Electrical

Question 1

TRU light logic (when it will illuminate)

• Ground
• Flight

Answer 1

Ground:
- any failure means a TR Unit light.

Flight:
- TR 1 Fails or
- TR 2 AND 3 Fails

Question 2

AC BUS TRANS(fer) switch

What does each option do?

Answer 2

AUTO:
- Allows BTB’s to configure to allow AC power to AC Transfer Buses via IDG’s, APU, or External Power. DC Cross Tie also will provide isolation as required.

*Usually an IDG powers it’s onside Transfer Bus.

OFF:
- Isolates AC Transfer Bus 1 from 2 (in the case one IDG is powering both)

Question 3

What does this light indicate?

Dim?
Bright?

Answer 3

Bright?
- APU Generator is operating above 95% RPM and NOT powering a bus.

Dim?
- APU GEN is powering the respective bus

Question 4

What does moving the 3 position APU switches to ON do when the AC transfer busses are unpowered (Dark Airplane, other than BAT ON)?

What would the light brightness be before and after?

Answer 4

• Spring loaded to neutral
• ON:
  Connects APU GEN to BOTH buses
  Disconnects GRD PWR
  Opposite Source of light illuminates until other APU GEN switch is selected ON, then dark.

LIGHTS
- Bright when OFF bus, Dark when ON bus

Remember 737 electrical rule! Only 1 source, and the previous source is kicked off.

Question 5

If the TRANSFER buses are unpowered can you select one bus to power with the APU and one with External (GRD PWR)?

Answer 5

No, an APU switch selected to ON actually connects both Transfer buses to the APU GEN when on external power

Question 6

What do the 3 position GEN 1 and GEN 2 switches do on the AC Transfer panel?

Answer 6

ON
- Powers respective transfer bus by IDG as long as the power is deemed good. Takes previous source off the bus.

OFF
- Terminates power to respective bus

Question 7

What does the GEN OFF BUS light indicate when BRIGHT or Dark

Answer 7

Bright: IDG not supplying power
Dark: IDG IS providing power to bus

Question 8

What does the TRANSFER BUS OFF light indicate?

Answer 8

The transfer bus is not powered.

It will cause a ELEC annunciator on the recall panel and MASTER CAUTION

Question 9

What does the SOURCE OFF light indicate?

Answer 9

• No manually selected source to the respective transfer bus (could be disconnected manually)

It will cause a ELEC annunciator on the recall panel and MASTER CAUTION

Question 10

What does the STANDBY POWER switch ‘AUTO’ position do?

Answer 10

AUTO: In the event of all engine or APU electrical failures the Batteries power the Standby Buses. This also occurs with the loss of power from AC Transfer Bus 1 or DC Bus 1.

• Standby Bus - Powered by Batteries via an inverter
• DC Standby, Battery Bus, Hot Battery Bus, & Switched Hot Battery Bus are powered by batteries

Question 11

What does the STANDBY POWER switch ‘OFF’ position do?

Answer 11

OFF: Removes power from both AC and DC Standby buses, which illuminates the STANDBY POWER OFF Light and associated annunciator.

Question 12

What does the STANDBY POWER switch ‘BAT’ position do?

Answer 12

Overrides automatic switching and connects the Stanby busses as well as the battery bus to batteries.

Question 13

Will you get a cargo fire indication with the aircraft at Standby Power (Turning off everything but the batteries).

Answer 13

No. You will not have any location information.

Question 14

On the Cruise check for the batteries, what are you looking for?

Answer 14

No Negatives

Question 15

Which side is DC and what side is AC on the electrical system monitor?

Answer 15

Noted on the bottom of the panel. DC left AC right.

Question 16

What are the sources of DC power?

(SM)

Answer 16

3 TRU’s
Main battery
Main battery charger

Question 17

What happens when an IDG is decoupled?

(SM)

Answer 17

If a generator drive DISCONNECT switch is selected to DISCONNECT with the engine start lever in idle the IDG shaft is retracted from the gearbox, decoupling the IDS from engine rotation.

This is irreversible, until on the ground by mtx personnel.

Question 18

What causes the illumination of a DRIVE light?

Are there associated lights?

(SM)

Answer 18

• IDG has low oil pressure caused by
  
  • IDG disconnected manually
  • IDG mechanical failure
  • IDG auto decouple due to high oil temperature.
  • engine shutdown

MASTER CAUTION with ELEC

Question 19

When is the GROUND POWER AVAILABLE light illuminated?

