US FAA regulators have formally incorporated new Boeing 777 operational procedures designed to reduce the risk of fuel-system icing on Rolls-Royce Trent 800-powered airframes.
The FAA's revised airworthiness directive covers new measures to counter the threat of ice build-up on the engine's fuel-oil heat exchanger, based on guidance from Boeing.
It also states that a modification is under development by the manufacturer - without specifying whether it refers to Boeing or Rolls-Royce - to address the condition.
"Once this modification is developed, approved and available, we might consider additional rulemaking," says the FAA.
The new directive follows investigation of a single-engine roll-back incident on a Delta Air Lines 777 in November which bore resemblance to the dual-engine roll-back that led to the British Airways 777 landing accident at London Heathrow last year.
While data confirmed suspicions, arising from the BA accident, that ice released after a high-thrust command could block fuel-flow at the heat exchanger, analysis also showed that ice accretes in the fuel system more rapidly - and at warmer fuel temperatures - than previously indicated.
The FAA says testing demonstrates that reducing the fuel-flow to minimum idle levels will clear ice accumulation at the heat exchanger "within a few seconds".
Boeing advised last September that Trent-powered 777s operating at the same altitude for three hours should, for a few seconds, be powered up to maximum thrust before descent. The new FAA measures bring this cruise period threshold down to two hours.
The directive also requires crews to retard the throttles to minimum idle for 30s at top of descent to ensure that ice accumulated on the heat exchanger melts while the aircraft is at high altitude.
Ground procedures for fuel circulation, issued in parallel last September, remain unchanged. The FAA's new directive becomes effective on 20 March.