Crashed BA 777 inquiry battles to understand fuel restriction

Flight International ^ | 15 May 2008 | David Kaminski-Morrow

[Yo-Yo]

UK investigators remain unable to explain the reduction in fuel flow that led to a British Airways Boeing 777-200ER losing power in both engines and landing short of the runway at London Heathrow in January.

The Air Accidents Investigation Branch has issued an update to the inquiry into the 17 January accident. Flight recorder information shows that the Rolls-Royce Trent 800s' engine control system detected the reduced flow and commanded fuel-metering valves to open fully. While the valve responded to this command, says the AAIB, there was "no appreciable change" in the fuel flow to either engine.

"Evidence to date indicates that both engines had low fuel pressure at the inlet to the high-pressure pump," it says. "Restriction in the fuel system between the aircraft fuel tanks and each of the engine high-pressure pumps, resulting in reduced fuel flows, is suspected."

Detailed examination of the fuel system and pipes, the AAIB points out, has found no unusual deterioration or physical blockage.

Boeing is concentrating on attempting to replicate cruise-altitude environmental conditions experienced by the Beijing-London flight at it passed over Siberia, to test a representation of the twinjet's fuel system.

The aircraft passed through a region of particularly cold air between the Russian Ural mountains and eastern Scandinavia, although the fuel temperature did not fall below -34°C (-29°F). The fuel on board had a measured freezing point of -57°C.

"These tests are collectively aimed at understanding and, if possible, replicating the fuel system performance experienced on the day and the potential for formation of restrictions," says the AAIB, which adds that work has begun "on developing a more complete understanding of the dynamics of the fuel as it flows from the fuel tank to the engine".

There is no evidence, it says, of core engine icing or any anomalous behaviour in the aircraft or engine systems that might indicate electromagnetic interference. Investigators have also ruled out fuel contamination, a wake vortex encounter and bird strike.

Neither Boeing, nor Rolls-Royce, nor the investigation team has put forward any recommendations for operational changes.

[Yo-Yo]

The aircraft passed through a region of particularly cold air between the Russian Ural mountains and eastern Scandinavia, although the fuel temperature did not fall below -34°C (-29°F). The fuel on board had a measured freezing point of -57°C.

I wonder if the fuel temp probe only measured fuel temp at one point in the tank, but that a layer of frozen fuel formed against the wing skins (which are also the tank walls.)

Once frozen, the layer of frozen fuel remained frozen until the aircraft lowered into warmer air on final approach. At that point, the frozen fuel released from the tank walls and got sucked into the pump intakes.

Or maybe some water ice at the bottom of the tanks did the same thing.

In either case, by the time the AAIB inspected the tanks and fuel, said frozen fuel or water ice would have long since been melted away.

[DuncanWaring]

Your theory’s roughly the same as mine.

To the best of my recollection, they spent considerable time enroute at outside air temperatures well below -57C.

[RobinOfKingston]

Last winter arrived while my tractor still had some summer formula diesel in it. A nice, cold day began to cause jelling of the diesel and the filter in the sediment bowl clogged. Tractor idled fine, but no power to do any work ‘til I added ‘juice’ to the fuel tank.

[LiveFree99]

This wouldn’t have happened if Algore’s promised Globull Warming had already arrived!

[sam_paine]

My thinking exactly.

When they say:

"although the fuel temperature did not fall below -34°C "

...they've already oversimplified the situation beyond resolution.

There's no such thing as one fuel temperature. There's multiple tanks, then what's the temp in the lines? Through various valves? before and after the pumps?

Furthermore, like you said...water/junk in the fuel.....can you say..." Beijing->London flight????"

[DuncanWaring]

In the interests of aviation safety, I’ll drive a few extra miles out of my way going home tonight.

[Yo-Yo]

http://www.flightglobal.com/articles/2008/02/20/221694/ba-777-accident-report.html

Air temperature over Sibera was measured at -76C (-105F), and the freezing point of the fuel was tested post-accident at -57C (-71F).

[DuncanWaring]

I didn’t recall the exact details, but I knew it was somewhere around -100F.

[Mr. Quarterpanel]

I somewhat agree, I was thinking the same thing. I am troubled by the fact that both engines had the same thing happen at the same time.

It is obvious that the AAIB is also thinking this, but unless both engines are sourced off the same tank, both inlets getting clogged by ice at the same time are pretty slim.

It is likely that ice will be ruled the cause, simply because nothing else was found that could have caused it.

It still is damn strange.

[ANGGAPO]

How do they vent the fuel tanks?