The plane, on a flight from Beijing, crashed short of the runway when its engines failed to respond to the crew’s throttle commands for increased thrust. The jet pancaked into the ground about 1,000 feet short of runway 27L. Although all 152 persons aboard successfully evacuated the airplane, it was damaged beyond the point of economical repair and was written off. (See Aviation Safety & Security Digest, ‘Crash May Stem From Sustained Exposure to Extreme Cold Weather,’ and ‘Scavenged Ice Theory,’ archives, February 2008, and ‘More Fuel Testing & New Additives May Be Necessary,’ briefs archives, May 2008.)

It is the first B777 to be lost in an accident since the airplane entered service worldwide in 1995. As the AAIB said in a 4 September 2008 statement accompanying release of the 21-page preliminary report, “Extensive data analysis has revealed that not only has there never been a previous occurrence of this type on the Boeing 777, but also that this is the first known occurrence of this nature in any large transport aircraft.”

Although the press statement said “fuel freezing or waxing” has been eliminated as a causative factor, a detailed reading of the AAIB interim report suggests the following conclusions, with water as the culprit:

(1) The AAIB strongly suspects that an unknown (or possibly forgotten – reference their mentioning of B52 operations in cold weather) fuel condition led to a blockage in the fuel delivery system.

(2) The AAIB does not fully understand the details of how the blockage occurred.

(3) The AAIB does not think any other organization has a current understanding of how very cold fuel behaves, as evidenced by this statement:

“As the fuel temperature is further reduced, it reaches the Critical Icing Temperature, which is the temperature at which ice crystals will start to stick to their surroundings. When the fuel temperature reduces to approximately -18ºC (0ºF), the ice crystals adhere to each other and become larger. Below this temperature, little is known about the properties of ice crystals in fuel and further research may be required to enable the aviation industry to more fully understand this behavior.”

(4) The AAIB strongly suspects that aircraft type, operator, manufacturer (airframe and engines) are irrelevant to the root cause of this accident. That is, the accident could, and may, happen to any aircraft in similar operating conditions.

(5) Given point 4, this accident is extremely important to the industry, for both existing and future-design aircraft and their operation. Put another way, this accident – and the assured avoidance of recurrence – may be very expensive.

The AAIB interim report focuses on the Rolls Royce Trent 800 engines, and a specific device on the engine known as a Fuel Oil Heat Exchanger (FOHE). The FOHE serves the dual purpose of cooling the engine lubricating oil and, in the process, warming the fuel above 0ºC so that ice crystals do not affect the “downstream” components, mainly the low-pressure (LP) filter and the high-pressure (HP) pump (the stage at which the fuel pressure is raised and the fuel is atomized for injection through the burners inside the engine’s combustion chamber).

Through extensive testing, the AAIB found that restricting the fuel flow “upstream” of the HP pump caused the engine to respond in a similar way to that recorded during the accident. Water in the fuel, turned to ice crystals, was swept up in high concentrations and, at higher fuel flow rates and larger quantities of water, substantially blocked the FOHE with ice.

There is a caveat to this AAIB scenario that must be pointed out: how could so similar an ice-blockage event occur in two separate (left and right engine) fuel feed systems with only a minor time difference between them?

There seems to be no scenario or explanation that takes into account the actual occurrence – the slight delay of a few seconds in the engines rolling back to idle power, but the nearly identical rolled-back thrust on each (1.06 and 1.07 engine pressure ratio, or EPR).

A number of hypotheses come to mind:

• The fuel flow rates were slightly different, which may have led to the ice’s detaching behavior and subsequent blockage of each engine’s FOHE right at the time during landing that the throttles are advanced to thrust-compensate for the drag increment attendant to the landing gear and flaps being deployed.

• Another plausible explanation for the difference in timing could be attributable to one side of the fuselage being in sunlight and the other in shadow during the daylight part of the long flight from Beijing, leading to a cold-soak differential between the wings.

• A composite of these two factors.

• Water would become dissolved in the fuel or freeze in non-designed water-trap areas and no longer be detectable as water or ice. This would explain the (perhaps undeserved) reputation of the water detecting system on the B777 as being unreliable and inconsistent. There were center wing tank warnings of water on the previous two flights, as well as on the accident flight out of Beijing. The response to water-in-the-tank warnings was to drain the sumps (low points) during turnaround servicing. This course, self-evidently, would be totally ineffective if done after a long-haul high-altitude flight with the water still frozen, or with the melted ice redistributed as water by the refueling operation to areas well away from the tank low point sumping drain-valves.

Where might such an accumulation occur? Perhaps at the collector box, where the boost and transfer pumps normally reside. There it stagnates, and because it is in “still waters,” unstirred and continually exposed to the super-cooled metal surface, freezing over a lengthy period into a mass of ice is more likely. The larger the mass, the more slowly it would melt, detach and move into the fuel-flow on descent. Differences in ice quantity and shape between the port and starboard accumulations would explain the timing difference between the port and starboard engine events.

From the report, “ice” accumulation was likely to have interposed between the low pressure pump and the FOHE. The solution to such blockages may be to move the FOHE closer to the low pressure pumps. This course would involve a lot of re-plumbing. It would be interesting to know the difference between manufacturers – Rolls Royce, General Electric, Pratt & Whitney – for fuel line diameter and length between the low pressure pump and the FOHE.

The AAIB report states that military aircraft use a fuel additive (FSII or Fuel System Icing Inhibitor) that lowers the freezing point of water in fuel. The additive was introduced in the military following several B-52 bomber accidents when engine fuel filter icing led to restricted fuel flow and subsequent engine rollbacks and flame outs. FSII is approved for use on the B777, and is supposed to be between 0.10% and 0.15% by volume of the fuel. It may not be used because the additive adds nothing to the calorific value of the fuel, vital for maximum air nautical miles per pound of fuel.

Longer-term design solutions, in consultation with the engine manufacturers, may be to have fuel-stirrers installed. Another design modification would feature multi-ported pump inlets, so that any one blockage would be inconsequential.

Fuel warming would also be a sane use of the otherwise wasted oxygen byproduct of the nitrogen enriched air (NEA) inerting process being proposed for airliner fuel tanks. The hot oxygen could be kept separate from the cold fuel in much the same way that engine oil is kept separate in the FOHE. Failing all else, diverted engine bleed air could be used in this fashion to stop the temperature within tanks from falling to critically low levels. Indeed, given the alternative – prohibiting flights at ultra low temperatures and forcing long range airplanes to operate at much lower inefficient cruise levels for safety reasons – fuel warming could prove the most economic solution.

The AAIB issued three recommendations as part of its interim report:

Safety Recommendation 2008-047: For the Federal Aviation Administration (FAA), the European Aviation Safety Agency (EASA), Boeing and Rolls Royce to reduce the risk of constrained fuel flow resulting from ice.

Safety Recommendation 2008-048: For the FAA and EASA to look at other airframe/engine combinations where icing in fuel may be a hazard.

Safety Recommendation 2008-049: For the FAA and EASA to review certification standards. As the AAIB said, “(T)he current requirements do not appear to address the scenarios identified during this investigation, such as the sudden release of accrued ice, which could lead to a restricted fuel flow.”

Once again, the standards against which airplanes are designed and built seem to be too restricted.