Carriers are on track to complete, within six months, modification of nearly 1,900 Trent engines considered vulnerable to the fuel icing which brought down a British Airways Boeing 777-200ER two years ago.

UK investigators have formally concluded that flight BA038 from Beijing crashed on approach to London Heathrow after a highly-unusual set of circumstances conspired to restrict the fuel flow to both Trent 800 engines.

In its final report today the Air Accidents Investigation Branch says an "avalanche" of accumulated soft ice was flushed along the fuel lines as the aircraft's throttles were adjusted in the late stages of the approach. Instead of passing through the fuel-oil heat exchanger plate, the ice smothered it and blocked the fuel flow to the powerplants.

Analysis of 175,000 flights showed that BA038 had been "unique" in having a low fuel-flow during cruise but a high fuel-flow, at low fuel temperature, during approach.

"The [exchanger], although compliant with the applicable certification requirements, was shown to be susceptible to restriction when presented with soft ice in a high concentration, with a fuel temperature that is below minus 10°C and a fuel flow above flight idle," says the AAIB.

BA-777 Crash London Heathrow

US investigators are studying a similar fuel-restriction event, 10 months after BA038, involving a Trent-powered Delta Air Lines Boeing 777 in cruise.

Rolls-Royce has since changed the design of the heat-exchanger face, and this simple 'drop-in' modification is being applied to all affected engines - not just the 570 Trent 800s, but also some 700 Trent 700s and 610 Trent 500s.

Modification involves replacing the exchanger face plate, which features an array of short protrusions on its surface, with one which is smooth. This can be achieved either with a new component or by machining the existing face plate.

Trent modification

The Trent 800 modification was certified in July last year, followed by the Trent 700 and Trent 500 in September and October respectively.

Sources familiar with the process state that the engines on British Airways' and Delta's fleets have already been modified.

By April the global fleet of Trent-powered twin-jets will have been undergone modification on at least one engine - two in the case of Trent 500-powered Airbus A340s - which, Rolls-Royce believes, will eliminate the risk of a total loss of thrust.

"The changes we have made to the affected part mean that this kind of incident cannot happen again," says a spokesman for the manufacturer.

Rolls-Royce has tested the modification at temperatures down to minus 44°C and, crucially, with ice levels five times greater than those involved in the BA038 accident.

Replacement of the heat-exchanger plate across the Trent fleet will be completed "well ahead" of a January 2011 regulatory deadline, the spokesman says.

Air Transport Intelligence understands that the modifications, which are being performed as aircraft are checked-in for routine maintenance, could be completed by August.

Investigators have yet to comprehend completely the icing processes which led to the crash of BA038 on 17 January 2008. The AAIB notes that the BA aircraft suffered a rapid onset of ice, while the Delta aircraft experienced a more progressive fuel restriction.

"Testing does offer some explanation, with the observation of 'randomness' in the formation of ice, indicative that there may also be a variance in the quantity of ice generated during similar flights," it says.

But it admits that the reasons why other 777s have remained unaffected, despite operating similar flight profiles, are "not fully understood". The AAIB says this indicates that factors other than fuel flow and temperature might affect the release of ice in fuel pipes.

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Source: Air Transport Intelligence news