(SM)

Answer 19

External power is attached and power is within tolerance and available for connection to the AC buses. It remains illuminated regardless of whether external power is powering the AC system.

Question 20

What are the basic principles of operation of the 737 electrical system?

(SM)

Answer 20

No paralleling of AC sources
A source of power bring connected to a transfer bus automatically disconnects an existing source.

Question 21

How is the AC standby bus normally powered?

(SM)

Answer 21

via Transfer Bus 1

Question 22

What is indicated but he GEN OFF BUS light?

(SM)

Answer 22

The bus is unpowered by the IDG

Question 23

Is it possible to power one AC transfer bus with external power and the other with APU power?

Answer 23

No. Refer to 737 AC principal number 1.

Question 24

What is the load shedding logic when only one source of AC power is available?

On Ground?

(SM)

Answer 24

• Main and Galley buses are load shed depending on the remaining sources of power.

APU generator on the ground only?
-APU attempts to carry full electrical load. If unable, the APU incrementally sheds Main and Galley power until within load limits.

 - Manual restoration of bus power can be attempted by cycling the CAB/UTIL power switch OFF then ON.

Question 25

What is the load shedding logic when only one source of AC power is available? In Flight? (SM)

Answer 25

**Single IDG, NO APU:** Incremental load shed based on load sensing. - Transfer Bus 2 Main & Galley buses - Transfer Bus 1 Main & Galley buses - IFE Buses If more power sources are added (2nd IDG or APU GEN), automatic load restoration occurs. **APU Only** (*Think MEL'd IDG and the other fails or the working IDG's onside engine fails*) - All Main and Galley buses shed - If load still exceeds limits, IFE is shed. (Similar but with a single IDG its incremental on Transfer Buses)

Question 26

What is the automatic generator on-line feature? (SM)

Answer 26

If the aircraft takes off with the APU powering both buses and the APU is shut down or fails, the IDGs are automatically connected to their onside transfer bus. *This only occurs once per flight cycle*

Question 27

How long can two fully charged batteries provide standby power? (SM)

Answer 27

60 minutes.

Question 28

When does the DC cross tie relay open? (SM)

Answer 28

With the BUS TRANS switch in AUTO, the cross tie is normally closed, except when the G/S is active during a F/D or coupled ILS approach.

Question 29

What does illumination of the BATTERY DISCHARGE light on the AC and DC Metering panel indicate? (SM)

Answer 29

Excessive discharge of the main battery and or auxiliary battery.

Question 30

How is the DC Standby Bus normally powered? (SM)

Answer 30

TR1, TR 2, or TR3

Question 31

The BAT switch controls which buses? (SM)

Answer 31

Closing the switch (ON) powers: - Switched Hot Battery Bus - Battery Bus. (The HOT Battery bus is always powered off the main battery even when the switch is off, for fueling and fire extinguishing.)

Question 32

What is the purpose of the static inverter? (SM)

Answer 32

Power the AC standby bus via the batteries (which are DC). With STANDBY POWER switch in AUTO the Standby Power Control Unit relays the standby busses to their battery power source in the event transfer bus 1 or DC bus 1 power is lost.

Question 33

What does the illumination of the TR UNIT light indicate? (SM)

Answer 33

Ground: Any TR unit fails Flight: TR1, OR TR2 AND TR3 fails

Question 34

With the loss of all AC power, if automatic switching does not occur, how can the standby power buses be powered? (SM)

Answer 34

Manually switching the STANDBY POWER switch to BAT. This overrides the automatic relay and places the AC Standby, DC Standby, and Battery buses on battery power regardless of BAT switch position.

Question 35

What causes the illumination of the STANDY POWER OFF light? (SM)

Answer 35

One or more of the following buses are unpowered: -AC Standby -DC Standby -Battery Bus

Question 36

What does illumination of the ELEC light indicate when it is armed? Follow-up: Will the ELEC light illuminate in flight? (SM)

Answer 36

- Fault in the Standby or DC power system. - No it is inhibited in flight.

Question 37

If all generators are inoperative, and the main battery bus and the auxiliary battery are the only sources of power, which displays are available? (SM)

Answer 37

CA PFD, CA NAV, Captains clock.