if TR 3 fails. With loss of AC transfer bus 1
or the source power to DC bus 1, the AC and DC standby busses are powered by the main and
auxiliary battery/battery chargers.
5. STANDBY POWER SYSTEM
Normal Operation
¡ The standby system provides 11 5V AC and 28 V DC power to essential systems in the event of loss
of all engine or APU-driven AC power. The standby power system consists of :
- the battery
- static inverter
- AC standby bus
- DC standby bus
- battery bus
- hot battery bus
- switched hot battery bus.
¡ During normal operation the guarded standby power switch is in AUTO and the battery switch is ON.
This configuration provides alternate power sources in case of partial power loss as well as complete
transfer to battery power if all normal power is lost.
¡ Under normal conditions the AC standby bus is powered from AC transfer bus 1.
¡ The DC standby bus is powered from DC bus No.1; (On B737 NG, The DC standby bus is powered
by TR 1, TR 2 and TR 3) the battery bus is powered by TR 3
¡ The hot battery bus and switched hot battery bus are powered by the battery/battery charger.
B737 Classics - Alternate Operation
¡ Alternate or standby power source for the standby busses is the battery. With a complete generator
power failure, the 11 5 volt AC standby bus is powered by the battery through the static inverter. The
28-volt DC standby bus is powered by the battery bus. A fully charged battery has sufficient capacity
to provide power to essential flight instruments, communication and navigation equipment for a
minimum of 30 minutes.
¡ In flight, automatic switching is provided from the normal power sources to the alternate power
source when the Standby Power Switch is in the AUTO position. If either the No. 1 DC bus or the
No. 1 transfer bus loses power, both standby busses automatically switch to the battery and battery
bus. The air/ground safety sensor prevents the battery and the battery bus from powering the
standby busses on ground. This prevents discharging the battery. The function of the air/ground
safety sensor may be bypassed by placing the Standby Power Switch to the BAT position.
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¡ When the Standby Power Switch is OFF, the STANDBY PWR OFF Light will be ON indicating the
AC standby bus is de-energized.
B737 NG - Alternate Operation
¡ The alternate power sources for standby power are the main battery and auxiliary battery. With
standby power switch in the AUTO position, the loss of all engine or APU electrical power causes the
batteries to power the standby loads, both in flight and on ground.
¡ The AC standby bus is powered from the batteries via the static inverter. The DC standby bus,
battery bus, hot battery bus, and switched hot battery bus are powered directly from the batteries.
¡ The standby power switch provides for automatic switching to alternate power or allows manual
control of some parts of the standby power system.
¡ In the AUTO position, automatic switching from normal to alternate power source occurs if either AC
transfer bus 1 or the power source for DC bus 1 loses power. Positioning the switch to BAT
overrides automatic switching and places the AC standby bus, DC standby bus and battery bus on
battery power. The battery switch may be ON or OFF. If the battery switch is OFF, the switched hot
battery bus is not powered.
¡ Positioning the standby power switch to OFF de-energizes both the AC standby bus and the DC
standby bus and illuminates the STANDBY PWR OFF light.
Static Inverter
¡ The static inverter converts 28 volt DC power from the battery to single phase, 115 Volts / 400 Hertz
AC power to supply the AC standby bus during the loss of normal electrical power. The power
supply to the inverter is controlled by the Standby Power Switch on the overhead panel.
5. PANELS
AC & DC METERING PANEL
¡ DC VOLTMETER : indicates voltage of source selected by the DC Meters Selector.
¡ DC AMMETER : indicates current of BAT, TR1, TR2 or TR3 as selected by the DC Meters Selector.
¡ FREQUENCY METER : indicates frequency of source selected by the AC Meters Selector.
¡ AC VOLTMETER : all series : indicates voltage of source selector by AC meters selector.
B737 classics :
- 130V SCALE : indicates voltage of source selected.
- 30V SCALE : indicates residual voltage of generator selected when Residual
Volts switch is pressed.
¡ AC METERS SELECTOR : selects the AC source for the AC Voltmeter & frequency Meter
indications. (TEST : used by maintenance to monitor the selections of the power system test module
In the P6 panel). (selecting STBY PWR shows the AC Standby bus readings.
