European regulators, in consultation with US counterparts, have agreed to define interim measures for Rolls-Royce Trent-powered Boeing 777 aircraft to reduce the risk of icing causing an engine roll-back.

The decision follows UK Air Accidents Investigation Branch findings which point to water icing as having restricted fuel-flow to both engines on the British Airways 777 which crashed at London Heathrow in January.

While the nature of the interim measures has yet to become clear, the AAIB has today put forward two preliminary suggestions.

One option, it says, would be to use fuel additives designed to inhibit icing. At concentrations of 0.10-0.15% these inhibitors migrate to undissolved water in fuel, lowering its freezing point to around minus 43°C and preventing its becoming ice.

Investigators estimate that fuel uploaded to the BA aircraft before its departure from Beijing contained up to 3l of dissolved and 2l of undissolved water.

The tanks may have contained residual water from previous flights, and a maximum of 0.14l would also have been drawn in through the fuel-tank vents during the Beijing-Heathrow service. The AAIB says these levels are normal.

Although icing inhibitors are not commonly used in commercial aircraft - and none was detected in the fuel used on the BA 777 - they have routinely been used by armed forces on military types for decades.

An alternative measure, says the AAIB, would be the introduction of operational changes to reduce the risk of ice formation during critical stages of flight.

"Such changes could be implemented quickly but must not compromise the safe operation of the aircraft," it adds.

While the European Aviation Safety Agency and the US FAA are to assess the implications of the investigation, the AAIB states that any design changes to make fuel systems more tolerant to icing would "certainly not be available" in the near-term.

It adds: "Therefore, to reduce the risk of recurrence, interim measures need to be adopted until such design changes to the fuel system are available."

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Source: Air Transport Intelligence news