Safety Investigation Report 2018:1 Factual Information/1.6/1.6.8 Aircraft Systems/3
SAFETY INVESTIGATION REPORT MH370 (9M-MRO)
3) Electrical Power
The electrical system generates and distributes AC and DC power to other aircraft systems, and is comprised of: main AC power, backup power, DC power, standby power, and flight controls power. System operation is automatic. Electrical faults are automatically detected and isolated. The AC electrical system is the main source for aircraft electrical power. Figure 1.6H (below) shows the cockpit electrical panel where electrical switching can be made. It also shows the associated lights.
As the various aircraft systems rely on electrical power, failure of the electrical buses will affect the systems operation which will in turn trigger the corresponding fault messages. These messages are collected by the CMCS which will transmit the messages, via the ACARS, to the Maintenance Control Centre (MCC).
| Note: Diagram not included | |||
| Electrical Power Panel Switches/Lights | |||
| 1. | Battery Switch | 11. | Backup Generator OFF Lights |
| 2. | Battery OFF Light | 12. | Backup Generator (BACKUP GEN) Switches |
| 3. | APU Generator (APU GEN) Switch |
13. | External Power AVAIL Lights |
| 4. | APU Generator OFF Light | 14. | External Power ON Lights |
| 5. | BUS TIE Switches | 15. | External Power (EXT PWR) Switches |
| 6. | BUS Isolation (ISLN) Lights | 16. | CABIN/UTILITY Power OFF Light |
| 7. | Generator Control (GEN CTRL) Switches |
17. | Cabin/Utility (CABIN/UTILITY) Power Switch |
| 8. | Generator OFF Lights | 18. | IFE/PASS SEATS OFF Light |
| 9. | Drive Disconnect Switches | 19. | In Flight Entertainment System/ Passenger Seats (IFE/PASS SEATS) Power Switch |
| 10. | Generator DRIVE Lights | ||
Figure 1.6H - Electrical Power Panel Switches/Lights
- a) Electrical Load Management System
The Electrical Load Management System (ELMS) provides load management and protection to ensure power is available to critical and essential equipment. If the electrical loads exceed the power available (aircraft or external), ELMS automatically shed AC loads by priority until the loads are within the capacity of the aircraft or ground power generators. The load shedding is non-essential equipment first, then utility busses. Utility busses are followed by individual equipment items powered by the main AC busses. When an additional power source becomes available or the loads decrease, ELMS restores power to shed systems (in the reverse order). The message LOAD SHED displays on the electrical synoptic when load shed conditions exist.
- b) Alternating Current Electrical System Power Sources
The entire aircraft alternating current (AC) electrical load can be supplied by any two main AC power sources. The main AC electrical power sources are:
- left and right engine integrated drive generators (IDGs)
- APU generator
- primary and secondary external power
The power sources normally operate isolated from one another. During power source transfers on the ground (such as switching from the APU generator to an engine generator) operating sources are momentarily paralleled to prevent power interruption.
- c) Integrated Drive Generators
Each engine has an Integrated Drive Generator (IDG). Each IDG has automatic control and system protection functions. When an engine starts, with the GENERATOR CONTROL switch selected ON, the IDG automatically powers the respective main bus. The previous power source is disconnected from that bus.
The IDG can be electrically disconnected from the busses by pushing the GENERATOR CONTROL switch to OFF. The IDG can also be electrically disconnected from its respective bus by selecting an available external power source prior to engine shutdown. The DRIVE light illuminates and the EICAS message ELEC GEN DRIVE L or R displays when low oil pressure is detected in an IDG. The IDG drive can be disconnected from the engine by pushing the respective DRIVE DISCONNECT switch. The IDG cannot be reconnected by the flight crew. High drive temperature causes the IDG to disconnect automatically.
- d) Auxiliary Power Unit Generator
The Auxiliary Power Unit (APU) generator is electrically identical to the IDG generators. The APU generator can power either or both main busses and may be used in flight as a replacement to an IDG source. If no other power source is available when the APU generator becomes available, the APU generator automatically connects to both main AC busses. If the primary external source is powering both main busses, the APU powers the left main bus, and the primary external source continues to power the right main bus. If the primary external source is powering the right main bus, and the secondary external source is powering the left main bus, the APU then powers the left main bus and the primary external source continues to power the right main bus. If the secondary external source is powering both main busses, the APU then powers both main busses.
The APU generator OFF light illuminates when the APU is operating and the APU generator breaker is open because of a fault or the APU GENERATOR switch is selected OFF. When the APU GENERATOR switch is ON and a fault is detected, the APU generator cannot connect to the busses.
In flight, when both transfer busses are unpowered, the APU starts automatically, regardless of APU selector position.
- e) Alternating Current Electrical Power Distribution
The AC power is distributed through the left and right main busses and the ground service bus. The right IDG normally powers the right main bus and the left IDG normally powers the left main bus. The APU normally powers both main busses when they are not powered by any other source.
Bus tie relays, controlled by BUS TIE switches, isolate or parallel the right and left main busses. When both BUS TIE switches are set to AUTO, the bus tie system operates automatically to maintain power to both main busses.
