Electrical System
๐ Sources: FCOM Ch.6 (R53), QRH Ch.6 (R53)
1. Overview
The electrical system generates and distributes AC and DC power to all airplane systems. It comprises five subsystems: (FCOM 6.20.1)
- Main AC Power โ Two engine IDGs, APU generator, primary/secondary external power
- Backup AC Power โ Two variable-frequency backup generators (one per engine)
- DC Power โ Four TRUs, flight control DC system
- Standby Power โ Main battery, standby inverter, RAT generator
- Flight Controls Power โ Dedicated PMG-fed PSAs for fly-by-wire
System operation is automatic. Electrical faults are automatically detected and isolated. (FCOM 6.20.1)
2. Components
AC Power Sources
| Component | Rating | Description |
|---|---|---|
| L/R Integrated Drive Generators (IDGs) | 120 kVA each | Engine-driven, constant-speed drive. Auto-connects when engine starts with GEN CTRL ON. High oil temp โ auto-disconnect. Cannot be reconnected by crew. (FCOM 6.20.1) |
| APU Generator | 120 kVA | Electrically identical to IDGs. Can power either or both main busses. In flight: auto-starts if both transfer busses lose power regardless of APU selector position. (FCOM 6.20.2) |
| Primary External Power | Ground only | Powers R main bus (normally). Can power both busses if sole source. (FCOM 6.20.3) |
| Secondary External Power | Ground only | Powers L main bus (normally). Can power both busses if sole source. (FCOM 6.20.3) |
Backup AC Power
| Component | Description |
|---|---|
| Backup Generators (L/R) | One variable-speed, variable-frequency generator per engine. Frequency converter โ constant 400 Hz. Only one powers the converter at a time. Contains two PMGs for flight control DC system. (FCOM 6.20.7) |
IMPORTANT
If both IDGs AND APU generator are inoperative, backup generator powers essential equipment only. Inoperative systems on backup power (FCOM 6.20.7):
- TCAS, SATCOM, Right HF radio
- Center tank override/jettison pumps (center fuel unusable, cannot jettison)
- Position and exterior lights (except nose gear landing lights)
- All non-essential cabin equipment (galleys, IFE, etc.)
- Passenger cabin lighting (except night, galley, cross-aisle)
- Cabin temp control (degraded mode only)
DC System
| Component | Description |
|---|---|
| L TRU | Powers L main DC bus โ secondary source for L flight control PSA, R main DC bus |
| R TRU | Powers R main DC bus โ secondary source for R flight control PSA, L main DC bus |
| C1 TRU | Powers captainโs flight instrument bus and battery bus โ secondary for center PSA, F/O flight instrument bus |
| C2 TRU | Powers F/O flight instrument bus โ secondary source for captainโs flight instrument bus |
| Main Battery | Connected to hot battery bus. Powers standby system for minimum 5 minutes. (FCOM 6.20.13) |
| APU Battery | Connected to APU battery bus. Dedicated to APU electric starter. (FCOM 6.20.9) |
Standby System
| Component | Description |
|---|---|
| Main Battery | Standby power to hot battery bus, battery bus, captainโs FLT INST bus, L/C PSAs, standby inverter |
| Standby Inverter | DCโAC conversion. Powers AC standby bus if L transfer bus unpowered |
| RAT Generator | Standby power to C1/C2 TRUs. No operating time limits. All airspeeds/altitudes. Auto-deploys if both AC transfer busses lose power in flight. (FCOM 6.20.14) |
CAUTION
Main battery can power standby system for a minimum of 5 minutes. (FCOM 6.20.13, QRH 6.6)
3. Controls and Indicators
