Boeing 777 G-YMMM
London Heathrow
17 Jan ’08
BRIEFING TO IASCC 9 September 2010
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Slide 1
LHR - BEI – LHR
Fuel 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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Slide 2
1,600 ft agl
to TD
4 sec grid
spacing
Thrust
Levers
EPR actual
and command
FMV (QAR)
Fuel Flow
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Slide 3
FDR Data
PALT
Min Fuel Temp in
cruise -34 deg C
Fuel temp at
590 ft agl was
-22 deg C
Takeoff fuel temp
was -2 deg C
TAT
Accident Flight Track from FDR
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Slide 4
Accident Site
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Slide 5
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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Slide 6
Fuel system description
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Slide 7
Reconstruction of Left Wing Fuel
System
Strut pipes
Engine
Main tank
Centre tank
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Slide 8
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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Slide 9
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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Slide 10
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 11
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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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Slide 12
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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Slide 13
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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Slide 14
Beaker Tests
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Slide 15
Significant temperatures
0C
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)
-50C
Lowest temperature super cooled water can exist in aviation fuel
(-51C, -60F)
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Slide 16
Layout of fuel Components on the
Boeing Fuel Rig
Sight
Glass
FOHE
Spar Valve
Boost Pump
Sight Glass
Flexible Hose
Inlet Screen
LP/HP Pump
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Boeing Proprietary
Slide 17
FOHE fitted to fuel rig
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Slide 18
Ice collecting on pump inlet screen
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Slide 19
Icing of FOHE
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Slide 20
Aircraft fuel pipes
LP pump
Fuel pipes in
main tank
Strut pipes
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Slide 21
Fuels Lab Test #156
Tube Inspections (Cont.)
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Slide 22
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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Slide 23
Data Mining
"the extraction of previously unknown, and
potentially useful information from significant
quantities of data“
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Slide 24
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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Slide 25
BA/United/Cathay ~60,000 flights
(Apr06 to Mar 08)
Minimum fuel temperature, -12 deg C and below
Fuel Temp vs TAT
10,00
0,00
-45
-40
-35
-30
-25
-20
G-YMMM
-34 Fuel
-45 TAT
-15
-10
-5
0
-10,00
-20,00
TAT
TAT
TAT1
-30,00
-40,00
-50,00
Fuel Temp
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-60,00
Slide 26
The accident flight WAS NOT unique with
respect to fuel temperatures experienced
during takeoff, cruise or approach phases
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Slide 27
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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Slide 28
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.
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Slide 29
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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Slide 30
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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Slide 31
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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Slide 32
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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Slide 33
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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Slide 34
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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Slide 35
Questions?
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Slide 36