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¡ GALLEY POWER SWITCH :
- ON: Electrical power is provided to the galleys. Galley power is available only when both
generator busses are powered. (both transfer busses on 737 NG).
- OFF : No electrical power is supplied to the galley.
Note : Galley power will automatically goes off if APU gen is > 167 amps (on ground).
¡ RESIDUAL VOLTS SWITCH (B737 Classics only) :
This switch is used to confirm a CSD disconnect (if disconnected voltmeter should show 0 - 6 V)
PRESS =
- 30V scale of AC Voltmeter indicates residual voltage of generator selected.
- Associated Generator Switch must be OFF. (With associated Generator Switch ON, AC Voltmeter
drives off scale and residual voltage cannot be read : AC Voltmeter will drive off scale)
Note : Switch has been removed on B737 NG : to confirm a CSD disconnect, use AC Meters to
Check AC VOLTS & CPS FREQ = 0.
¡ BATTERY SWITCH
- OFF : No power to the battery bus (with Electrical Standby Power Switch in OFF or AUTO).
- ON (B737 Classics) :
Connects switched hot battery bus to the battery.
Energizes relays to provide automatic switching of standby electrical system to battery power with
loss of normal power.
a) With main bus No. 2 energized, TR 3 supplies power to the battery bus.
b) If main bus No. 2 is not powered, the hot battery bus powers the battery bus.
- ON (B737 NG) :
Connects switched hot battery bus to the battery.
Energizes relays to provide automatic switching of standby electrical system to battery power with
loss of normal power.
a) With AC Transfer No. 2 energized, TR 3 supplies power to the battery bus.
b) If AC Transfer No. 2 is not powered, the battery powers the battery bus
¡ DC METERS SELECTOR : selects the DC source for the DC Voltmeter & DC ammeter indications
(TEST : used by maintenance to connect to BITE).
- STBY PWR = volts only
- BAT bus = volts only
no amp indication since these indications are not sources of power.
- BAT = volts & amps of battery or battery charger when charging.
- TR1 & TR2 = volts from DC bus while TR3 = volts & amps directly from TR2
Note : Do not leave the selector in BAT position overnight or you may drain the battery !!
GENERATOR DRIVE & STANDBY POWER PANEL
¡ B737 Classics - STANDBY POWER SWITCH
1) AUTO (guarded position)
Condition : in flight or on ground & AC busses powered.
- The AC standby bus is powered by AC transfer bus No. 1. The DC standby bus is powered by DC
bus No.1.
Condition: in flight, loss of all AC power.
- The AC standby bus is powered by the battery through the static inverter. The DC standby bus is
powered by the battery bus directly.
- A fully charged battery will provide a minimum of 30 minutes of standby power.
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Condition : on ground, loss of all AC power.
- No automatic transfer of power to the standby busses.
2) OFF (center position):
- STANDBY POWER OFF light illuminates.
- Standby busses and static inverter are not powered.
3) BAT (unguarded position):
- The AC standby bus is powered by the battery through the static inverter.
- The DC standby bus is powered directly by the battery bus.
The battery bus is powered by the hot battery bus, regardless of Battery Switch position.
¡ B737 NG - STANDBY POWER SWITCH
1) AUTO (guarded position)
Condition : AC busses powered.
- The AC standby bus is powered by AC transfer bus No. 1. The DC standby bus is powered by TR
1 and TR 2 (TR 3 is a backup source).
Condition: Loss of all AC power.
- The AC standby bus is powered by the battery through the static inverter. The DC standby bus is
powered by the battery bus directly.
- A fully charged battery will provide a minimum of 60 minutes of standby power.
2) OFF (center position):
- STANDBY POWER OFF light illuminates.
- Standby busses and static inverter are not powered.
3) BAT (unguarded position):
- The AC standby bus is powered by the battery through the static inverter.
- The DC standby bus is powered directly by the battery.
- The battery bus is powered by the battery, regardless of Battery Switch position.
¡ GENERATOR DRIVE DISCONNECT SWITCH (guarded)
- Disconnects Generator Drive.
- Generator Drive cannot be re-engaged in flight.
¡ GENERATOR DRIVE TEMPERATURE SWITCH (guarded / B737 Classics only)
- Selects RISE or IN temperature to be displayed on the Generator Drive Oil Temp Indicator.
