BA B777 Incident @ Heathrow (merged)

[OverRun]

The investigation is looking closely at the thronomister which failed when electrical power was lost, and excessive aft C of G due to waste tank overflowing on the long flight, leading to low speed handling problems and a stall.

Thronomister
The engine is managed by a dual channel FADEC system. The EEC is part of this system (the heart). The EEC controls engine systems, starts and autostarts, and T/R operation. Interruption of electrical power to either channel of the EEC causes a reset of the EEC. Interruption of power to the FADEC activates the EEC inhibit reset circuit and is intended to control spurious commands in the event of circuitry over-voltage such as lightening strikes. The fuel supply to this engine is from airplane fuel system through the engine fuel pump (two stage) to the carburettor. The pump supplies fuel to Fuel Metering Unit and servo fuel for actuators. Fuel flow transmitter and thermocouples supply fuel flow and temperature readings to cockpit via EEC. The power to the fuel pump circuit is via an independent bus. Loss of the thronomister due to electrical power loss causes the fuel pump to revert to one of four fixed settings. At low engine speed the fuel pump revert setting is idle. The electrical power loss affects many systems in the 777:

_115VAC STANDBY BUS-MAIN_
ADC LEFT POWER (34-12-01)
NAV RADIO RMI (Optional)
NAV RADIO MKR VOR LEFT (34-51-01)
NAV RADIO ADF RIGHT (can also be found on 115AC Bus #1 on some airlines)
NAV RADIO ATC LEFT (34-53-01) Optionally found on 115VAC Main Bus #3
STANDBY IGN ENG 1 (74-31-01)
STANDBY IGN ENG 2 (74-31-02)
STANDBY INSTR LIGHTS (33-11-04)
FLIGHT CONTROL ELEC 1L AC (Also known as FCE Power Supply or PSM. NAV RADIO MLS-L (Optional)
EFIS CONT LEFT (31-61-01)
UPPER EICAS (31-61-01)
THRONOMISTER FUEL ENG 1 (79-31-01)
THRONOMISTER FUEL ENG 2 (79-31-02)
EIU LEFT (31-61-01)
FLIGHT CONTROL ELEC 2L AC (Also known as FCE Power Supply or PSM. One of four. See Notes)
MAWEA POWER A (31-51-01) Modularized Avionics Warning Electronics Assembly power supply A.
FMCS CDU-LEFT (34-61-01)
MAIN AC STBY BUS VOLTAGE (24-34-02)
MMR LEFT (MULTI MODE RECEIVER) 34-31-01

_115VAC STANDBY BUS- APU_
PFD-LEFT
ND-LEFT
FMCS FMC-LEFT

_28VDC APU BATTERY BUS_ (Normally supplied with DC power from TRU #3 unless forced to use APU Battery Charger or APU Battery)
ENG START AIR CONT (Provides elec power to Start Switches and Pneumatic Start Valve solenoids. Valves are electrically controlled, but pneumatically driven... They won't open without bleed air). Power for light in Start Switches comes from the #1 DC Bus.
APU DC FUEL PUMP (Required if #2 AFT MAIN BOOST PUMP is not providing pressure. APU switch must be in ON or START and the following CB's must be set: APU PRIME CONTROL (or APU ALT CTRL) and APU START)
THRONOMISTER FIRE WARN HORN (Horn located in RH Body Gear Well)
NACELLE LE ANTI-ICE 1 (30-21-01) Also known as NAI. Provides power to VALVE solenoid). Auto Anti-Ice Systems appear to use Main Bus power however
NACELLE LE ANTI-ICE 2
LOOP A FIRE 1 OVERHEAT 2
LOOP A FIRE 2 OVERHEAT 1
LOOP B FIRE 1 OVERHEAT 2
LOOP B FIRE 2 OVERHEAT 1
FIRE DET APU LOOP A
FIRE DET APU LOOP B
ENG 1 SPEED SENSOR 1 (77-12-05) Provides power to engine #1 "N2 speed card" Channel 1. N2 > 50%, N2 > 52%, N2>83% data is used for airplane system logic.)
ENG 1 SPEED SENSOR 2 (As above, Channel 2))
ENG 2 SPEED SENSOR 1
ENG 2 SPEED SENSOR 2
EEC CH A RESET/INHIBIT 1 (73-21-15) EEC Hardware reset
EEC CH A RESET/INHIBIT 2

Excessive aft CofG

The original design of the 777 was for the A market: 7,200 to 9,630 km range. When the longer range versions were introduced, the potable water system and lavatory waste system capacities were increased by an additional 383 litres of potable water, and 513 litres of waste-tank capacity. The larger tanks are located in the rear cargo compartment area and have reduced the available rearmost cargo area to 9.77 m3. This is offset by increases in forward and aft cargo compartment capacity. The original tank was 33 metres aft of the nose. With the extended range 777, the new toilet tank is 55 metres aft of the nose. The effect of this larger tank and more rear location on the centre of gravity is significant.

On very long flights (such as China-London), as the toilet is used over time, the tank overfills and spills, and the CofG can go outside the aft limit. In cruise, operating near the aft CG limit, the download on the tail is minimized and angles of attack and drag are reduced which improves performance. However, moving the CG aft reduces the longitudinal static stability of the airplane. At low speeds, at the end of a very long flight when the waste tank is over full, waste has spilt into the rear cargo area, and the CofG is outside aft limits, the aircraft is difficult to control and is tail heavy; it is more likely to stall at very low speeds.

This had been a problem before, and the incident affecting Boeing 777-236, G-VIIC, in the cruise between Barbados and London. Approximately ten and a half hours into a scheduled flight from Barbados to London, a crew member noticed an unusual odour in the cabin. Two minutes later a toilet smoke warning sounded and smoke was seen emanating from beneath the door of toilet 'N'; located by the R4 door. Fire fighting equipment was gathered and the toilet door was partially opened revealing flames and smoke. A BCF fire extinguisher was discharged into the toilet compartment and the door closed. When the fire was confirmed as extinguished, the area was investigated. The source of the fire was found to be overflowing of the waste transfer pump due to the waste tank being too full and an electrical short circuit causing the fire.

Boeing have just issued a service letter 777-SL-38-009 which requires that for Boeing 777s after 8 hours flight duration, all the economy toilets except one for emergency use should be closed off to prevent overflowing until changes to the waste tankage system can be made.