The investigators may wish to focus on the waxing in fuel that can occur above the temperature at which the fuel freezes, which can lead to disrupted fuel flow and loss of engine thrust.

To quote from the Special Bulletin (S3/2008) put out by the UK’s Air Accidents Investigation Branch (AAIB):

“In view of the sustained interest within the aviation industry, and amongst the traveling public, it is considered appropriate to publish an update on the continuing investigation into the accident involving a Boeing 777, G-YMMM [the registration of the British Airways jet] …

“The reduction in thrust on both engines was the result of a reduced fuel flow and all engine parameters after the thrust reduction were consistent with this. …

“The high pressure (HP) fuel pumps from both engines have unusual and fresh cavitation damage to the outlet ports consistent with operation at low inlet pressure. The evidence to date indicates that both engines had low fuel pressure at the inlet port to the HP pump.”

In other words, the HP pumps “puckered up” trying to suck paraffin globules of high viscosity and low fluidity. This is a point of interest that could prove quite conclusive, and could lead one to ask whether there is any real operational relevance to fuel freezing temperatures vis-à-vis the higher temperatures at which fuel “waxing” will occur, which quite logically would disrupt fuel-flows to both the B777’s engines’ high pressure fuel systems. The evidence of this disrupted fuel-flow is provided by the cavitation damage to both engines’ HP fuel pumps caused by the low pressure system’s inability to supply fuel at the demanded rate.

Significantly, the AAIB bulletin seems to rule out fuel freezing, with the rather awkward phrasing, “The specified fuel freezing temperature for Jet A-1 [fuel] is not above -47˚C.” Although the temperatures during the flight were unusually low compared to the average, they were not exceptional, and the lowest total air temperature (TAT) recorded during the flight was -45˚C.

Despite the rather defensive claims that the high quality fuel was above its freezing temperature, this latest AAIB release may bring some clarity to the situation aboard the British Airways jet. The pilots put their complete faith in monitoring the TAT, which explains why they remained at 39,000 feet while just about every other crew on that route on that day took the more prudent action of descending to warmer ambient temperatures. The eventual proof may likely be in the wax pudding that filled the B777’s fuel lines.

This problem of “paraffinization” is not unknown to the oil drilling industry, particularly regarding deep offshore wells and the cold temperatures they experience at depth.

The problem of waxing in aviation fuels has also been mentioned in the past, notably an Airworthiness Notice issued in 1994 by Transport Canada (see box below).

Note also that the fuel-flows in cruise at height would be quite low (and quite static) and the problem of waxing might not show up until the auto-throttle demanded much higher engine-accelerating fuel-flows in final approach – once the Flight Management Computer System (FMCS) sensed the extra drag of landing gear and flap deployment at low altitudes on approach.

While the average freezing temperature of aviation fuels is -47˚C (-53˚F), tests showed that the fuel used on the British Airways jet does not turn to ice until -57˚C (-71˚F). Tests also found that the fuel temperature throughout the flight theoretically never dropped below -34˚C (-29˚F). Is it possible for cold-soaked fuel to start depositing wax on the interior of fuel feed lines, in much the same way that plaques gradually foul and constrict blood flow in your arteries?

During the cruise phase of flight, a high fuel flow rate may prevent this constriction from occurring. But what about for descent, when the engines are throttled back, and a lower fuel flow rate results. According to Bernoulli’s law, reduced velocity of the fluid equates to higher effective pressure on the sidewalls of the fuel pipes. Cold-soaked fuel may start building up somewhere along the inside of a low pressure fuel feed line, hence the restriction in flow. After the fact of the restricted fuel flow, wax buildup melts away, and the evidence vanishes.

Given the problem with cold fuel, reader John Archer wondered if some sort of fuel heating system is needed. “I was surprised to note that, given the very low temperatures jetliners can experience, fuel lines are not heated as a matter of course,” he wrote.

To this, contributing editor John Sampson replied:

“To beat the fuel-cooling, the fuel would have to be heated en masse (i.e., in its totality in the tank, particularly the center wing tank). Waxing will otherwise defeat flowability through filters, valves and pumps. It’s all about the totality of the calorifics involved, and the engines just don’t provide enough bleed-air based heat to do the job, particularly when at low power and on a constant descent approach (CDA), which applied at Heathrow. Of note, the times when the greatest heating is required are often those times when the engine is not operating at maximum, such as during a lengthy descent at idle from high altitude.

“Fuel System Icing Inhibitors (anti-icing additives, or FSII) reduce the freezing point of water precipitated from jet fuels due to cooling at high altitudes. However, emulsification due to other additives (biocides, wax depressants, etc.) ‘separating out,’ and a consequent change in flight of the physical properties of the constituents of the fuel may not be completely understood. Heating the fuel with electrical heating elements would require a huge generator load, although that’s one possible solution for keeping the fuel ‘together’ at tolerable temperatures or at least slowing the fuel’s cooling during lengthy enroute exposure at well below sub-zero ambients.

“Alternatively, a heating system using the oxygen cast off by an onboard nitrogen generating system for inerting the ullage space in the fuel tanks, combined with a small amount of fuel, the mixture ignited in a burner jacket, could provide a heating system. Such fuel heaters were quite popular for cabin heating in the aircraft designs of the 1940s and 1950s. The P-2 Neptune, which I flew, had one for heating both the cabin and the empennage.

“Or, utilizing an auxiliary power unit’s (APU’s) electrical and/or diverted jet exhaust output might also be a possible solution, particularly as modern APUs are being designed to operate at heights near their aircraft’s operational ceiling.

“Think of the problem in a similar manner to a large capacity water reservoir embedded in frozen tundra. Heating may stop pipes from bursting downstream; however, if the source is icy slush (or frozen water), the pipe-flows will still be inhibited. Most jetliners now have fuel/oil heat exchangers to heat the fuel with hot engine oil, but they can still be defeated, as this accident demonstrates, by very low fuel temperatures and cold-soaking over extended periods.”

The AAIB bulletin says, “(W)ork has commenced on developing a more complete understanding of the dynamics of the fuel as it flows from the fuel tank to the engine.” This line of investigation should lead to a better appreciation for paraffin precipitating out of cold fuel and gumming up pipes and pumps. As maintained here, the problem isn’t one of fuel freezing but of the emulsification (and higher viscosity) of fuel waxing, leading to “stalled out” and cavitation-damaged pumps.

(For more on waxing in aviation fuels, see brief, this issue, titled 'More Fuel Testing & New Additives May Be Necessary'; for an informative dissertation on the problems of waxing in diesel fuels, see http://extension.usu.edu/files/publications/factsheet/FM-16.pdf)