AIRCRAFT RELIABILITY AND MAINTENANCE

Boeing 777-236; G-YMMM

History

Following an uneventful flight from Beijing, China, the aircraft was established on an ILS approach to Runway 27 L at London Heathrow. The aircraft was correctly configured for landing on runway 27 L and both the autopilot and the auto throttle were engaged. Initially the approach progressed normally until the aircraft was at a height of approximately 720 ft and 2 miles from touchdown.  

The auto throttles commanded an increase in thrust from both engines and the engines initially responded. However the thrust of the right engine reduced to approximately 1.03 EPR (engine pressure ratio) and then 7 seconds later the thrust on the left engine reduced to 1.03 EPR. 

The reduction in the in thrust on both engines was a result of a less than commanded fuel flows and all engine parameters after the reduction were consistent with this. 

Systems on the aircraft that monitor parameters such as the FDR (flight data recorder) and QDR (quick data recorder) indicated that the engine control system detected the reduced fuel flows and commanded the FMVs (fuel metering valves) to open fully. The FMVs responded to this command and opened fully with no appreciable change in the fuel flow to either engine.

The aircraft then descended rapidly and struck the ground, some 1,000 ft short of the paved runway surface, just inside the airfield boundary fence. The aircraft stopped on the very beginning of the paved surface of Runway 27 L. 

During the short ground roll the right main landing gear separated from the wing and the left main landing gear was pushed up through the wing root. A significant amount of fuel leaked from the aircraft but there was no fire. An emergency evacuation via the slides was supervised by the cabin crew and all occupants left the aircraft, some receiving minor injuries.

Accident overview 

Furthermore the aircraft had previously operated a flight on 14 January 2008 from Heathrow to Shanghai, with the return flight arriving on 15 January 2008. The aircraft was on the ground at Heathrow for 20 hours before the departure to Beijing on the 16th January 2008. Prior to these flights the plane had been in maintenance for two days. During which the left engine EEC was replaced and left engine ground runs carried out.

Fuel Pathway

The fuel on the Boeing 777-200ER is stored in three fuel tanks: a centre tank, a left main tank and a right main tank. The centre tank contains two override/ jettison pumps (OJ) and each main fuel tank contains two boost pumps which can be identified as forward and aft. 

Each of the pumps inlets is protected by mesh screen and the pumps are also equipped with a check valve fitted in the discharge port, to prevent fuel in the fuel feed manifold flowing back through the pump. 

A pressure switch, mounted between the pumps impellor and check valve, monitors the fuel pressure and triggers a warning in the flight deck if the pressure rise across the pumps and drops to a value between 4 and 7 psi.

Override/ Jettison Pumps

The fuel feed manifold runs across the aircraft and connects to the engine fuel feed lines. The manifold is split between the left and right system by two cross-feed valves. When these valves are closed, and the centre tank is the source of the fuel, the left jettison pump feeds the left engine and the right jettison pump feeds the right engine.

Spar Valves

In the fuel manifold provide a means of shutting off the fuel supply to the engines, and are controlled by the engine run/ cut-off switches. The spar valves also move to the closed position when the fire switch is operated.

Fuel Tank

To prevent large amounts of free water building up in the fuel tanks the aircraft is fitted with water scavenge system that uses jet pumps operated by motive flow from the over-ride jettison pump and boost pumps. 

The jet pumps draw fluid from the lowest sections of each tank and inject it close to the inlet of each aft boost pump and both override jettison pump inlets. Moreover each tank is vented to atmosphere through channels in the roof of the fuel tanks, which are connected to surge tanks mounted outboard of each of the main tanks.

Low Pressure Pump

The airframe fuel system supplies fuel to the low pressure engine driven pump, this raises the fuel pressure (and fuel temperature slightly) and pumps the fuel through a fuel/oil heat exchanger. Moreover the lower pressure pump is also there to prevent cavitation's occurring in the high pressure pump.

Fuel Filters

The fuel is filtered by a high and low-pressure filter. If the first (low-pressure) filter becomes clogged with contaminates, fuel will bypass the filter allowing contaminated fuel to enter the control unit. The purpose of the second (high-pressure) fuel filter located downstream of the fuel flow governor is to catch debris from a deteriorating second stage (high-pressure) fuel pump.

Fuel Pathway; Boeing 777,

Rolls Royce Trent 800 Fuel System

Fuel/Oil Heat Exchanger

This serves the dual purpose of cooling the engine lubricant and raising the temperature of the fuel. It does this by preheating the fuel from the heat of the engine oil as it flows through the fuel/oil exchanger. Moreover it prevents ice occurring and affecting the downstream components, including the low pressure filter.

