06 - Electrical

Question 1

ELECTRICAL

1. What are the sources of DC power?

Answer 1

3 TRUs
Main battery
main battery charger

Question 2

2. What happens when an IDG is decoupled?

Answer 2

The splined IDG drive shaft is retracted out of the gearbox for complete drive decoupling from engine rotation

Question 3

3. What causes the illumination of the DRIVE light?

Answer 3

IDG experiencing low oil pressure caused by:
- Engine shutdown
- IDG mechanical failure
- IDG disconnected by DISCONNECT switch
- IDG auto-decouple due to high oil temperature

Question 4

4. When is the GRD POWER AVAILABLE light illuminated?

Answer 4

External power cord is connected to the aircraft and the power is checked to be within tolerances and available for connection to the AC buses

Question 5

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

Answer 5

• There is no paralleling of AC power sources
• The source of power being connected to a transfer bus automatically disconnects an existing source

Question 6

6. How is the AC standby bus normally powered?

Answer 6

By transfer bus 1

Question 7

7. What is indicated by a GEN OFF BUS light?

Answer 7

An IDG is not supplying power to the transfer bus

Question 8

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

Answer 8

No. As soon as the GRD PWR switch is selected ON (with GRD POWER AVAILABLE illuminated), the BTB will close supplying external power to both transfer buses

Question 9

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

Answer 9

APU;
- on ground it attempts to carry the full electrical load
- in flight, if the APU is the only source of power, all main and galley buses are automatically shed
Single IDG;
- sheds transfer bus 2 main and galley buses
- sheds transfer bus 1 main and galley buses
- IFE buses

Question 10

10. What is the automatic generator on-line feature?

Answer 10

In the event an aircraft takes off with the APU powering both transfer buses and the APU is subsequently shut down or fails, the IDGs are automatically connected to their related transfer buses. This feature only occurs once per flight cycle.

Question 11

11. How long can two fully-charged batteries provide standby power?

Question 12

12. When does the DC cross bus tie relay open?

Question 13

13. How is the DC standby bus normally powered?

Question 14

14. The BAT switch controls which bus(ses)?

Question 15

15. What does the illumination of the BAT DISCHARGE light on the AC and DC Metering
    panel indicate?

Question 16

16. What is the purpose of the static inverter?

Question 17

17. What does the illumination of the TR UNIT light indicate?

Question 18

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

Question 19

19. What causes the illumination of the STANDBY PWR OFF light?

Question 20

20. What does the illumination of the ELEC light indicate and when is it armed?

Question 21

21. If all generators are inoperative, and the main battery and the auxiliary battery are the only sources of power, which Display Units are available?