Guy Norris/LOS ANGELES
Rolls-Royce is to issue service bulletins to all Trent 800-powered Boeing 777-200/300 operators which will revise, and in most cases alleviate, the tight inspection intervals for the engine's fan blades which have been in place since the failure of a blade on an Emirates 777-300 in Melbourne, Australia earlier this year.
Intensive tests conducted by the engine maker have confirmed the failure was caused by differential stresses imposed on the blade root by patchy lubricant, rather than by a fundamental design flaw, as some operators had initially feared.
The fix is the use of Metco 58, a copper-nickel-indium alloy coating material, which had been introduced into the Trent fan blade production process last July. The failed blade was an earlier production unit which had been coated with a graphite-based paint called PL 237. Tests showed that under high thrust settings, this becomes patchy and causes stresses to build up in the blade root, resulting in cracks.
Rolls-Royce director Boeing programmes Phil Hopton says a fleet wide inspection of all fan sets (124 aircraft in service plus spares) revealed four cracked blades. Hopton says that, after the completion of tests, the UK Civil Aviation Authority is likely to "move the inspections out to 1,200 cycles in most cases," adding: "The maintenance burden will disappear."
Under an amendment to a Federal Aviation Administration airworthiness directive issued in February, operators have been required to inspect Trent 800s at intervals as often as every 200 cycles in the case of the higher thrust 892 and 892B versions. Even the lower powered Trent 875s required inspection every 400 cycles, while 877 and 884 engines called for checks every 350 cycles.
Although Metco 58 has been standard on the Trent 800 since July, and will now become the standard across all Rolls-Royce wide chord fan engines, Hopton says: "Obviously we have got some sets that have run with PL237. So the question is how healthy are those blades?
"We have around 150 fan sets, and we are working to categorise those on a risk basis. It might mean swapping some fan sets," he adds.
Tests proved the phenomenon only affected the higher power engines, so anything at Trent 884 rating or less is "out of the debate".
The fan root requires lubricating because, under acceleration, the energy of the fan forces the disc to dilate. The blade, which has a curved root, then sits further out in the dovetail fitting. Under deceleration, the blade root slides back as the disc returns to its original shape.
Lubrication is required to ease the friction between the blade root and the dovetail, and the cracks occurred when the original lubricant became patchy under high thrust loads. This allowed parts of the surfaces to stick, which caused differential stresses to build up and eventually cracks to appear.
Source: Flight International