Overhead Electrical Panel
| # | Control / Indicator | Function |
|---|---|---|
| 1 | BATTERY Switch | ON: allows APU start (no AC); powers standby busses & emergency lighting if AC lost. OFF: battery bus not powered (ground only) |
| 2 | Battery OFF Light (amber) | Battery switch OFF while airplane has AC power |
| 3 | APU GEN Switch | ON: APU generator breaker armed. OFF: breaker open |
| 4 | APU Generator OFF Light (amber) | APU GEN switch OFF, or APU running with auxiliary power breaker open |
| 5 | BUS TIE Switches (L/R) | AUTO: bus tie breaker armed. OFF: bus tie open |
| 6 | ISLN Lights (amber) | Bus tie breaker open (BUS TIE OFF or fault in AUTO) |
| 7 | GEN CTRL Switches (L/R) | ON: generator breaker armed. OFF: breaker open, resets fault trip circuitry |
| 8 | Generator OFF Lights (amber) | Generator breaker open |
| 9 | DRIVE DISC Switches (L/R) | Push: disconnects IDG from engine. Irreversible โ requires maintenance to reconnect |
| 10 | DRIVE Lights (amber) | IDG oil pressure low |
| 11 | BACKUP GEN OFF Lights (amber) | Backup gen switch OFF, fault, engine shut down, or fire switch out |
| 12 | BACKUP GEN Switches (L/R) | ON: backup generator converter armed. OFF: relay open, resets fault circuitry |
| 13 | EXT PWR AVAIL Lights (green) | External power plugged in, quality acceptable |
| 14 | EXT PWR ON Lights (green) | External power connected to bus |
| 15 | EXT PWR Switches (PRI/SEC) | Push: connects/disconnects external AC power |
| 16 | CABIN/UTILITY OFF Light (amber) | CABIN/UTILITY switch OFF |
| 17 | CABIN/UTILITY Switch | ON: powers cabin and utility systems. OFF: disconnects (some cabin lighting remains) |
| 18 | IFE/PASS SEATS OFF Light (amber) | IFE/PASS SEATS switch OFF |
| 19 | IFE/PASS SEATS Switch | ON/OFF: controls IFE, passenger seat motors, PC outlets, telephones |
Overhead Maintenance Panel
| Control | Function |
|---|---|
| STANDBY POWER Switch | OFF: AC standby bus unpowered. AUTO (guarded): standby busses transfer to battery if normal AC lost. BAT (momentary): powers standby AC bus from battery via inverter; initiates DC/standby self-test with AC available |
Electrical Synoptic Display (ELEC)
Displayed by pushing ELEC synoptic display switch on display select panel: (FCOM 6.10.5)
| Display Element | Meaning |
|---|---|
| Wide green line | Power flowing |
| Thin white line | Power not flowing |
| Green generator/bus | On / available / powered |
| White generator/bus | Off / state unknown |
| Amber X in amber box | Generator failed or drive disconnected with engine running |
| Amber ISLN | Bus tie open (fault or BUS TIE OFF) |
| Amber bus | Unpowered |
| Amber DRIVE | Drive oil pressure low |
| LOAD SHED | ELMS load shedding active |
NOTE
Electrical synoptic displays switch/contactor positions, not actual electrical flow. Not included in MMEL/DDG. Not required for any normal or non-normal procedure. (FCOM 6.10.5)
4. Electrical Load Management System (ELMS)
The ELMS provides automatic load management and protection to ensure critical/essential equipment power. (FCOM 6.20.1)
Load Shedding Order (when loads exceed available power):
- Galleys (first)
- Utility busses
- Individual equipment items on main AC busses
Restoration Order: Reverse (when additional power becomes available or loads decrease)
ELMS also monitors fuel pump operation during climb/cruise โ EICAS alerts for unintentional dry fuel pump operation when center tank is empty.
