Describe the Electrical System (FCOM)
- AC and DC Power is generated and distributed automatically.
- Faults are detected and isolated automatically as well.
- AC is the main source of airplane electrical power.
What are the AC Power Sources?
L and R Engine IDGs (Main Source)
APU (Main Source)
EXT PWR (Ground Ops)
HDG (Back-up for Dual Main AC Bus Fail / ETOPS)
Describe IDG operation.
- Switch Normally Kept ON
- Automatically Controlled Priority Source for Main AC BUS.
- When connected, previous source is auto disconnected.
- Can be disconnected with the switch, or EXT PWR ON.
- OFF displays anytime the Gen Control Breaker is OPEN.
What does an Illuminated DRIVE light indicate, what should accompany it, and what should the pilot do?
- IDG High Oil Temp or IDG Low Oil Pressure.
- L or R GEN DRIVE EICAS message.
- Pull QRH. Pushing DRIVE DISC disconnects the IDG from the N2 rotor.
Describe the APU GEN.
- Identical to the Engine IDGs.
- Can supply all NECESSARY power for the A/C.
- Connected to both Main AC Busses.
- Connected to the Ground Handling Bus, and a backup for the Ground Service Bus.
When does the APU GEN OFF light illuminate?
- When the APU is operating and the GEN SWITCH is selected OFF, or
- FAULT.
What does an EXT PWR AVAIL light indicate?
- Adequate
- Voltage
- Frequency
What happens when you push the EXT PWR switch ON?
- External Power connects to both main busses.
- IDGs and APU disconnected from busses.
What are the three main components of the BUS TIE System?
- The AC BUS TIE breakers
- The DC BUS TIE.
- The Fight Instrument TX Busses.
What are the three functions of the Bus Tie System?
- Automatically Reconfigure Electrical System in case of a PWR Source FAIL.
- Automatically Isolate a FAULT.
- Automatically Prevent Paralleling Power Sources.
When would a BUS TIE switch ISLN light illuminate?
- Bus Tie OPEN due to
- Fault, or
- Selected OPEN.
When would a L/R BUS OFF light illuminate?
- Whenever the L/R Main AC Bus is not powered.
What is the Source order for powering the Main AC Busses?
- Respective IDG
- APU GEN
- Opposite IDG
What Busses provide FIRE DETECT and PROTECT?
DETECT - Battery Bus
PROTECT - Hot Battery Bus
When, Why, and What is shed for an OVERLOAD?
- When the Captain does his B4 Start Flow,
- HYD and FUEL Pumps are selected, and there is normally only one power source (APU), and therefore,
- The C2 ELEC HYD Pump and Galley Loads are shed.
When, Why, and What is shed for an ENG START?
- When GRND is selected and yet increases the load more,
- The Utility Bus and the L ELEC DMND Pump Shed.
- For the Second Engine Start, the On-Side Galley Load is Shed.
What is shed if a GEN is lost?
- Only “Non-Normal” Shedding Condition
- Shed system armed when Thrust Levers are advanced.
- Galley First, then Utility, then individual equipment items.
How is the HDG Powered?
What does it power?
- Powered by the Center Air Demand Pump.
- 4 AC Busses - L/R TX, CA FLT INST TX, STBY AC
- 3 DC Busses - STBY DC, BATT, HOT BATT
- Available instruments will generally be those with “Standby” in their name; L of Alternate Gear, except EICAS and Autobrakes.
What is available with Battery Power only?
Busses: HOT, BATT, STBY AC and DC.
- STBY Attitude Indicator, Air Speed, Altimeter and CA DMI
General Description of the Electrical System.
- Four Power Sources
- Two Sides (L/R)
- Each side - Main AC, Utility (non-essential high power), Instrument Transfer, and DC Busses.
- Bus Tie System Tries to run each side separately, but can use any one source to power both sides.
- STBY and BATT busses normally powered by Left Side, but if there’s no PWR there, then through the HOT Bus.
- Load Shedding gets rid of high power non-essential items during peak demands - utility busses, HYD and FUEL pumps, Galley Stuff.
- Isolates systems for independent power sources during autoland.
How could you power the airplane service outlets, some galley power, most cabin lighting, and the battery chargers with EXT PWR?
The Entire Aircraft?
- Check AVAIL Light indicating acceptable voltage and frequency; This means the GRND SERVC Bus is powered.
- The Bus Switch ON, on the FA Panel next to 1L door will then power the GROUND SERVICE Bus via the GROUND HANDLING BUS.
- If you then select external power ON, then the entire aircraft will be powered (and the GND SRV Bus will switch back to the R MAIN AC Bus.)
Describe a Single Source Shed.
- Before Engine Start, with one AC Source (usually APU) selecting all the HYD and FUEL Pumps ON will cause the #2 C HYD Pump to shed. (PRESS light)
Describe an Engine Start Load Shed.
- Moving the ENG START SELECTOR to GND will shed both utility busses, and as a result, the RECIRC FAN INOP lights illuminate.
- GND also opens the Start Valve (VALVE in-tx flash) the APU goes to high flow (inc in duct press) arms selected ignition, and closes the engine bleed air valve, if opened.
- The Center Fuel Pumps will show PRESS (inhibited) until their associated engine is at 50% N2.
- The IDG will trigger the GEN BREAKER CLSD and the OFF Light Extinguishes, the R AC BUS TIE BREAKER OPENs to isolate the two sides. The #2 C ELEC HYD Pump comes back on because we have two sources, and the Center Boost Pumps because we have adequate power.
Walking into a cold dark airplane, what is the first step in powering the aircraft?
- Battery Switchlight ON and STBY PWR AUTO.
- This powers the HOT, BATT, and STBY Busses.
- STBY ADI, AI, ALT, ENG Indications, L VHF, L VOR, C ILS
