Flight VS400 - Virgin Atlantic Airways. Airbus A340-642 (A346). 4 Jan 2013. London, Heathrow (LHR) to Dubai (DXB).
The incident is under investigation within the airline, and not yet by AAIB. There were no injuries or damage reported and it is not a serious incident because the circumstances did not indicate that an accident nearly occurred. The data from the FDR has been analysed and the circumstances are that operations were normal as the aircraft was flying normally over Romania, 37,000' 493 kts, track 126o. Across the Black Sea it turned to track 120o and on entering Turkish airspace it accelerated to 517 kts and rapidly climbed to 39000' before entering an incipient high speed stall. The aircraft recovered without incident and there were no injuries or damage.
The investigation is looking closely at the thronomister which failed early in the flight, leading to excessive aft C of G after the rearmost toilet waste tank overflowed, and it is thought that this caused the sharp climb with increased angle of attack and incipient high speed stall.
The original design of the A340-300 was a fuselage length of 63.7m and 335 pax. When the longer fuselage -600 versions were introduced with 75.3m length and 380 pax, 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. All the tanks are now located in the rear cargo compartment area and this has reduced the available rearmost cargo area to 9.77 m3. This is offset by increases in forward and aft cargo compartment capacity as the forward tanks were removed. The original tank was 33 metres aft of the nose. With the A340-600, the new toilet tanks are 70 metres aft of the nose. The effect of the extra tanks and rearwards location on the centre of gravity is significant.
Waste flow to the tanks is controlled by the thronomister, which maintains aircraft balance relative to the Centre of Gravity by directing waste flow to an appropriate tank and controlling the pumps. 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. The thronomister is controlled by the FMS to keep the CG near the aft limit. However, moving the CG aft reduces the longitudinal static stability of the airplane. The failure of the thronomister meant that the aft-most waste tank was over filled and waste spilt into the rear cargo area. The CG moved outside aft limits, the aircraft was difficult to control and tail heavy, and this led to the sharp climb with increased angle of attack and incipient high speed stall. The crew recovered the aircraft after it had climbed 2'000'.
This had been a problem before, with an incident affecting Airbus A340-642, ZS-SNE, in the cruise on a flight between JFK and JNB. Approximately ten and a half hours into the scheduled flight, 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 in the rear cargo area causing the fire. The overflow was again due to a thronomister failure.
The thronomister is being redesigned. Airbus have just issued a service letter 340-SL-03-2013 which requires that for Airbus A340-600s after 6 hours flight duration, all the rear economy toilets should be closed off to prevent overflowing until the revised thronomister is installed and changes to the waste tankage system made.
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