The US National Transportation Safety Board (NTSB) on 11 March released urgent safety recommendations for the redesign by Rolls-Royce of the fuel-oil heat exchanger on Trent 800 engines powering Boeing 777s.

NTSB says Rolls-Royce is already working on a redesign that could be available in twelve months, but still felt compelled outline the need for the heat exchanger overhaul.

The January 2008 dual engine roll-back of a British Airways 777 and a single-engine roll-back in November 2008 of a Delta Air Lines 777 prompted two NTSB recommendations.

The first is the redesign of the engine fuel/oil heat exchanger (FOHE) to prevent ice accumulation on its face to ensure fuel flow is not restricted to achieve commanded thrust.

 FOHEface main
NTSB has released this image of ice accumulation on the inlet face of a Trent 800 fuel/oil heat exchanger during testing

After the design overhaul is approved by certification authorities, NTSB says those entities need to mandate installation of new FOHEs during scheduled maintenance or within six months of the heat exchanger certification.

FAA on 5 March issued an airworthiness directive that incorporated new Boeing 777 operational procedures to reduce fuel system icing on Trent 800-powered aircraft.

Citing tests, FAA said reducing fuel-flow to minimum idle levels clears ice accumulation in the FOHE within a few seconds. FAA in the directive also said carriers operating 777s at the same altitude for two hours should power the engines to maximum thrust for a few seconds before descent. Boeing has previously issued guidance outlining a three hour threshold.

FAA also mandated the retarding of throttles to minimum idle for 30s at the top of descent to ensure the melting of ice built up on the heat exchanger at higher altitudes.

NTSB reasons while those actions may cut the risk of a roll-back they add complexity to flight crew operations and argues the level of risk reduction is not established. The board also feels since recovery procedures require a descent, "the aircraft may be exposed to other risks such as rising terrain or hazardous weather, or the inability to achieve maximum thrust during a critical phase of flight, such as a missed approach".

The UK Air Accidents Investigation Branch (AAIB) is expected to release its final report on the BA Heathrow crash in the next few weeks.

Rolls-Royce says: "This continues to be an ongoing investigation by the NTSB and the AAIB and we are working in full support of these efforts. Any advice provided by the NTSB to regulatory authorities such as the FAA which is then mandated or advised for Rolls-Royce will be implemented with full regard to their comments and direction."

Source: Air Transport Intelligence news