High Pressure Pump

After the low pressure filter, the fuel travels to the high pressure pump where its pressure is raised higher to the values needed for injection through the burners in the combustion chamber.

Fuel Metering Unit

The high pressure fuel is ported into the fuel metering unit, where it contains a fuel metering valve. This regulates a fuel flow to match a thrust demand as dictated by the thrust lever position. The electronic engine control trims the metered fuel to prevent over boost when operating at or near the thrust limits.

The fuel from the fuel metering unit is routed to the burners via a flow-meter and a relatively coarse high pressure strainer.

Fuel waxing

The freezing point of aviation turbine fuel is established by cooling the fuel until wax has formed and then warming the fuel until the last crystal of wax is seen to disappear. It can also be described as a jelly like substance.

Following the accident, 66 fuel samples were taken from the aircraft and the engines. A number of these samples were tested and critical properties such as freezing points, density, flash points, viscosity, contamination, fuel additives and presence of water were tested. The outcome of the test showed that the fuel samples complied fully with the fuel specification for Jet A-1 as well as passing the test for quality assurance certificate.

The freezing point of the fuel sampled from G-YMMM was measured using both automatic and manual test. 

Neither test could detect any wax crystals in the fuel at temperatures warmer than -57⁰C (the freezing point of the fuel sampled from G-YMMM). The Boeing 777 also has a fuel temperature probe located in the inboard section of the left main tank. After examining the data the conclusion was that the data indicates that the fuel did not reach a low enough temperature to cause the fuel to wax during the accident flight.

 

Moreover after further investigation of this accident the design of the fuel/oil exchanger for the Trent 800 was changed to prevent ice accumulation on its inlet face to ensure fuel flow is not restricted to achieve commanded thrust. The re-design involved removing the fuel tube that acted like ridges/grooves on the inlet face of the fuel/oil heat exchanger and be replaced with a smooth surface to prevent dumped ice building up on the fuel tubes.

Static and total air temperature

The total air temperature (TAT) is a measure of the air temperature at the point the air is brought to rest relative to the aircraft’s forward motion (‘stagnation point’). It is always higher than the static air temperature (SAT) when the aircraft is in flight. The Boeing 777 has two TAT probes, one located on each side of the nose section of the aircraft just below the rear-most cockpit window. The Air Data Inertial Reference System calculates the SAT from the temperature provided by the TAT probes, Computed Air Speed (CAS) and Mach number.

Furthermore on long flights the temperature of the fuel in the main wing tanks will tend towards the temperature of the boundary layer around the wing, which can be up to 3⁰C than that of TAT (total air temperature).

On the accident flight the minimum TAT was -45 C. Due to the position of the centre fuel tank, the temperature of the fuel in this tank is warmer than the fuel in the main tanks.

Combination of Water and Fuel

Water is always present, to some extent in aircraft fuel systems and can be introduced during re-fuelling or by condensation from moist air which has entered the fuel tanks through the tank vent system.

The water can take the form of:

Dissolved Water

This occurs when a molecule of water attaches itself to a hydrocarbon molecule. As the fuel is cooled the dissolved water is released and takes the form of either entrained water or free water.

Entrained Water

Is when water that is suspended in the fuel as tiny droplets and can, with time, settle as free water?

Free Water

This takes the form of droplets, or puddles, which collect on the bottom of the fuel tanks or in stagnation points within the fuel delivery system.

It is estimated that the fuel loaded at Beijing would have contained up to 3litres or 40 parts per million of dissolved water and a maximum of 2 litres or 30 parts per million of entrained or free water. Water will always be present in aviation turbine fuel at 35 to 40 ppm the total water content measured in the fuel samples taken from G‑YMMM was similar to that in the samples taken from another B777, G‑YMMN.

Icing inhibitors

Fuel system icing inhibitors is a fuel additive that, when used in concentration of 0.10-0.15% by volume can prevent the formation of water ice down to a temperature of -40^oC.Icing inhibitors are not commonly used in large public transport aircraft as they do not require them as they have fuel heaters already on board. Moreover airlines will find that it is more cost effective to have heaters as icing inhibitors are more expensive.

Fire Fighters

It was not possible to establish the condition of the fuel in the centre tank at the time of the accident as it had been contaminated with fire fighting foam/water.

The technique that was used to disregard the foam/water that was sprayed by the fire fighters was the Karl Fischer test. This test uses a chemical method to establish the total amount of both dissolved and entrained water in the fuel. This test was carried out on the samples taken from the left main tank sump, the APU fuel line and the right engine variable sector (samples that had been still sealed in the tank or uncontaminated by the foam).

The Karl Fischer test indicated the total amount of water in the samples, dissolved and entrained was below 40 parts per million which is a very low level. 