- RISE scale (outer) = displays the temp rise within the CSD (differential between outlet & inlet)
Higher than normal temp rise indicates excessive generator load or poor condition of the
generator drive & lack of adequate cooling will generally cause the temp rise to decrease…
max CSD temp = 157°C / max CSD rise = 20°C.
- IN scale (inner) = displays the temp of oil entering into the CSD.
GROUND POWER & GENERATOR AMMETERS PANEL
¡ GENERATOR AC AMMETER
- Indicates engine generator load.
¡ GROUND POWER SWITCH :
OFF :
- Disconnects ground power from both generator busses (both transfer busses on 737 NG).
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ON :
If momentarily moved to the ON position & ground power is available :
- Removes previously connected power source from both generator busses (both transfer busses on
737 NG).
- Closes external power contactors & connects ground power to both generators busses (both
transfer busses on 737 NG) if power quality is correct.
- Switches the ground service bus to the No.1 generator bus. (to the (both transfer busses on
737 NG).
- Deactivates the Ground Service switch.
¡ B737 CLASSICS – GROUND SERVICE SWITCH :
(solenoid held ON, spring-loaded to OFF)
- Provides manual control of the ground service bus. Enables servicing the aircraft using external
power without powering the generator
- The ground service busses power the battery charger(s), Equipment cooling fans (NORMAL) &
miscellaneous service lights & outlets.
ON :
- Connects the external power to the ground service bus.
- Does not remain ON if the ground power switch is ON
OFF :
- Disconnects the ground service bus from the external power.
- Trips OFF when the ground power switch is positioned to ON
To power down an aircraft but leave the Ground Service Bus powered for the cleaning team :
1) hold GROUND SERVICE Switch ON while someone turns the BAT switch OFF (no interruption
in cabin lighting)
OR
2) turn BAT switch OFF then turn the GROUND SERVICE switch ON
¡ B737 NG – GROUND SERVICE SWITCH :
(solenoid held ON, spring-loaded to OFF)
- Provides manual control of the ground service bus. Enables servicing the aircraft using external
power without powering the AC transfer busses.
- The ground service busses power the battery charger(s), Equipment cooling fans (NORMAL) &
miscellaneous service lights & outlets.
PUSH :
- Connects the external power to the ground service busses.
PUSH AGAIN :
- Disconnects the ground service bus from the external power.
To power down an aircraft but leave the Ground Service Bus powered for the cleaning team :
- Place GRD POWER Switch OFF, press GROUND SERVICE switch ON, then BAT switch OFF
- If ground power is plugged in, the battery charger is not powered until either the Ground Service
switch or the External power is switched to ON.
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BUS SWITCHING PANEL
¡ B737 CLASSICS - BUS TRANSFER SWITCH :
AUTO (guarded position) :
- Allows automatic transfer of transfer bus upon failure of generator bus, allows TR2 & TR3 to supply
No. 1 DC bus.
- A TR failure can be detected by a zero reading on the DC ammeter of the selected TR.
OFF :
- Isolates transfer busses by preventing operation of the bus transfer relays, & opens TR3
disconnect relay.
- Prevents the battery charger from switching to its alternate source of power (main bus 2).
¡ B737 NG - BUS TRANSFER SWITCH :
AUTO (guarded position) :
- BTBs operate automatically to maintain power to AC transfer busses from any operating generator
or external power.
- DC cross tie relay automatically provides normal or isolated operation as required.
OFF :
- Isolates transfer bus 1 from transfer bus 2 if one IDG is supplying power to both AC transfer
busses,
- DC cross tie relay opens to isolate DC bus 1 from DC bus 2.
¡ B737 CLASSICS - GENERATOR SWITCH : (Three position switch, spring-loaded to center position)
- OFF : disconnects generator from the generator bus.
- ON : connects the generator output to the generator bus when power quality is correct.
¡ B737 NG - GENERATOR SWITCH : (Three position switch, spring-loaded to center position)
- OFF : disconnects IDG from the transfer bus.
- ON : connects the IDG output to the generator bus when power quality is correct.
