Guy Norris/LOS ANGELES
Pratt & Whitney says that an upgrade effort to counter reliability problems on more than 1,600 PW4000 engines is showing results, with a "dramatic reduction" to in-flight shutdown rates.
The upgrade effort, known as the Number 1 reliability programme, involves around 100 service bulletins. More than 70 PW4000 customer airlines have incorporated around 80% of the improvements into their engines, says P&W. It adds that the 12-month rolling average in-flight shutdown rate for the fleet has dropped from 0.014% in January 1997 to 0.008% in August as a result.
The package affects the nine major PW4000 family members, including the three PW4150 versions for the Airbus A300/310, the four PW4050/60 engines for the Boeing 747/767 and the PW4460 /62 for the Boeing MD-11. The first elements of the package were introduced in 1995 to tackle a series of problems which were driving up both the removal and in-flight shutdown rate of all versions. These included surges, high-pressure compressor (HPC) blade and dovetail cracking, high-pressure turbine blade-root cracking, fan-blade fractures and failures of the angle gearbox bearing.
The bulk of problems are being solved using modifications based on design improvements developed for the larger PW4168 and PW4084 families. "We looked at how we could improve the reliability of all our products as an outgrowth of the A330 and 777 engines," says PW4000 programme vice-president Tom Davenport. The cracking problem on the fifth HPC stage, for example, was handled with a revised design developed by using computational fluid-dynamics analysis techniques used to design the PW4084 compressor. The revised blades have been fitted into around half of the fleet.
Validation tests of a revised fan- blade leading-edge design with a thicker radius, again based on the larger engines, are also under way in efforts to improve tolerance to foreign-object damage. The baseline PW4000 fan has shown susceptibility to leading-edge cracking after foreign-object impacts. P&W plans to issue a service bulletin covering the revised design this year, says Davenport.
Solutions are also being introduced to solve binding, wear, failure and leakage problems with the "2.5" bleed-valve located between the high- and low-pressure compressors. The company is strengthening the tracks which support the fore and aft movement of the bleed-ring rollers, as well as strengthening the pivot pins.
P&W confirms that it is replacing low-pressure turbine (LPT) stage-five vanes from PW4077 engines in service on the Boeing 777. "We saw this in the validation tests for ETOPS [extended-range twin-engined operations], and we saw some areas in the LPT vane that needed improvement," says Davenport. All operators were told about the impending vane change at, or before, handover. Japan Air Lines says that it "-received information about the stage-five vanes at first delivery in February 1996".
According to Boeing, 11 out of 15 removals on the P&W-powered fleet were caused by this change in 1996, and a further 16 out of 34 for the year to date.
Source: Flight International
