The FAA published a revised Airworthiness Directive today for Boeing 777-200s and -300s equipped with Rolls-Royce Trent 800 engines. The new AD adds further procedures to prevent ice buildup that can clog fuel lines. In addition to new fuel circulation procedures required by an earlier AD, the new directive “revises AFM in-flight procedures by reducing the step climb from three to two hours before descent, and by requiring flight crews to retard the throttles to minimum idle for 30 seconds at the top of descent to ensure any ice accumulation on the face of the FOHE melts while at higher altitudes.” The FAA has given airlines 10 days to comply with the new directive. It affects 56 aircraft in the U.S. fleet, and a total of 228 worldwide.
The British investigation of a Boeing 777 accident at Heathrow in January 2008 found that ice can accumulate in the main fuel tank system of the aircraft at the low fuel temperatures and low-power fuel flows associated with long flights. Investigators found that when the pilot increases power, the increased fuel flow in the tank can cause the ice to break free, which can then block the flow of fuel to the engines.
Initial review of other 777 engine combinations has not revealed the same vulnerability to ice buildup in the fuel lines, said the FAA.
Meanwhile, the agency is working with European aviation safety organizations to consider the implications of this investigation on other aircraft/engine combinations.