Friday, September 05, 2008

Missing the implications?

In May last, we ran a report on how global cooling had possibly caused the crash of a Boeing 777 at Heathrow in January of this year.

Now, the interim report of the British Air Accidents Investigation Branch has been published, reported widely in the media, including The Guardian, which has produced as good a summary as any.

The official report rules out any idea of the fuel freezing in the tanks, due to the exceptional cold (as was originally postulated), and instead prefers an unprovable theory of ice forming in the fuel lines – a phenomenon which could occur at very much higher temperatures than those to which this aircraft was exposed.

By contrast, in the original report we cited, reference was made to the exceptionally low temperatures encountered, down to -76° Celsius – "the coldest ever experienced" – while the freezing point of the fuel used in the 777 was -57°C (The report records a minimum of -73°C static air temperature).

Strangely enough, in entirely different circumstances, I have had personal experience of the effect of ice while flying. Tooling around North Yorkshire in a Jet Provost one day – as one does – emerging from a high-speed aerobatic manoeuvre, I found that the trim wheel had jammed. Pushing hard to release it, it suddenly freed itself and spun most of the way forward, then locking in position, totally unmovable.

For those unused to the technicalities of flying, the trim wheel is used to balance the controls, allowing it to be flown in level flight with fingertip pressure. The trim nearly fully forward, it now took some considerable physical effort to keep the aircraft in level flight and prevent it plunging to earth.

However, after orbiting the field for some time at low altitude, the trim freed itself and I made an uneventful landing. To my potential embarrassment, though, a minute technical inspection showed up no faults and for a while my CO was deeply suspicious that I had invented the whole incident.

To my rescue came the engineering officer, who surmised that ice must have built up round the trim cable and pulley, jamming it in place, it having become free when the ice had melted at low altitude. That too was unprovable, but entirely plausible, and the incident went no further.

One would hesitate to cast aspersions at the current investigation report, which concludes that "the data indicates (sic) that the fuel did not reach a low enough temperature to cause the fuel to wax during the accident flight." This, however, is at such variance with the original findings we reported, that we could be forgiven for wondering whether "ice" in this case – conveniently melted after the accident, as inconveniently as had mine – gave the investigators the opportunity to play down the effects of extreme cold conditions, which may become the norm and have implications for the whole of commercial aviation.

While the investigators do acknowledge that their findings might have lessons for other aircraft types, it is unarguable that dealing with an ice problem is far easier (and cheaper) than having to address the implications of extremely low temperatures, sufficient to freeze aircraft fuel.

That said, the British Air Accidents Investigation Branch has unparalleled expertise and a world-wide reputation for technical excellence, but their personnel would not be the first to have difficulty coming to terms with the implications of global cooling.