5. Power Distribution
AC Bus Structure
graph TB LIDG["L IDG<br/>120 kVA"] --> LMAIN["L MAIN BUS"] RIDG["R IDG<br/>120 kVA"] --> RMAIN["R MAIN BUS"] APU["APU GEN<br/>120 kVA"] --> LMAIN APU --> RMAIN PRIEXT["Primary EXT PWR"] --> RMAIN SECEXT["Secondary EXT PWR"] --> LMAIN LMAIN --> LXFR["L TRANSFER BUS"] LMAIN --> LUTIL["L UTILITY BUS"] RMAIN --> RXFR["R TRANSFER BUS"] RMAIN --> RUTIL["R UTILITY BUS"] RMAIN --> GSVC["GROUND SERVICE BUS"] LMAIN ---|"BUS TIE"| RMAIN LBKP["L BACKUP GEN"] --> CONV["CONVERTER<br/>400 Hz"] RBKP["R BACKUP GEN"] --> CONV CONV --> LXFR CONV --> RXFR style LMAIN fill:#1a5e1a,color:#fff style RMAIN fill:#1a5e1a,color:#fff style LXFR fill:#1a4a6e,color:#fff style RXFR fill:#1a4a6e,color:#fff
Source Priority (in flight) for L/R main busses: (FCOM 6.20.5)
- Respective IDG
- APU generator
- Opposite IDG
AC Main Bus Loads (typical)
| Load Category | Examples |
|---|---|
| Direct loads | Cooling vent fan, recirculation fans, lavatory/galley fans, electric hydraulic pumps, IFE |
| Transfer bus | DC system TRUs, AC standby bus |
| Utility bus | Fwd galley heater, gasper fan, PF/PM foot/shoulder heaters, door area heaters, lavatory heaters |
| Galley busses | Galley power |
DC Bus Structure
graph TB LXFR["L TRANSFER BUS<br/>(AC)"] --> LTRU["L TRU"] RXFR["R TRANSFER BUS<br/>(AC)"] --> RTRU["R TRU"] LXFR --> C1["C1 TRU"] RXFR --> C2["C2 TRU"] LTRU --> LDC["L MAIN DC BUS"] RTRU --> RDC["R MAIN DC BUS"] C1 --> CFIT["CAPT FLT INST BUS"] C1 --> BATBUS["BATTERY BUS"] C2 --> FOFIT["F/O FLT INST BUS"] BATBUS --> HOTBAT["HOT BATTERY BUS"] MAINBAT["MAIN BAT"] --> HOTBAT GSVC["GROUND SVC BUS"] --> MAINCHG["MAIN BAT CHG"] GSVC --> APUCHG["APU BAT CHG"] LDC ---|"Secondary"| LPSA["L PSA"] RDC ---|"Secondary"| RPSA["R PSA"] CFIT ---|"Secondary"| CPSA["C PSA"] LPSA --> LFCTL["L FCTL DC BUS"] CPSA --> CFCTL["C FCTL DC BUS"] RPSA --> RFCTL["R FCTL DC BUS"] style LDC fill:#5a3a1a,color:#fff style RDC fill:#5a3a1a,color:#fff style LFCTL fill:#6a1a1a,color:#fff style CFCTL fill:#6a1a1a,color:#fff style RFCTL fill:#6a1a1a,color:#fff
Flight Control DC System
Dedicated power for primary flight control (fly-by-wire): (FCOM 6.20.12)
| PSA | Primary Source | Secondary Source | Backup Source |
|---|---|---|---|
| L PSA | L Backup Gen PMG | L Main DC Bus | Hot Battery Bus |
| C PSA | L or R Backup Gen PMG | Captainโs FLT INST Bus | Hot Battery Bus |
| R PSA | R Backup Gen PMG | R Main DC Bus | โ |
Each PSA has a dedicated battery for brief power interruptions during source transfers.
