Guy Norris/LOS ANGELES
CERTIFICATION of the CFM International (CFMI) CFM56-7B2 for Boeing's next-generation 737 family is expected to be delayed by almost two months, to give the engine maker more time to validate modifications to the troublesome fan-blade retention system.
CFMI says that the delay "-will not affect the flight test programme for the 737, which is scheduled [to begin in] February". US Federal Aviation Administration certification of the engine/airframe is due in September 1997, a month before entry into service with Southwest Airlines.
Problems emerged with the fan-blade-retention device earlier this year during a blade-off test. The test was aimed at proving the integrity of the containment system, which had been strengthened following earlier trials. Although the revised containment worked well, the test revealed problems with blade retention when "several" trailing blades were detached.
The problem was traced to the heavier weight of the blades and the higher loads placed on the retention device. The wide-chord blades are made from solid titanium and are around 35% heavier than the blades used on the current 737 engine. The axial retainers on the dovetail assembly at the base of each root were redesigned.
The test was re-run in August, but problems persisted because the redesigned retainer was now too strong. "During a spin-pit test conducted on the fan-blade axial-retention system, the redesigned blade retention adversely impacted the load distribution into the blade shank," says CFMI.
"Since then, CFMI has been evaluating methods of optimising the load distribution to limit the impact on the trailing blade during blade-out conditions. We are conducting trials on an optimised blade/retainer design," CFMI adds.
CFMI now plans to perform a final certification test to validate the redesigned axial-retention system in November, and expects certification "before year's end". The company's problems with the retaining device are in contrast to the achievements of the otherwise successful programme. To date, 105 of the 107 reports needed for FAA certification have been submitted, and performance has "-met or exceeded expectations with an exhaust-gas-temperature margin bet- ter than expected", says CFMI.
Source: Flight International
