boeing 777 g-ymmm london heathrow 17 jan ’08

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Boeing 777 G-YMMMBoeing 777 G-YMMM

London HeathrowLondon Heathrow

17 Jan ’0817 Jan ’08

BRIEFING TO IASCC 9 September 2010BRIEFING TO IASCC 9 September 2010

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LHR - BEI – LHRFuel 79,000 kg (No 3 Jet Fuel, PRC)

3 flt - 13 cabin - 136 pax

Uneventful flight ( Min temps: SAT -76ºC; TAT -45ºC; Fuel -34ºC )

TOD predicted fuel 10,000 kg

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Thrust Levers

EPR actual and commandFMV (QAR)

Fuel Flow

1,600 ft agl to TD4 sec grid spacing

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PALT

Min Fuel Temp in cruise -34 deg C

TAT

Accident Flight Track from FDR

Takeoff fuel temp was -2 deg C

Fuel temp at 590 ft agl was -22 deg C

FDR Data

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Accident Site

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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.

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Fuel system description

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Reconstruction of Left Wing Fuel System

Engine

Centre tank

Main tank

Strut pipes

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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

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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.

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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.

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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.

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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.

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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.

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Beaker Tests

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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

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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

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FOHE fitted to fuel rig

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Ice collecting on pump inlet screen

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Icing of FOHE

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Aircraft fuel pipes

Strut pipes

LP pump

Fuel pipes in main tank

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Fuels Lab Test #156Tube Inspections (Cont.)

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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.

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Data Mining"the extraction of previously

unknown, and potentially useful information from significant

quantities of data“

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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

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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

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The accident flight WAS NOT unique with respect to fuel temperatures

experienced during takeoff, cruise or approach phases

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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

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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 pph3.Fuel Temperature during approach less than -15°C 4.Max Fuel Flow greater than 10,000 pph during

approach5.Max Fuel Flow during descent less than 10,000 pph

ONLY MMM ACCIDENT FLIGHT MET ALL 5 CRITERIA FROM ~13,000 FLIGHTS.

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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

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Testing showed:

• Ice can accrete on:– Fuel tank surfaces– Boost pump Inlet screen– Pipework – both rigid and flexible– Valves within the fuel system

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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

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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?)

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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

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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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Questions?