US FAA safety regulators have issued an airworthiness directive formalising Boeing's interim advice to operators on reducing the risk of water-in-fuel icing on Boeing 777 aircraft.
Investigators have identified water icing as a likely cause of the fuel restriction and dual-engine roll-back on a Rolls-Royce Trent 800-powered British Airways 777 which crashed at London Heathrow in January.
The FAA directive applies only to 777-200 and -300 aircraft equipped with Trent engines. It requires revision of aircraft flight manuals to include in-flight procedures for pilots to follow during particular cold-weather conditions, as well as specific fuel-circulation procedures on the ground.
A spokesman for Rolls-Royce says: "The focus is on Trent-powered 777s because that's the combination in the incident."
But the directive states: "Initial review of [777s with other engines] has not revealed the same vulnerability to the identified unsafe condition."
While the mechanism for the icing has yet to be determined, a spokesman for General Electric says the GE90 and Trent 800 have differing fuel-system architectures, notably in the location of the main fuel/oil heat exchanger.
These differences, he says, generate a difference in fuel temperature at the heat exchanger inlet.
Investigators of the BA accident have performed several tests to determine whether ice is capable of blocking the fuel flow in the vicinity of the heat exchanger.
The GE spokesman adds: "There are no anticipated field actions for the GE90-powered 777s."
In its directive the FAA says: "We are issuing this airworthiness directive to prevent ice from accumulating in the main tank fuel-feed system which, when released, could result in a restriction in the engine fuel system.
"Such a restriction could result in failure to achieve a commanded thrust, and consequent forced landing of the aircraft."
It says the directive becomes effective on 29 September and that operators should revise their procedures within 10 days of this date.