What is the difference between the amber “TR UNIT” light indicated in flight, opposed to on the ground?
FCOM 6.10.3
The amber ‘TR UNIT’ (Overhead) and ELEC master caution light will only illuminate INFLIGHT: if TR1 fails OR, if BOTH TR3 AND TR2 both fail.
On the ground: ‘TR UNIT’ light will illuminate if ANY TR fails.
Note: ‘TR UNIT’ light will always illuminate if TR1 fails, on the ground or in the air (but will only illuminate INFLIGHT, if TR2 and TR3 fail together.)
Can the amber ‘ELEC’ (Overhead panel - AC/DC metering panel) light illuminate in flight if a fault is encountered?
FCOM 6.10.3+6.10.4
NO.
Incase of an electric overload (Engine Generators), what will be the first thing to be shed?
FCOM 6.20.4
The galleys and main bus on transfer bus 2 are shed first.
If an overload is still sensed, the galleys and
main bus on AC TRANSFER BUS 1 are shed; if overload still exists, the IFE
buses are shed.
If the APU is the sole source of electrical power what difference does this make in flight compared to on the ground.
FCOM 6.20.4
In flight, ALL galley and main busses are automatically shed. On the ground, the APU will attempt to supply full electrical load. In other words, partial power is supplied inflight.
If the BATT switch is selected to OFF, can the STBY power supply power?
FCOM 6.10.6 & 6.20.11
Yes, by switching the ‘STANDBY POWER’ switch to BATT.
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.
Note: If the BATTERY switch is OFF, the switched hot battery bus is
not powered.
What is required in order to disconnect IDG’s?
FCOM 6.10.5
Engine start levers must be in ‘IDLE’ detent with electrical power available.
What is the output of the IDG’s?
1. Phases?
2. KVA?
3. Voltage?
4. CPS?
FCOM 6.20.1
3 phases, 90 KVA, 115 Volts AC & 28V DC @ 400 cycle AC power.
What will cause the IDG’s to automatically disconect?
FCOM 6.10.5
IDG high oil temperature (not engine oil).
How is the IDG oil cooled?
FCOM 6.20.5
By a selfÐcontained oil system.
What is difference between the “SOURCE OFF” light and the “TRANSFER BUS OFF” lights?
FCOM 6.10.7 & 6.20.3
SOURCE’ light indicates that no source has been manually selected to power the related
transfer bus, or the manually selected source has been disconnected. The SOURCE light refers to the power (generator) supplied to the bus.
The ‘TRANSFER BUS OFF’ light refers to the transfer bus’ power supply. This indicates the Transfer bus is not receiving power.
Note: The “SOURCE OFF” light can also indicate that the transfer bus is being powered thought the BTB by another generator until an AC power is manually selected. In this particular scenario, the TRANSFER BUSS OFF light will not illuminate.
What is the purpose of a Bus tie breaker (BTB)
FCOM 6.20.3
The BTB allows one generator to supply power to both transfer busses.
Can the entire electrical system be powered by the APU generator? (One generator)
FCOM 6.20.1 &
Yes, the APU generator can meet electrical power requirements for all ground conditions and most flight conditions.
How does the APU power the transfer busses?
FCOM 6.20.2
Through bus tie breakers (BTB’s)
How many batteries does Qantas 737-800 have, and how long can a fully charged battery provide standby power?
FCOM 6.20.8
ONE - 30 minutes, however it also has another seperate battery dedicated to the ISFD that provides power to the Standby instrument (ISFD). This dedicated battery can provide power to the ISFD for 150 minutes (Two hours and 30 minutes).
Once the Main battery runs flat, the ISFD will still have power for another 2 hours. The ISFD receives power from its dedicated battery system AFTER the loss of power to the battery bus.
Which busses makes up one AC power system?
FCOM 6.20.2-3
Each AC power system consists of a transfer bus, a main bus, two galley busses, and a ground service bus. AC TRANSFER BUS 1 also supplies power to the AC standby bus (1 x Transfer bus, 1x Main bus, 2x Galley bus, 1x Ground service bus and 1x AC STBY bus).
11 AC busses in total: 2x Transfer bus, 2x Main bus, 4x Galley bus, 2x Ground service bus and 1x AC STBY bus.
Note: Only the left AC power system has a standby AC bus. The right side does not have one.
What powers the AC standby Bus?
FCOM 6.20.2-3
Transfer bus 1 from either IDG or the Inverter from the Battery.
Is it possible for number 2 IDG to power transfer bus 1, while powering transfer bus 2, and vice versa?
Through the bus tie breakers (BTB’s)
Is it possible to power one transfer bus with external power and the other transfer bus with the APU’s generator?
FCOM 6.20.3
Yes, either generator or the APU can supply power to both transfer busses, however only when the system automatically detect s loss of source. This cannot be selected manually. The last selected power source will power (either GND or APU) both busses.
Note: 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.
Is it possible to power one transfer bus with a respective IDG and the other transfer bus with the APU’s generator or external power?
FCOM 6.20.3
Yes, 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). The BUS TRANSFER switch must be in the AUTO position.
Note: 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.
If a transfer bus loses power from its respective IDG, do the BTB’s close automatically?
FCOM 6.20.3
Yes, if the ‘BUS TRANSFER’ switch is in AUTO position, BTBs automatically close to allow the
other engine generator to supply both transfer busses through the tie bus and BTBs.
How many transfer busses can the APU power in flight?
FCOM 6.20.3
Two/Both
What happens if you take off with the APU powering both transfer busses and the APU shuts down/fails?
FCOM 6.20.3
The IDG’s automatically connect o their respective transfer busses through the ‘Auto generator-online’ feature.
Can one IDG provide power to both transfer busses at the same time in flight?
FCOM 6.20.3
Yes, however 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.
What causes the “DRIVE” amber caution light to illuminate? (1 item)
FCOM 6.20.5
Low oil pressure, which is indicative of an IDG failure, engine failure or IDG disconnect.
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Chapter 3 Anti ice, Rain42
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Chapter 7 Engines & APU67
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Chapter 10 Flight instruments & display57
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Chapter 14 Landing gear67
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Flight crew training manual (FCTM)1
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B737NG Fuel System16
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B7373NG Flight Management and Navigation32
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Aircraft Parameters (NG)8
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Bulletins (FCOM)7
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# Limitations (FCOM & QRH Memory)23
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Chapter 6 Electrical system V283
