Slide 6 Aircraft examination No pre-existing defects found with electrical, hydraulic, autoflight, navigation systems or flying controls HIRF/EMI eliminated by testing – the power levels required to affect the EEC would have affected the electrical, navigation and communications system first.
Slide 8 Reconstruction of Left Wing Fuel System Engine Centre tank Main tank Strut pipes
Slide 9 Engine - HP pump cavitation marks Fuel Pump 0019 (LH Engine) –Built Oct 99, Never overhauled –A/C Boost pump debris on Impellor –Abnormal cavitation markings on bearing thrust faces and HP housing at discharge window
Slide 10 Types of water in fuel Dissolved water –Molecule of water attached to a hydrocarbon molecule. As temperature drops becomes entrained water. Undissolved water –Entrained water, often referred to as suspended. Suspended as tiny droplets in the fuel settles to form free water. –Free water Visible water that collects on bottom of tanks.
Slide 11 Water ice in fuel Only entrained and free water form ice. Ice crystals form at -1 to -3 Deg C. –Density similar to fuel, so float in fuel. Critical Icing Temperature ~ -8 Deg C. –When ice crystals start to stick to their surroundings. -18 Deg C –Crystals adhere to each other and become larger. Below -20 Deg C little is known about the properties of the ice.
Slide 12 Fuel testing Fuel was of good quality. Fuel freezing point was -57 Deg C. Compared with 1,245 batches of Jet A-1 tested in UK during 2007. –Distillation range average. –Freezing point slightly below average.
Slide 13 Estimated water content in fuel during accident flight Dissolved water, 3 ltr (40ppm). Undissolved water (entrained and free), maximum of 2 ltr (30 ppm). This Water would have been evenly spread across the fuel system at the start of the flight. Water also introduced through the vent system during the flight, approximately 0.14 ltr. Plus any water remaining from previous flights.
Slide 14 Testing by Boeing Beaker Test –Small scale test in climatic chamber. –Used simulated fuel system components. –Establish how ice might accumulate and restrict flow. Fuel rig testing –Actual components from B777. –Establish if ice could build up in the system and restrict the flow. –Use fuel preconditioned with water or inject water directly into boost pump inlet.
Slide 16 Significant temperatures Water ice forms (-1 to -3C, 31 to 27F) Sticky range (-8 to -20C, 23 to -4F) Ice starts to adhere to metal (-9C, 16F) Ice at most stickiness (-12C, 10F) Ice adheres strongly to metal surfaces (-17C, 0F) Ice takes on a more crystalline appearance below -20C, (-4F) Ice lacks the properties to bridge orifices (-25C, -23F) Spontaneous formation of ice crystals from super cooled water (-24C, -11F) Lowest temperature super cooled water can exist in aviation fuel (-51C, -60F) 0C -50C
Slide 17 Spar Valve Boost Pump FOHE Sight Glass Inlet Screen Flexible Hose Sight Glass LP/HP Pump Boeing Proprietary Layout of fuel Components on the Boeing Fuel Rig
Slide 21 Aircraft fuel pipes Strut pipes LP pump Fuel pipes in main tank
Slide 22 Fuels Lab Test #156 Tube Inspections (Cont.)
Slide 23 Findings of rig test Ice can accrete on the inside of fuel pipes and on inlet screens. –Thickness depend on fuel temp and flow. It is possible to restrict the flow through the FOHE with cold fuel and low levels of water simulating a sudden release of ice. Blocks of ice unlikely to have caused restriction. Problems with repeatability of some of the results.
Slide 24 Data Mining "the extraction of previously unknown, and potentially useful information from significant quantities of data
Slide 25 Facts from the accident flight Fuel temperature at takeoff -2 degC Minimum fuel temperature in the cruise - 34 degC Minimum TAT in the cruise -45 degC Fuel temperature on final approach -22 degC
Slide 26 BA/United/Cathay ~60,000 flights (Apr06 to Mar 08) Minimum fuel temperature, -12 deg C and below G-YMMM -34 Fuel -45 TAT Fuel Temp TAT
Slide 27 The accident flight WAS NOT unique with respect to fuel temperatures experienced during takeoff, cruise or approach phases
Slide 28 Through the complementary use of data mining and laboratory tests, efforts were focused on the activity of two parameters: Fuel Flow and Fuel Temperature The following slide identifies the combination of Fuel Flow and Fuel Temperature parameters which were unique to the accident flight
Slide 29 1.Fuel Temperature at take off below 0°C and remaining below 0°C during all phases of flight 2.Max Fuel Flow in cruise less than 10,000 pph 3.Fuel Temperature during approach less than -15°C 4.Max Fuel Flow greater than 10,000 pph during approach 5.Max Fuel Flow during descent less than 10,000 pph ONLY MMM ACCIDENT FLIGHT MET ALL 5 CRITERIA FROM ~13,000 FLIGHTS.
Slide 30 Investigation Summary Engines rolled back due to reduced fuel flow with no increase although FMV opens fully. No technical problem with the aircraft or its systems that could explain the event Only physical evidence – HP pump cavitation Reasons for HP pump cavitation – a restriction of the fuel flow to the pump
Slide 31 Testing showed: Ice can accrete on: –Fuel tank surfaces –Boost pump Inlet screen –Pipework – both rigid and flexible –Valves within the fuel system
Slide 32 Testing also showed The FOHE can become blocked when water is introduced to cold fuel creating a snowball The effect of the blockage changes at different fuel temperatures & fuel flows (above certain temperatures and below certain fuel flows, the FOHE can successfully melt the ice) The system needs to be ~95% blocked to cause the reduced fuel flow Ice can accrue within the airframe fuel system and be released, dependent on fuel temperatures and flow rates
Slide 33 Summary Fuel flow restricted due to ice formed from water that was naturally occurring in the fuel The ice accreted over a long period, with low fuel flows whilst the fuel temperature was in the sticky range The ice was suddenly released, probably due to demands for higher fuel flow during final approach, but could be due to other factors such as increasing temperatures or turbulence The sudden avalanche of ice blocked the FOHE, which was unable to melt it G-YMMM was always within its certificated operating envelope and there was no evidence of abnormal water quantities in the fuel No tests for this threat existed at the time of certification (and will they in the future?)
Slide 34 AAIB Safety Recommendations. These included: Interim flight crew procedures to clear accumulated ice at a safe altitude Modifying the FOHE to resist this unforeseen threat (already underway by Rolls-Royce) Investigating other airframe/engine combinations for vulnerability to this phenomenon Changing certification requirements to accommodate the new threat
Slide 35 Safety Recommendation 2009-032 – Issued 12 March 2009 It is recommended that the Federal Aviation Administration and the European Aviation Safety Agency jointly conduct research into ice accumulation and subsequent release mechanisms within aircraft and engine fuel systems.
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