Air transportation safety investigation report A98H0003
Supporting technical information (STI)
Electrical system description
Three 120 kVA, three-phase 115 V AC, 400 Hz engine-driven IDGs are the primary source of power for the in-flight operation of the AC and DC electrical system. Each IDG is attached to an engine accessory gearbox by a quick attach/detach adapter. Each IDG incorporates a hydro-mechanical constant speed drive and a generator. The constant speed drive portion is designed to provide a constant output speed, irrelevant of the input speed from the engine accessory gearbox. This constant output speed produces a constant 400 Hz output.
The electrical system may also be powered by an APUor an external source through two external power receptacles (main and galley). The APU is installed in the tail section and provides electrical power for ground operation and also serves as a supplemental electrical power source in flight, when required.
AC power and distribution
Each IDG is connected through a generator relay to its own generator bus. Each generator bus is then connected to its associated AC buses. Parallel operation of the generator buses occurs through the AC tie bus. This permits an IDG or IDGs to supply power for all the electrical loads.
Generator Bus 1 normally powers the left emergency AC bus; Generator Bus 2 normally powers the AC ground service bus through the AC ground service tie relay; and Generator Bus 3 normally powers the right emergency bus. The ground service bus permits the supply of APU generator power or external power for use in ground operations. The AC generator buses and the AC ground service bus supply most of the high-current and centrally located loads or both, such as the hydraulic pumps, most of the fuel pumps, and the galley power. Power for the lower current, non-centrally located loads, and essential loads is supplied through the three AC buses, cabin AC buses, ground service bus, and the two emergency AC buses. Separate instrument buses supply their respective component loads.
The AC generator buses distribute power to the four, 75 A unregulated TRs. The TRs change three-phase 115 V AC, 400 Hz power to 28 V DC power. The total DC power system load is less than the power output capacity of two TRs. The four TRs usually operate in parallel; however, it is possible to isolate a TR such that it only supplies power to its bus. When isolated, TR1 is powered by Generator Bus 1, TR2A is powered by Generator Bus 2, TR2B is powered by the generator ground service bus, and TR3 is powered by the right emergency AC bus.
Control and protection
Electrical supply and control normally occurs automatically through the EPCU, the three GCUs, and the APU GCU. If the automatic system fails, the flight crew can control the electrical system from the forward overhead panel. The EPCU transmits system and status and failure data to the EAD and to the system display. The applicable system alerts are shown on the EAD and the system display.
Each of the four GCUs control their respective generator relay, bus tie relays, IDG disconnect (crew commanded), and DC ties 1 and 3, and regulate the IDG and APU generator voltage, IDG frequency, and current limit and load control. Each GCU protects its respective generator from electrical faults. The GCUs automatically operate IDG resets resulting from generator protective trips and AC generator bus fault resets. The GCUs also maintain IDG oil temperature and pressure indications for IDG fault indicating.
The EPCU protects
- load shedding;
- main external power; and
- galley external power.
The left emergency power system consists of one main 28 V DC battery, a battery charger, a static inverter and a manually deployed ADG. The main battery consists of two 14 V DC nickel-cadmium batteries connected in series for a 28 V DC system. The battery charger converts AC input into a controlled DC output to keep the battery fully charged. The static inverter is a 2 400 VA inverter installed in the avionics compartment. It changes battery DC power into 115 V AC, 400 Hz, single-phase power. The ADG is an air-cooled, 25 kVA, three-phase, 115 V AC, 400 Hz turbine generator that consists of an air turbine unit, brushless generator, and voltage regulator.
The EMER PWR selector is located in the forward overhead panel. The switch has three positions: OFF, ARM, and ON. The ON or OFF position control the left emergency control relay. The function of each position is as follows:
- OFF: Battery is not allowed to supply power to the static inverter for the left emergency AC bus and the left emergency DC bus.
- ARM: EPCU is allowed to automatically transfer battery power to the static inverter and DC bus when system conditions are correct.
- ON: Battery supplies power to the left emergency AC bus via the static inverter and to the emergency DC bus.
When the EMER PWR selector is in the OFF position and the aircraft electrical power is on, the Emergency Power OFF light in the forward overhead panel will illuminate amber.
With the EMER PWR selector in the ARM position and the BATTERY switch in the ON position, a loss of power to the left emergency AC or DC buses will cause the EPCU to automatically transfer main battery power to the left emergency DC bus and to the static inverter, which powers the left emergency AC bus. This loss of power assumes the SMOKE ELEC/AIR selector is not in the 1/2 OFF position. The battery alone will supply approximately 15 minutes of emergency power to the captain's flight essential equipment during an all-engines or all-generators failed situation. Rotating the EMER PWR selector to the ON position will also result in the same power transfers to the battery.
When deployed, the ADG operates in one of two modes: Hydraulic or Electrical. In Hydraulic mode, the ADG ELECTRIC switch is in the OFF position and the ADG supplies electrical power to the electrically driven auxiliary Hydraulic Pump 1. When the EMER PWR selector is selected ON and the ADG is in the Hydraulic mode, the ADG will supply electrical power to the auxiliary Hydraulic Pump 1 and to the left emergency AC bus.
