1/18/23 Electrical Flashcards
What are the basic principles of operation of the 737 Electrical System?
There are two basic principles of operation for the 737 Electrical System:
• There is no paralleling of the AC sources of power
• The source of power being connected to a Transfer Bus automatically disconnects an existing source
What happens when an IDG is decoupled?
The IDG drive shaft is mechanically retracted of out of the gear box. A Generator DRIVE light illuminates as well as a MASTER CAUTIONs and a ELEC light
The Generator Drive DISCONNECT switch, when placed to DISCONNECT with the Engine Start Lever in IDLE, provides Battery Bus power to a solenoid-operated pawl that mechanically interfaces with a worm-gear. The worm-gear will retract the splined IDG drive shaft out of the gearbox for complete drive decoupling with engine rotation.
NOTE: A decoupled IDG can only be reset on the ground by Maintenance personnel.
The Generator DRIVE (amber) light, when illuminated, indicates the respective IDG has experienced low oil pressure caused by:
- IDG failure
- engine shutdown
- IDG disconnected by activation of the DISCONNECT switch
- IDG auto-decouple due to high oil temperature (182°C)
What does a generator DRIVE (amber) light indicate?
The Generator DRIVE (amber) light, when illuminated, indicates the respective IDG has experienced low oil pressure caused by:
- IDG Mechanical failure
- Engine shutdown
- IDG disconnected by activation of the DISCONNECT switch
- IDG auto-decouple due to high oil temperature (182°C)
NOTE: An auto-decouple of the IDG occurs when the internal temperature reaches 182°C. To reset this decouple, the IDG must be removed from the aircraft.
The illumination of the DRIVE light will cause the ELEC annunciator and MASTER CAUTION lights to illuminate.
What does the GRD POWER AVAILABLE (blue) light indicate?
it indicates the external power cord is connected to the aircraft and power (checked within tolerances) is available for connection to the aircraft busses.
Is it possible to power AC Transfer Bus 2 with APU power and AC Transfer Bus 1 with External Power?
No,
It is not possible to power one Transfer Bus with external power and the other Transfer Bus with APU power
What does positioning the BUS TRANSFER switch OFF do?
Placing the BUS TRANSFER switch to OFF isolates AC Transfer Bus 1 from AC Transfer Bus 2 if one IDG is supplying power to both Transfer Busses.
What is the load shedding logic for single generator operation?
Galleys and Main Bus 1st
- Engine Gen sheds them on Transfer Bus 2 first then Transfer Bus 1
- APU sheds them all in flight
Then IFE (if needed)
For single generator operation, the system is designed to shed electrical load incrementally based on actual load sensing. The Galleys and Main Bus on Transfer Bus 2 are shed first; if an overload is still sensed, the Galleys and Main Bus on Transfer Bus 1 are shed; if overload still exists, the In-Flight Entertainment (IFE) Busses are shed. When configuration changes to more source capacity (two generator operation), automatic load restoration of the Main Busses, Galley Busses and IFE Busses occurs; manual restoration of Galley and Main Bus power can be attempted by moving the CAB/UTIL Power Switch to OFF, then back ON (on the ground).
In flight, if the APU is the only source of electrical power, all Galley Busses and Main Busses are automatically shed. If electrical load still exceed design limits, both IFE Busses are also automatically shed. On the ground, the APU attempts to carry a full electrical load. If an overload condition is sensed, the APU sheds Galley Busses and Main Busses until the load is within limits. Manual restoration of Galley and Main Bus power can be attempted by moving the CAB/UTIL Power Switch to OFF, then back ON (on the ground).
What does the GEN OFF BUS (blue) light indicate?
Indicates an IDG is not supplying power to the respective Transfer Bus.
Once the IDG Generator Control Breaker closes (connecting IDG power to the Transfer Bus), the respective GEN OFF BUS light will
extinguish.
What is the Automatic Generator Online feature?
The system also incorporates an automatic generator on-line feature in case the aircraft takes off with the APU powering both Transfer Busses. If the APU is either shut down or fails, the engine generators are automatically connected to their related Transfer Busses.
What are the sources of DC power?
- 3 Transformer Rectifier Units powered by the AC Transfer Busses provide DC power.
- Dual nickel-cadmium (NICAD) batteries provide DC power to loads required to be operative when no other source is available
What does an ELEC (amber) light indicate?
Indicates that a fault exists in DC Power System or Standby Power System.
NOTE: The ELEC light is inhibited in flight.
The illumination of the ELEC light causes the ELEC annunciator and the MASTER CAUTION light to illuminate
What would cause the illumination of a TR UNIT (amber) light?
- On the ground, with a failure of any TR
- In flight, if:
• TR1 fails, or
• TR2 and TR3 have failed
The illumination of the TR UNIT light causes the ELEC annunciator and MASTER CAUTION lights to illuminate.
When will the Cross Bus Tie Relay open automatically?
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
Positioning the BAT switch to OFF deenergizes which electrical busses?
When the BAT switch is placed to OFF:
- removes power from Battery Bus and Switched Hot Battery Bus when operating with normal power sources available
- removes power from Battery Bus, Switched Hot Battery Bus, DC Standby Bus, Static Inverter, and AC Standby Bus when battery is only power source
What does the BAT DISCHARGE (amber) light indicate?
excessive battery discharge is detected in the Main Battery and/or Auxiliary Battery.
The illumination of the BAT DISCHARGE light will cause the ELEC annunciator and MASTER CAUTION lights to illuminate.
Under normal conditions, how is the AC Standby Bus powered? DC Standby Bus?
• the AC Standby Bus is powered from AC Transfer Bus 1
• the DC Standby Bus is powered by TR1, TR2, or TR3
What causes the STANDBY PWR OFF (amber) light to illuminate?
Indicates one or more of
the following busses are unpowered:
• AC Standby Bus
• DC Standby Bus
• Battery Bus
The illumination of the STANDBY PWR OFF light will cause the ELEC annunciator and MASTER CAUTION to illuminate.
What does the STANDBY POWER switch do? Which displays are available on Standby Power?
The STANDBY POWER switch provides for automatic or manual control of power to the Standby Buses.
- Captain’s Outboard Display Unit with Primary Flight Display
- Captain’s Inboard Display Unit with Navigation Display
- Upper Display Unit (N1, N2, fuel flow, EGT, fuel quantity, oil pressure, oil
temperature, oil quantity) - clocks
- left EFIS Control Panel
- Standby Instruments
Radio Magnetic Indicator (RMI), Standby Airspeed/Altimeter, Standby Attitude Indicator, Standby Magnetic Compass, Integrated Standby Flight Display (ISFD) - left FMC
- left CDU