Furthermore if required they could have tested the fuel at Beijing and run some test on the fuel to check for quality and contamination.

Sumping

Sumping is when the amount of free water is controlled by regularly draining the water out of the fuel tank sumps.

G-YMMM was last sumped at London Heathrow two days prior to the accident. Moreover the aircrafts fuel tanks had also been sumped at London Heathrow whilst on maintenance on 14^th January 2008 (three days prior to the accident).

Sumping is best carried out in warm hangers as any ice present in the fuel tanks would have melted and migrated to the drains. The drain valves would be clear (free of ice) when the fuel is warm, the flow of fluid through the drains will also be quicker.

Experiment by Rolls Royce and Boeing

As part of the investigation the manufactures under the direction of the AAIB undertook small scale fuel testing in a climate chamber and full scale testing on an adapted fuel rig.

The aim of the two tests I will mention in the next paragraph was to establish if ice could build up with the fuel delivery system and cause a restriction of the fuel flow. The tests were carried out using either preconditioned fuel with a known quantity of water, or injecting quantities of ice or water directly into the boost pump inlets.

The Beaker test was a small scale test to establish the behaviour of water when introduced into cold-soaked fuel. The test used a number of simulated fuel systems component to see how ice might accumulate in a fuel system and restrict the fuel flow.

The Fuel rig testing was the full scale testing and consisted of a storage tank containing 3520litres of Jet A fuel that could be cooled to -40^oC as well as all the components in the aircraft fuel system. The flexible fuel feed pipes from the G-YMMM were also fitted to the rig.

Moreover to validate how engine reacts to a restricted fuel flow, two other test facilities were used: firstly a systems test facility (STF), and secondly a Trent 800 engine mounted in a fully instrumented engine test cell.

The STF provided valuable data, particularly concerning the manner in which the EEC reacts to the FMV moving to fully open and the fluctuations in fuel flow and burner pressure.

Rollback

Rollback is an un-commanded reduction of engine thrust and is the primary cause of the accident.

The factors I think that would have caused roll back was originally due to engine surge.An engine surge is normally caused by the fuel management system.

In the report it had emphasised that neither engine had surged. After examining the QAR it was found that the response of the electronic engine control unit (EEC) was considered to be quite explicable and no abnormalities were apparent.

Therefore my original view on what would have caused rollback other than icing was proven to be mistaken as the EEC had followed the correct logic to avoid engine surge.

Cavitation

On examining the accident scene it was found that that the flight pumps (high pressure pump in engine) had cavitation marks. Tests were conducted on new pumps in an attempt to replicate these marks.

The test revealed that running the pumps with an abnormally low inlet pressure and restricted fuel flow of 5000ppm for 60 seconds gave identical marks to those seen on G-YMMM and thereby as a result of these conditions causing pitting of the pump vanes.

The cavitation marks (pitting of the pumps) were not an indication of a fault in the pumps but a symptom of either low pressure or fuel aeration and not have affected operation of the pumps.

Safety recommendations 

2008-047

It is recommend that the FAA and EASA, in conjunction with Boeing and Rolls Royce, introduce interim measures for the Boeing 777, powered by Trent 800 engines, to reduce the risk of ice formed from water in aviation turbine fuel causing a restriction in the fuel feed system. 

2008-048 

It is recommended that the FAA and EASA should take immediate action to consider the implications of the findings of this investigation on other certified airframe/engine combinations.

2008-049

It is recommend that the FAA and EASA review the current certification requirements to ensure that aircraft and engine fuel systems are tolerant to the potential build up and sudden release of ice in the fuel feed system.

Conclusion

The investigation has shown that the fuel flow to both engines was restricted. Most probably due to ice within the fuel feed system. The ice is likely to have formed from water that occurred naturally in the fuel whilst the aircraft operated for a long period.

This has also been back up by the cavitation marks found on the pressure outlet ports of the high pressure pumps on both engines. From testing and in service experience it is concluded these marks were fresh and therefore most probably occurred from the accident.

To prevent history from repeating itself I think airliners should invest in icing inhibitors; this opinion was formed after agreeing with the safety recommendation 2008-048.

As mentioned earlier in the report it can be more expensive and although there is fuel heaters on board more investigation need to be carried out on air frame / engine combination. And therefore to prevent anymore similar accident occurring again, it is worth investing icing inhibitors for the time being until more experiments are done.

Moreover further investigation on properties of fuel and water should be carried out so engine manufacturer are aware of how certain properties of fuel in different condition will affect the performance of the engine. By knowing the properties of fuel and how it behaves in different condition it can prevent engines from experiencing unfamiliar problems that may not have be known prior to a potential accident.

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