By Jon Ostrower and Robert Wall 

Bombardier Inc. still expects its new CSeries jetliner to enter service in 2015 in the wake of a major engine failure during testing last week that an initial probe suggested may stem from an oil-related problem.

Other aircraft makers including Airbus Group NV due to use versions of the same engine type also played down the May 29 incident during testing on the ground of the new Bombardier jet.

The Pratt & Whitney unit of United Technologies Corp. has sold more than 5,500 of the new fuel-efficient engines. A preliminary assessment of last week's incident points to an oil issue inside the new engine, according to two people briefed on the incident. One said a design issue, not an isolated factor on the affected engine, like a manufacturing flaw, appears to have been related to its cause.

Bombardier officials said further details should be ready within days. "From what I know today we are still on track for our [entry into service] in the second half of 2015," said Guy Hachey, chief operating officer at Bombardier Aerospace, on the sidelines of an airline conference in Qatar.

A person familiar with the probe said Pratt has a "good grasp on what the issue is," and the problem could be addressed "fairly rapidly" if its initial assessment proves correct. The failure manifested in the engine's core, the person added, and is unrelated to the new gear system that allows components to spin at different speeds, reducing how much fuel it consumes.

Pratt spokesman Jay DeFrank said it was premature to speculate on a final cause or what, if any, change might be required as an outcome of the pending investigation with Bombardier.

The incident occurred on the Canadian plane maker's lead flight test aircraft during a routine maintenance test at Bombardier's assembly plant at Mirabel, Québec. Mr. Hachey called the damage to the aircraft "manageable," without detailing the extent of the repairs. However, the two people briefed on the incident said debris was ejected out of the sides of the aircraft, damaging the jet's fuselage. There were no injuries.

The Canadian Transportation Safety Board has deployed a team to Mirabel to investigate.

Bombardier has shipped the engine back to Pratt for inspection at one of its Connecticut facilities and had shifted its sidelined test fleet to ground testing to keep on schedule, said Mr. Hachey. Four CSeries prototypes had logged about 330 hours in flight since September before the hiatus.

Aerospace industry officials have taken a cool-headed approach to the May 29 failure, citing the testing phase as the time to address such issues before they are ever experienced in service, but the potential outcome of the investigation underscores how central the engine's readiness is to airlines counting on its touted fuel efficiency.

Airbus, whose A320neo uses a similar version of the Pratt & Whitney engine, said Sunday its plan to field the upgraded single-aisle aircraft are unaffected by the problem. The A320neo is due for its first flight in September. Any knock-on effect to the Pratt engine selected by Airbus, still in testing, isn't yet clear, but both it and the Bombardier engine share a similar core design.

Canada's aviation regulator certified the CSeries engine in February 2013, and Pratt & Whitney says the engine design has undergone more than 9,000 hours of testing, including more than 1,300 hours of flight.

Pratt has staked its future on the new engine design, which competes head to head with CFM International, a joint venture between General Electric Co. and Safran SA in the multitrillion-dollar market for single-aisle jets.

New engine technology has sparked an unprecedented wave of demand for new jets offering familiar work-horse aircraft from Boeing Co., Airbus and Embraer SA, new life that will keep them in production well into the 2020s. Boeing and Airbus alone have sold nearly 5,000 jets with next-generation engines.

Bombardier was first to select Pratt's new engine design in 2007 for the all-new 100 to 149-seat CSeries, setting in motion a sea-change for the major aircraft makers.

The company has 203 firm orders for its CSeries, since launching the program 2008, but Airbus, acknowledging the Canadian jet's potential as a new-entrant into its market, launched the A320neo in 2010 with the same geared engine and an all-new design from CFM, In turn, Boeing launched its single-aisle 737 Max in 2011 with CFM and Embraer with the Pratt engine.

Bombardier shares closed down 2.4% at 3.69 Canadian dollars Friday in the wake of The Wall Street Journal's report on the incident. The stock is down 20% so far this year, the worst performer among the four main aircraft manufacturers.

Write to Jon Ostrower at jon.ostrower@wsj.com and Robert Wall at robert.wall@wj.com

Subscribe to WSJ: http://online.wsj.com?mod=djnwires

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