Following two engine thrust rollbacks on Boeing 777s powered by Rolls-Royce RB211 Trent 800 powerplants, the National Transportation Safety Board (NTSB) has issued an urgent safety recommendation calling for redesign of an engine component—work that is already underway at Rolls-Royce. The Safety Board also recommends that the redesigned unit be installed on all affected B-777s at the next maintenance check or within six months of availability.
The two recommendations to the Federal Aviation Administration (FAA) and European Aviation Safety Agency (EASA) were issued in response to two investigations involving Boeing 777-200ERs.
In both cases, a buildup of ice (from water normally present in all jet fuel) on the fuel/oil heat exchanger (FOHE) restricted the flow of fuel to the engine, resulting in an uncommanded engine rollback.
The first event occurred on January 17, 2008when a B-777 (G-YMMM) experienced a dual engine rollback on final approach and crashed short of the runway at London’s Heathrow International Airport. British Airways Flight 38 was arriving from Beijing.
UK accident investigators noted that during the flight the Boeing 777 passed through a region of particularly cold air (with ambient temperatures as low as -76 degrees C) between the Urals and Eastern Scandinavia. The UK’s Air Accident Investigation Branch (AAIB) said ice in fuel lines “most probably” caused the Trent-equipped B-777 to lose all power just prior to landing on Runway 27L at London Heathrow.
The second event occurred on November 26, 2008, when a Delta Air Lines B-777 (N862DA) experienced a single engine rollback during cruise flight over Montana while en route from Shanghai to Atlanta.
Testing in support of the UK accident investigation led Boeing to develop procedures to help prevent ice accumulation, and to recover thrust in cases of ice blockage. As more information from the Delta rollback event was developed, Boeing modified the procedures, which became the basis of an airworthiness directive (AD) issued by the FAA.
The AD requires revision of the aircraft flight manual to include in-flight procedures for pilots to follow in certain cold weather conditions and requires fuel circulation procedures on the ground when certain conditions exist.
While the procedures may reduce the risk of a rollback in one or both engines due to FOHE ice blockage, they add complexity to flight crew operations, and the level of risk reduction is not well established. And because the recovery procedure requires 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 during a missed approach.
The Safety Board concludes that there is risk of ice crystal blockage of FOHEs on Trent 800 series engines installed on 777-200s at temperatures commonly encountered by long- range transport airplanes.
The only acceptable solution to this safety problem, the NTSB believes, is a redesigned FOHE that would eliminate the potential of ice buildup. In February, Rolls-Royce indicated that a redesign of the FOHE was underway, and that they anticipated the redesign to be tested, certified and ready for installation within 12 months.
“Because the adequacy of the design, the speed and appropriateness of the FOHE testing, and the installation of the redesigned FOHE are critical to the flight safety of Boeing 777-200 airplanes powered by Rolls-Royce RB211 Trent 800 series engines, the Safety Board believes that the FAA and EASA should take an oversight role and direct the development and installation of the new FOHE design,” said the NTSB.
“With two of these rollback events occurring within a year, we believe that there is a high probability of something similar happening again,” said NTSB Acting Chairman Mark V. Rosenker.
“We are encouraged to see that Rolls-Royce is already working on a redesign, and we are confident that with the FAA and EASA overseeing the process, this flight safety issue – even one as complex as this – will be successfully and expeditiously resolved,” he added.