¡ B737 CLASSICS - APU GENERATOR SWITCH : (3 position switch, spring-loaded to center position)
- OFF : disconnects the APU generator from the generator bus
- ON : connects the APU generator output to the generator bus when power quality is correct.
¡ B737 NG - APU GENERATOR SWITCH (3 position switch, spring-loaded to center position)
- OFF > APU generator powering both AC transfer busses.
1) Moving a single APU GEN switch to OFF illuminates related SOURCE OFF light.
APU continues to power AC transfer busses.
2) Subsequently moving other APU GEN switch to OFF disconnects APU generator from tie
bus and removes APU power from AC transfer busses.
> APU generator powering one AC transfer bus; IDG powering one AC transfer bus.
1) Moving related APU GEN switch to OFF disconnects APU generator form tie bus & AC
transfer bus. IDG powers AC transfer busses.
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- ON > Neither AC transfer bus powered by I DG; moving a single APU GEN switch to ON :
1) Connects both AC transfer busses to the APU generator.
2) Disconnects external power, if connected.
3) Opposite SOURCE OFF light illuminated until the other APU GEN switch is moved to ON.
> Both AC transfer busses powered by IDGs; Moving an APU GEN switch ON :
1) Powers the related AC transfer bus from the APU generator.
2) Other AC transfer bus continues to receive power from the IDG.
ALL GENERATORS INOP (B737 Classics)
- Battery is the only source of power (without optional aux battery)
AIRPLANE GENERAL
Standby compass Iight
White dome Iight (BATTERY BUS)
Emergency instrument flood Lights
Entry lights (HOT BATTERY BUS)
Position lights (BATTERY BUS)
Flight crew oxygen (BATTERY BUS)
Passenger oxygen (BATTERY BUS)
AIR SYSTEMS
A/C pack valves (BATTERY BUS)
Altitude warn horn (BATTERY BUS)
BLEED TRIP OFF Lights (BATTERY BUS)
Manual Pressurization control (BATTERY BUS)
PACK TRIP OFF Lights (BATTERY BUS)
COMMUNICATIONS
VHF. #1 (28 V DC STANDBY BUS)
Flight interphone captain & F/O (BATTERY BUS)
Manual pressurization control (BATTERY BUS)
Passenger Address (BATTERY BUS)
ELECTRICAL
APU gen. Control (SWITCHED HOT BATTERY BUS)
Engine gen. Control (SWITCHED HOT BATTERY BUS)
Hot Battery Bus indication (HOT BATTERY BUS)
Inverter Voltage Indication (115 V AC STANDBY BUS)
STBY PWR OFF Lights (HOT BATTERY BUS)
ENGINES & APU
APU operation (BATTERY BUS)
EGT (28 V DC STANDBY BUS)
LOW OIL PRESS Lights (BATTERY BUS)
N1 (BATTERY BUS)
Right igniters (115 V AC STANDBY BUS)
Starter valves (BATTERY BUS)
Thrust reversers (BATTERY BUS)
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FLIGHT INSTRUMENTS
Capt's EFIS (115 V AC STANDBY BUS)
Clocks (HOT BATTERY BUS)
Standby airspeed light (HOT BATTERY BUS)
Standby altimeter light (HOT BATTERY BUS)
Standby horizon (BATTERY BUS)
FIRE PROTECTION
APU fire bottles (HOT BATTERY BUS)
APU detection sys (HOT BATTERY BUS)
Engine detection sys (HOT BATTERY BUS)
Engine fire bottles (HOT BATTERY BUS)
FUEL
Crossfeed SOV (BATTERY BUS)
Engine fuel SOV (HOT BATTERY BUS)
Fuel quantity indicator (115 V AC STANDBY BUS)
FUEL VALVE CLSD Lights (SWITCHED HOT BATTERY BUS)
HYDRAULIC POWER
Engine hydraulics SOV (BATTERY BUS)
Standby rudder SOV (BATTERY BUS)
LANDING GEAR
Inboard antiskid system (BATTERY BUS)
ANTISKID INOP Iight (BATTERY BUS)
Landing gear air / ground relay (BATTERY BUS)
Parking brake (BATTERY BUS)
VOR/ILS Captain (28 V DC STANDBY BUS)
NAVIGATION
ADF #1 (115 V AC STANDBY BUS)
