Electrical Systems

Question 1

Explain the AC Electrical system?

Answer 1

• Consists of left and right IDG’s (integrated drive units)
• APU Generator
• The entire AC system can be powered by ground power

Question 2

What are IDG’s?

Answer 2

• Intergrated Drive Generator
• Can be disconected from the engine inflight by the GEN DRIVE DISC switch but cannot be reconnected.
• Can be electrically disconected from the busses by selecting the GEN CONT switch off or by selecting external power prior to engine shutdown.
• Each IDG can power the opposite bus via the bus tie switches.

Question 3

In what order do the bus tie brakers keep the busses powered?

Answer 3

• Associated IDG
• APU GEN
• Opposite IDG

Question 4

Explain load shedding

Answer 4

• Any one Gen can supply all essential flight loads.
• If only one Gen operating the system load sheds:
  
  • Galleys
  • Left and Right utility Busses
  • Recirculating fans
  • Center two hyd pump (prior to engine start)
  • Center tank fuel pumps ( prior to engine start)
  • 1 Center tank fuel pump (after engine start)

Question 5

Explain the DC electrical system?

Answer 5

• Uses TRU’s (Transformer Rectifier Units) powered from the main AC busses
• Two DC busses
• The TRU’s operate isolated so if one fails the bus tie braker closes and both busses remain powered by one TRU
• Also consists of the Battery

Question 6

Battery/ Standby power system?

Answer 6

• Hot battery Bus
• powers items that are continually powered like the clocks time reference
• prior to electrical power being suplied the battery powers the hot battery bus
• After electrical power the main battery charger provides the power via the left DC bus
• Battery Bus
• prior to electrical power, with the BATT switch ON, the main battery powers the battery bus
• left DC system powers it after electrical power.
• DISCH light illuminates when main battery is discharging
• Standby DC Bus
• prior to electrical power when BATT is ON and standby power selector is in auto, the main battery powers it
• After electrical power the left DC bus powers it with the main battery as a back up.
• When the standby power selector in BAT the main battery powers the standby DC bus.
• Stanby AC Bus
• Prior to electrical power with the BAT on and the standby power selector in AUTO the main battery powers the standby inverter which powers the AC standby bus
• After electrical power the left AC system powers the standby AC bus and the main battery and standby inverter is back up.

Question 7

What does the main battery power?

Answer 7

• Standby power system
• Pax address and interphone system
• Fuel quantity system

Question 8

What does the Hot Battery Bus power?

Answer 8

• Fire extinguisher discharge bottles
• IRS mode selector
• Alternate landing gear extension motor
• APU fuel shutoff valves
• Clock time base
• Equipment cooling ground warning and auto test
  *

Question 9

What happens to the electrical systems during an autoland?

Answer 9

• The busses isolate to allow three independent sources of power to power the three auto pilots.
• Left Main bus
  
  • _​_left A/P
  • CPT flight instrument transfer bus
• Batt/standby bus
  
  • center A/P
• Right Main bus
  
  • Right A/P
  • FO flight instrument transfer bus

Question 10

What happens if you suffer the loss of one generator above 200’ (alert height)?

What happens if you suffer the loss of one generator below 200’?

Answer 10

Above 200’

• Both bus tie breakers close and the operating IDG powers both left and right AC busses
• The left main system powers the center autopilot
• NO LAND 3 on ASA

Below 200’

• both bus tie breakers remain open
• the autopilot associated with the failed IDG is unpowered
• Flight instruments remain powered throught the instrument transfer bus
• The auotland continues using the two remaining autopilots
• When the auto pilots are disengaged or an autopilot go-around is performed the electrical systme reverts to normal, non-isolated operation.
  *

Question 11

What powers the flight instruments?

Answer 11

• The Left AC bus powers the Captains instruments and the Right AC bus poweres the FO’s instruments.
• The instrument transfer busses power the instruments from the oposite AC bus if power is lost to that bus.
• if power is lost to both buses then the cpatains intruments are powered by the HDG

Question 12

What does it mean if you see 4 in flight?

Answer 12

Illuminated (amber) –
• a fault has occurred, automatically opening the AC bus tie breaker

• the BUS TIE switch is OFF.

Question 13

What does this light mean?

Answer 13

• The AC bus is unpowered

Question 14

What happens if you push this switch?

What does light 7 mean?

Answer 14

• GEN CONT SWITCH
• ON (bar in view) – arms the generator breaker to close automatically when

generator power is available.

• OFF (bar not visible)
• opens generator breaker
• resets fault trip circuitry.
• Light 7 = lluminated (amber) – the generator breaker is open. the generator is electrically discontected from the bus.

Question 15

What are these switches, and what does the amber light mean?

Answer 15

• GEN DRIVE DISC SWITCHES
  
  • Mechanically disconect the generator from the engine
  • not reversable. Requires maintanence to reset on the ground
• DRIVE light
  
  • Illuminated (amber) –
    • the generator drive oil temperature is high

• the generator drive oil pressure is low.

Question 16