European regulators have today mandated that airlines operating Rolls-Royce-powered Boeing 777s fit a redesigned component in an attempt to avoid accidents like the loss of a British Airways (BA) aircraft at Heathrow Airport last year.

Rolls-Royce has redesigned the fuel-oil heat-exchanger (FOHE) on the Trent 800 engine which powers the 777.

The FOHE is intended simultaneously to warm fuel and to cool oil by transferring heat from the latter to the former but it is believed that on the flight in question, and possibly others, it was overwhelmed by ice in the fuel which allowed ice to collect on the front face of the FOHE and restrict the fuel flow.

The result was that both engines suffered serious power reductions and the aircraft landed heavily, inside the airport perimeter but short of the runway, and was subsequently written off.

The same phenomenon is suspected to have been behind an incident to a Delta Air Lines 777. UK and US investigators called on Rolls-Royce to undertake the redesign of the device.

In an airworthiness directive (AD) just published, the European Aviation Safety Agency (EASA) says operators must fit the redesigned FOHE by the end of next year or within 6,000 flight hours - whichever comes sooner.

The AD states: "Post incident analysis and investigation has established that, under certain ambient conditions, ice can accumulate on the walls of the fuel pipes within the aircraft fuel system, which can then be released downstream when fuel flow demand is increased. This released ice can then collect on the FOHE front face and limit fuel flow through the FOHE.

"This type of icing event was previously unknown and creates ice concentrations in the fuel system beyond those specified in the certification requirements.

"To mitigate the risk of dual engine FOHE blockage within the Trent 800 engine powered Boeing 777 fleet, which constitutes a potential unsafe condition, this Airworthiness Directive instructs replacement of the FOHE with a modified standard incorporating enhanced anti-icing and de-icing performance."

Kieran Daly recently spoke to scientists looking at the fuel-icing phenomenon. Read his blog.