Capt. RDMI (115 V AC STANDBY BUS)
F/O RDMI (115 V AC STANDBY BUS)
Left IRS (115 V AC STANDBY BUS)
Magnetic compass (BATTERY BUS)
VHF #1 (28 V DC STANDBY BUS)
WARNINGS
Aural warnings (BATTERY BUS)
Emergency Evacuation signal – as installed (HOT BATTERY BUS)
Flight Recorder (BATTERY BUS)
Master caution system (BATTERY BUS)
Stall warning system (28 V DC STANDBY BUS)
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BOEING 737 SYSTEMS REVIEW
ALL GENERATORS INOP (B737 NG)
- Battery is the only source of power (without optional aux battery)
AIRPLANE GENERAL
Emergency instruments flood Lights
Entry lights
Flight crew oxygen
Passenger oxygen
Position lights
Standby compass Iight
White dome Lights
AIR SYSTEMS
A/C pack valves
Altitude warn horn
BLEED TRIP OFF Its
Manual press cont
PACK TRIP OFF Its
COMMUNICATIONS
Flight interphone
Passenger address
VHF #1
ELECTRICAL
STBY PWR OFF light
ENGINES & APU
Fuel quantity
Hydraulics pressure & quantity
N1, N2, FF, EGT
Oil pressure, temperature & quantity
Right ignitors
Starter valves
Thrust reversers
Upper display unit
FIRE PROTECTION
APU detection system
APU fire bottles
Eng detection system
Engine fire bottles
FLIGHT INSTRUMENTS
Airspeed, attitude
altimeter, IVSI
Captain outboard display (Compact mode)
Clocks
Left EFIS control panel
Standby airspeed
Standby altimeter
Standby attitude indicator
Standby RDMI
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BOEING 737 SYSTEMS REVIEW
FUEL
Crossfeed SOV
Engine fuel SOV
FUEL VALVE CLSD Iight
Spar fuel SOV
HYDRAULIC POWER
Engine hydraulics SOV
Stby rudder SOV
LANDING GEAR
ANTISKID INOP light
Inboard antiskid system
Parking brake
NAVIGATION
ADF #1 - as installed.
Full HIS
Left GPS
Left IRS
Magnetic compass
Marker beacon
RDMI
VHF #1
WARNINGS
Aural warnings
Master caution system
Stall warning sys
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BOEING 737 SYSTEMS REVIEW
7. FAULTS & INDICATIONS
VALID FOR
LIGHT
APU GEN
OFF BUS
BAT
DISCHARGE
BUS
OFF
DRIVE
ELEC
GEN OFF
BUS
GRD POWER
AVAILABLE
SOURCE
OFF
STANDBY
PWR OFF
TRANSFER
BUS OFF
TR UNIT
INDICATION
300
400
500
600
700
800
900
X
X
X
X
X
X
X
X
X
DRIVE (amber)
- Generator Drive oil pressure is low due to IDG
failure, engine shutdown or IDG disconnect.
X
X
X
ELEC (amber)
- A fault exists in DC power system or standby
power system (operates only with aircraft on
ground)
GEN OFF BUS (amber)
- Generator is not supplying the generator bus.
X
X
X
X
X
X
X
X
X
X
X
X
TRANSFER BUS OFF (amber)
- Transfer bus is not powered.
X
X
X
TR UNIT (amber)
- On ground : any TR has failed.
- In flight : TR1 has failed or TR2 & TR3 failed.
X
X
X
APU GEN OFF BUS (blue)
- APU is running & is not powering a bus.
BAT DISCHARGE (amber)
- Illuminated with BAT switch ON, excessive battery
discharge detected
BUS OFF (amber)
- Generator bus is inactive
- The associated Main bus is inactive.
GROUND POWER AVAILABLE (blue)
- Ground power is plugged in.
- Irrespective of power quality (B737 Classics)
- Power is within limits (B737 NG)
SOURCE OFF (amber)
- No source has been manually selected to power
the related transfer bus, or the manually selected
source has been disconnected.
- If a source has been selected to power the opposite
transfer bus, both transfer busses are powered.
STANDBY PWR OFF (amber)
- AC standby bus and/or DC standby bus and/or
Battery bus is inactive.
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X
X
X
X
X
X
X
X
X
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