Guy Norris/LOS ANGELES
Pratt & Whitney is being forced to recertificate its PW4098 engine for the heavyweight Boeing 777-300, adding several months to the already delayed programme and making first deliveries to Korean Air Lines almost a year late.
The latest problems with the PW4098 emerged during flight tests on the testbed 777-300 when "heavy rubs" occurred during simulated single-engine rejected take-offs. Boeing simulated the engine failure by cutting off fuel, (fuel chop) causing the engine to surge.
"Those rub outs - on the low and high pressure compressors [HPCs] - don't give me confidence I can keep this engine on wing for the scheduled maintenance interval. We are not satisfied with the durability of this engine and, given what we saw, we need more margins," says P&W vice-president programmes Bob Leduc. "Obviously, Boeing is not very happy because it is unable to deliver the aircraft, and neither is the customer. But we have elected to make this tough choice," he says.
The company is making adjustments in the turbine area and dropping the operating line of the HPC to improve margins.
The resulting performance loss is "-a couple of tenths of a per cent, so it is minimal", says Leduc, who adds that the PW4098 originally "-beat its fuel burn specification by a couple of per cent anyway".
The revised certification programme is aimed at clearing the redesign in March , with flight tests restarting in April. Deliveries of three higher gross weight 777-300s to KAL are now expected to follow by September. The airline was originally scheduled to begin taking the aircraft in late September 1998.
"It would happen to any engine," says Leduc who adds: "Fuel chops are not a requirement of engine certification." The outcome of the tests, conducted flawlessly on the lower-power PW4084, could have repercussions for higher-power engines up to and beyond 100,000lb thrust (445kN) being studied by General Electric and Rolls-Royce, says Leduc.
"Boeing will ask all engine makers to add fuel chop to their development procedures. It is a phenomenon for the industry, not just for us. The differences seen in the higher thrust bracket result from the greater mass and inertia of the PW4098 LP spool [which has seven LPC stages compared to five on the PW4084]."
The PW4098 was originally due to be certificated in March 1998, but failed to meet this date because of technical problems. Design changes were certificated in July.
Source: Flight International