Power transfers are made without interruption when the aircraft is on the ground, except when switching between primary and secondary external power sources. The source order for powering left and right main busses in flight is the:
- f) Autoland
During autoland, the busses isolate to allow three independent sources to power the three autopilots:
- the left IDG powers the left AC transfer bus, the left main DC bus, and the captain’s flight instrument bus;
- the right IDG powers the battery bus and AC standby bus through the main battery charger; and
- the back-up system powers the right AC transfer bus, the right DC bus, and the first officer’s flight instrument bus.
- g) Backup Alternating Current Electrical System
The electrical system is highly reconfigurable to accommodate multiple failures. The electrical system is designed to automatically provide power to selected aircraft systems. The electrical system automatically powers one or both transfer busses when:
- only one main AC generator (includes APU) is available;
- power to one or both of the main AC busses is lost;
- approach (APP) mode is selected for autoland; and
- the system is automatically tested after engine starts
The system automatically transfers power without interruption.
- h) Backup Generators
Backup power is provided by one variable speed, variable frequency generator mounted on each engine. A frequency converter converts the generator frequency to a constant 400 Hz. Only one backup generator can power the converter at a time.
Each backup generator contains two permanent magnet generators (PMGs) that supply power to the flight control DC electrical system (refer to DC Electrical System). If both IDGs and the APU generator are inoperative, a backup generator powers essential aircraft equipment. To reduce electrical loading on the backup generator, the following systems are inoperative:
- i) Direct Current Electrical System
The direct current (DC) electrical system includes the main DC electrical system and the flight control DC electrical system. The main DC electrical system uses four transformer-rectifier units (TRUs) to produce DC power. The TRUs are powered by the AC transfer busses.
TRU DC electrical power is distributed to various DC busses as follows:
- (1) The left TRU powers the left main DC bus, which provides a second DC power source for:
- (2) The right TRU powers the right main DC bus, which provides a second DC power source for:
- (3) The C1 TRU powers the captain’s flight instrument bus and the battery bus. The captain’s flight instrument bus provides a second DC power source for:
- centre flight control PSA
- first officer’s flight instrument bus
The C2 TRU powers the first officer’s flight instrument bus, which provides a second DC power source for the captain’s instrument bus.
- (1) The left TRU powers the left main DC bus, which provides a second DC power source for:
- j) Batteries
The main battery is connected directly to the hot battery bus and provides standby power to other busses. The main battery charger normally powers the hot battery bus and maintains the main battery fully charged.
The APU battery is connected directly to the APU battery bus and provides dedicated power to the APU electric starter, which is used when sufficient bleed air duct pressure is unavailable for the APU air turbine starter. The APU battery charger normally powers the APU battery bus and maintains the APU battery fully charged.
- k) Flight Control Direct Current Electrical System
The flight control DC electrical system is a dedicated power source for the primary flight control system. Primary power for the flight control DC electrical system comes from permanent magnet generators (PMGs) housed within each backup generator. Variable frequency PMG AC power is used by individual power supply assemblies (PSAs) to provide DC power to the three flight control DC busses. To ensure a high level of system reliability, each PSA also has multiple DC power sources. If primary PMG AC power is not available, secondary power for the left and right PSAs is provided by the related main DC bus. Secondary power for the centre PSA is provided by the captain’s flight instrument bus. The hot battery bus provides additional backup power for the left and centre PSAs only. Each PSA also uses a dedicated battery to prevent power interruptions to the related flight control DC bus. The batteries have limited capacity and are incorporated to supply power for brief periods during PSA power source transfers.
- l) Standby Electrical System
The standby electrical system can supply AC and DC power to selected flight instruments, communications and navigation systems, and the flight control system, if there are primary electrical power system failures. The standby electrical system consists of:
- the main battery
- the standby inverter
- the RAT generator and its associated generator control unit
- the C1 and C2 TRUs
- (1) Main Battery
The main battery provides standby power to the:
- hot battery bus
- battery bus
- captain’s flight instrument bus
- left and centre flight control PSAs
- standby inverter.
Note:
The main battery can power the standby system for a minimum of 10 minutes.
- (2) Standby Inverter
The standby inverter converts DC power to AC power. The inverter powers the AC standby bus if the left transfer bus is not powered.
- (3) Ram Air Turbine Generator
The ram air turbine (RAT) generator provides standby power to the C1 and C2 TRUs. The RAT can supply electrical and hydraulic power simultaneously. If the RAT is unable to maintain RPM, the RAT generator electrical load is shed until RPM is satisfactory. Power for standby electrical loads is provided by the main battery during deployment of the RAT and when RAT generator loads are shed. The RAT is deployed automatically if both AC transfer busses lose power in flight. The RAT can be manually deployed by using the RAM AIR TURBINE switch on the overhead panel.
- (4) Cabin Systems and Utility Power
Electrical power to some cabin and utility systems are controlled from the cockpit. The IFE/PASS SEATS Power switch controls power to the IFE and passenger seats. The CABIN/UTILITY Power switch controls power to cabin and utility systems.