In Electrical mode, the ADG ELECTRIC switch is in the ON position. When the EMER PWR selector is selected ON, the ADG supplies electrical power to the left and right emergency AC buses, which power the captain's and the first officer's flight-essential equipment. The right emergency AC bus also supplies power to TR3, which supplies power to the battery bus and the left and right emergency DC buses. The right emergency AC bus supplies power to the battery charger, which in parallel with the battery, powers the battery direct bus. With the BATTERY switch in the ON position, the batteries are charged by the ADG.
CABIN BUS switch
The CABIN BUS is a guarded switch that is located in the forward overhead panel directly below the SMOKE ELEC/AIR selector. The CABIN BUS switch controls the electrical power to the cabin and ground service buses, and is active with the electrical system in either the auto or manual mode of operation.
When pushed, a level 1 alert message, "ELEC," is generated on the EAD and the alert "CABIN BUS SW OFF" is annunciated on the system display. The CABIN BUS switch illuminates amber "OFF" indicating that the following buses are de-powered: cabin AC buses 1 and 3 (galley buses, cabin individual air and recirculation fans, and cabin lights), forward and mid-cabin AC ground service bus, and the overwing and aft cabin AC ground service bus.
Activation of the CABIN BUS switch supplies power from the 28 V DC battery bus through the emergency panel miscellaneous lights and cabin bus control CB, B1-879, located at position C-28 on the overhead CB panel to the cabin bus control relays R2-5774 and R2-5775 located in the equipment rack forward relay panel in the avionics compartment. When energized, relays R2-5774 and R2-5775 open eight RCCBs to remove the cabin bus and ground service power. Removing power from these two relays will re-energize the cabin buses if primary power is available.
The emergency panel miscellaneous lights and cabin bus control CB was located in an area of high heat (as high as 343°C). The CB was not identified.
CB B1-112 at position C-30 (RCCB BACKUP POWER) was identified. The CB exhibited soot accumulation on the white indicator ring. The wire run from the emergency panel miscellaneous lights and cabin bus control CB to the cabin bus control relays was also located in an area of high heat.
The emergency panel miscellaneous lights and cabin bus control CB was not recovered; however, because of the high heat in this area, a thermal tripping of this CB is possible. If this CB were to trip, the power to the cabin buses would be restored, and in part, power to the recirculation and individual air fans and cabin lights would be restored.
The soot accumulation on CB B1-112 at position C-30 is consistent with the CB tripping prior to impact (it is unknown whether this CB had tripped as a result of an electrical arcing event or as a result of the surrounding temperature (a trip by ambient heat).
The effects of heat or fire on the wire run from the emergency panel miscellaneous lights and cabin bus control CB to the cabin bus control relays could cause a tripping of the CB or an opening of the circuit, which would enable the cabin buses to become re-energized.
Known electrical power distribution at the time of impact
|Electrical component operating||Aircraft bus||CB||CB location|
|Tail Tank Left Fuel Transfer Pump||115 V AC Generator Bus 1||B1-394
|B21 Upper Main CB Panel|
|Upper Aux Tank Right Fuel Transfer Pump||115 V AC Generator Bus 1||B1-1250
|B23 Upper Main CB Panel|
|Tank 1 Forward Fuel Boost Pump||115 V AC Generator Bus 1||B1-614
|B24 Upper Main CB Panel|
|Tank 3 Fuel Transfer Pump||115 V AC Generator Bus 1||B1-618
|B26 Upper Main CB Panel|
|Tail Tank Left Fuel Transfer Pump Control||115 V AC Bus 1||B1-542||E22 Upper Main CB Panel|
|Upper Aux Tank Right Fuel Transfer Pump Control||115 V AC Bus 1||B1-1206||E23 Upper Main CB Panel|
|Tank 1 Forward Fuel Boost Pump Control||115 V AC Bus 1||B1-615||E24 Upper Main CB Panel|
|Tank 3 Fuel Transfer Pump Control||115 V AC Bus 1||B1-619||E26 Upper Main CB Panel|
|CAC Cooling Fan 1||115 V AC Bus 1||B1-270 3-ø||L18 Upper Main CB Panel|
|Cabin Air Recirculation Fans||115 V AC Bus 1 and 115 V AC Bus 3 or both|
|Avionics Compartment Cooling Fan||115 V AC Bus 2||B1-357
|M11 Upper Main CB Panel|
|Tank 1 Aft Fuel Boost Pump||115 V AC Generator Bus 3||B1-612 3-ø||D24 Upper Main CB Panel|
|Tank 2 Fuel Transfer Pump||115 V AC Generator Bus 3||B1-756 3-ø||D25 Upper Main CB Panel|
|Tank 1 Aft Fuel Boost Pump Control||115 V AC Bus 3||B1-613||G24 Upper Main CB Panel|
|Tank 2 Fuel Transfer Pump Control||115 V AC Bus 3||B1-757||G25 Upper Main CB Panel|
|Avionics Overheat Light||28 V DC Bus 3||B1-320||S08 Upper Main CB Panel|