Airbus A330-243 incident near Seattle, Washington, November 7, 2017

The NTSB's narrative final · quoted from the NTSB record

A confluence of four fuel system anomalies caused a blockage of a fuel filter in the engine control system of the Rolls Royce Trent 700 that led to the loss of control of the left-hand (No. 1) engine thrust during approach and landing. The Rolls-Royce Trent 700 features several protective fuel filters throughout the many fuel components that constitute the engine’s fuel system. An extensive teardown of the fuel system components of the No. 1 engine revealed a significant blockage of the Variable Stator Vane (VSV) Torque Motor (TM) supply port (SP) filter. The VSV TM constantly modulates the VSV actuators to match the rotational speed, altitude, temperature, and power requirements of the engine. Without this matching, the internal airflow of the engine will be disrupted resulting in multiple surges, overspeed, over temperature and incorrect fuel scheduling. The reason for the blockage of the VSV TM was not readily apparent; however, it was concluded that long-term use of fuel containing higher concentrations of sulphur compounds caused debris to adhere and accumulate to the upstream side of the VSV Control Unit Main Inlet Filter (MIF) element, with sulfate acting as the binding agent. Laboratory testing and analytical assessment of the fuel system determined the most likely sequence was that a larger-than-normal quantity of water was introduced into the fuel lines dissolving the water-soluble sulphate and releasing the MIF deposits/debris. The MIF debris particles flowed downstream to the partially blocked VSV TM SP filter, completely blocking it. The EEC increased the fuel flow to maximum fuel flow based on an erroneous VSV position, causing fireballs and fire streaks from the exhaust pipe and subsequent thermal damage to the wing. Only the forced closing of the wing spar valve stopped the exhaust fire. The reason for the larger-than-normal quantity of water to be introduced into the fuel was the omission by the maintenance provider to ‘sump’ the wing tanks after ten days of the airplane being stationary for unrelated scheduled service. Examination of other components of the fuel system revealed the presence of aluminum sulfate (alum) nodules. Since aluminum sulfate can only be dissolved in water and not fuel this implies the presence of water in the fuel. The four conditions that precipitated this event were. Water in the fuel: The most likely scenario was that the airplane wing tanks contained water that had accumulated and settled during the 10 days while it was undergoing maintenance and the maintenance provider failed to manage routine fuel tank sumping on the airplane as recommended by the manufacturer. One hour, prior to start 4,300 pounds of fuel was uploaded to the airplane. This was considered to be sufficient time for any water to settle back to the bottom. The engines are started with 120 liters of the original fuel in the engine fuel system components and lines. The fuel consumed during engine start, idle and taxi was estimated to be about 87 liters. The selection of takeoff power requires high VSV servo flow for 500 milliseconds to position the VSVs quickly in response to the power demand, thus drawing the high-water-content fuel from the tanks through the VSVC MIF, thereby liberating previously deposited material from the filter into the servo passages. As the takeoff power was achieved, low fuel flow to the servo allowed the liberated debris near the VSV TM supply port filter. During the flight, there is low servo fuel flow in the VSVC since there is not a large change in power; however, during landing approach, high servo flow was again commanded, and a ‘cloud’ of liberated debris almost completely blocked the VSV TM SP filter, creating a slow VSV response, leading to increased N2 and subsequent N1 speed increase upon reverse thrust selection. Aluminum Sulfate (alum) was found throughout the left engine fuel system and is unique to this event. Alum is not used in the aviation industry, and it is not likely that the alum was introduced into the fuel system by simple fueling contamination. Recent industry research on the quality of fuel distributed in the United States indicated that fuel distributed on the west coast can, in the long-term cause more sulfate precipitate in the fuel system and be dissolved by water than fuel distributed on the east coast. These studies also suggest that sulfates increase a chemical binding phenomenon that was observed in the VSV torque motor filter. The long-term use of west coast fuel may increase the precipitation of sulfate salts in fuel lines. Examination of the left engine main high-pressure fuel pump revealed cavitation erosion. The elemental makeup of metallic debris found in the VSV TM SP filter was very similar in composition to that of the main high pressure fuel pump parts. Ordinarily this fine material should pass easily through the filters (MIF mesh size is 45 microns (µ) and the VSV TM SP filters mesh size is 80 µ; however, there appears to be an unexplained process in which the fine fuel pump particles clump together, using the fuel breakdown products (sulfate) as a binder. Because the normally smooth engine internal airflow was disrupted due to the VSV slow response, the engine became internally stalled and unstable during approach. Due to the inhibited cockpit warnings during final approach, the pilots were not aware of the increasing internal engine airflow instability. During landing the EEC logic changes modes so that it controls the N1 spool; however, because of the airflow disruption from the mis-scheduled VSVs, the N1 spool did not respond in accordance with the reverse power demand. The EEC kept increasing fuel flow until it reached the maximum flow rate, causing excess fuel in the combustor and unburnt fuel to exit the tailpipe where it mixed with fresh air and became ignited, causing thermal damage to the airplane skin.

The complete narrative as the NTSB published it. The NTSB's docket holds the report as a PDF and any photographs, statements and other documents from the investigation.