Autoland Bus Configuration
During autoland (APP mode), busses isolate for three independent autopilot power sources: (FCOM 6.20.5)
| Source | Powers |
|---|---|
| L IDG | L AC transfer bus, L main DC bus, captainโs flight instrument bus |
| R IDG | Battery bus, AC standby bus (via main battery charger) |
| Backup System | R AC transfer bus, R main DC bus, F/O flight instrument bus |
Standby Power Architecture
graph TB MAINBAT["MAIN BATTERY<br/>โ ๏ธ Min 5 min"] --> HOTBAT["HOT BATTERY BUS"] MAINBAT --> BATBUS["BATTERY BUS"] MAINBAT --> STBYINV["STANDBY INVERTER"] MAINBAT --> LPSA["L PSA"] MAINBAT --> CPSA["C PSA"] MAINBAT --> CAPTFI["CAPT FLT INST BUS"] STBYINV --> ACSTBY["AC STANDBY BUS"] RAT["RAT GENERATOR<br/>No time limit<br/>All speeds/altitudes"] --> C1["C1 TRU"] RAT --> C2["C2 TRU"] C1 --> CAPTFI C2 --> FOFI["F/O FLT INST BUS"] style MAINBAT fill:#8a4a0a,color:#fff style RAT fill:#4a0a6a,color:#fff
WARNING
When both transfer busses are lost: (FCOM 6.20.14)
- F/O displays temporarily unpowered while RAT deploys
- Captainโs displays, EICAS, L CDU โ battery power โ then RAT
- F/O forward displays โ RAT (once online)
- F/O lower display and R CDU remain unpowered
Cabin Systems Power Control
| Switch | OFF removes power from |
|---|---|
| IFE/PASS SEATS | All IFE, seat motors, PC outlets, telephones |
| CABIN/UTILITY | Ground service bus (except battery chargers & L fwd fuel pump), utility busses, galleys, fluorescent cabin lighting, beacon/logo/wing lights |
CABIN/UTILITY OFF still powers: night lights, reading lights, attendant work lights, some galley/crew rest lights (FCOM 6.20.16)
6. EICAS Messages
| Message | Level | Aural | Logic |
|---|---|---|---|
| ELEC AC BUS L, R | โ ๏ธ Caution | Beeper | AC bus is unpowered |
| ELEC BACKUP GEN L, R | Advisory | โ | Backup generator has failed |
| ELEC BACKUP SYS | Advisory | โ | Backup power system has failed |
| ELEC BATTERY OFF | Advisory | โ | Battery switch is OFF |
| ELEC BUS ISLN L, R | Advisory | โ | Bus tie breaker open (fault or BUS TIE OFF) |
| ELEC CABIN/UTIL OFF | Advisory | โ | Cabin/utility power switch is OFF |
| ELEC GEN DRIVE L, R | Advisory | โ | Generator drive oil pressure low |
| ELEC GEN OFF APU | Advisory | โ | APU generator control breaker open |
| ELEC GEN OFF L, R | Advisory | โ | Generator control breaker open |
| ELEC GND HDLG BUS | Advisory | โ | Ground handling bus relay failed |
| ELEC IFE/SEATS OFF | Advisory | โ | IFE/PASS SEATS power switch OFF |
| ELEC STANDBY SYS | Advisory | โ | Standby power system fault |
| MAIN BATTERY DISCH | Advisory | โ | Main battery discharging or hot battery bus unpowered |
7. Limitations
- IDG disconnect (DRIVE DISC): irreversible โ requires maintenance ground action (FCOM 6.10.3)
- Main battery standby time: minimum 5 minutes (FCOM 6.20.13)
- RAT generator: no operating time limits, all airspeeds and altitudes (FCOM 6.20.14)
- Backup generator: electrical load shed if RAT cannot maintain RPM (FCOM 6.20.14)
8. QRH Non-Normal Checklists
| EICAS Message | QRH Ref | Page |
|---|---|---|
| ELEC AC BUS L, R | QRH 6.1 | QRH p.129 |
| ELEC BACKUP GEN L, R | QRH 6.2 | QRH p.130 |
| ELEC BACKUP SYS | QRH 6.3 | QRH p.131 |
| ELEC GEN DRIVE L, R | QRH 6.4 | QRH p.132 |
| ELEC GEN OFF APU | QRH 6.4 | QRH p.132 |
| ELEC GEN OFF L, R | QRH 6.5 | QRH p.133 |
| MAIN BATTERY DISCH | QRH 6.6 | QRH p.134 |
Cross-References
- RAM Air Turbine (RAT) โ Emergency hydraulic/electrical power
- APU System โ APU generator operations
- Flight Controls โ Flight control DC electrical system
- Hydraulic System โ Electric hydraulic pumps, RAT deployment