By Tim Hepher
PARIS, Oct 24 (Reuters) - A probe into a series of engine failures on Airbus's smallest jet, the A220, is studying whether a software change set off unexpected vibrations that damaged fast-moving parts and forced three emergency landings, people familiar with the case said.
The airline Swiss briefly halted its fleet of A220 jets for checks on Oct 15 after a third flight in as many months was forced to divert with engine damage. Engine maker Pratt & Whitney also expanded checks on similar engines worldwide.
Investigators are focusing their attention on recent changes in engine software that may have caused parts that compress air inside the engine to be set in a way that caused mechanical resonance or destructive vibrations, two of the people said.
Neither the aircraft nor the engine has been grounded but pilots have been told to avoid certain combinations of thrust settings and altitude to avoid the risk of a new problem until the root cause of the three Swiss engine failures can be found.
A third source said it may take until December to confirm the cause, while other scenarios have also not been ruled out.
Airbus, which was starting a Pacific tour to promote the A220 on Thursday, had no immediate comment on the investigation.
Officials at the U.S. National Transportation Safety Board (NTSB), which is leading an investigation into the recent failures involving the U.S.-built engine, declined comment.
The European Union Aviation Safety Agency (EASA) said it was monitoring the situation closely and coordinating with the U.S. Federal Aviation Administration.
The head of Pratt & Whitney parent United Technologies UTX.N said earlier this week it was working on finding the cause and remained confident in the new fuel-saving engine.
"Clearly, any time you get an issue like this, we're on top of it. The guys are working through it," Chief Executive Greg Hayes told analysts on a conference call.
Nobody was hurt in the three incidents, which all took place while flying over France between London and Geneva. But parts from two of the engines were found on the ground.
France's BEA air accident agency has launched a rare appeal for 150 volunteers to help look for a key component from one of the Swiss engines in a wood in eastern France in coming weeks.
Pratt & Whitney set off a chain reaction of new aircraft designs or upgrades when it announced its new Geared Turbofan engine in 2008, promising 16% fuel savings. But it has wrestled with a spate of performance or reliability problems and delays.
However, although software problems can take time to fix, experts say there are better chances that they can be rectified with a software update rather than forcing a new part design.
Formerly known as the CSeries, the 110-130-seat A220 was designed by Canada's Bombardier BBDb.TO and was one of the first to adopt the new Pratt & Whitney technology. Bombardier sold the programme to Airbus last year due to heavy losses.
It now sits just under the European planemaker's A320 range which also offers a version of the same family of engines designed by Pratt. The Airbus version of the engines is not affected by the recent problems and is not subject to checks.
Some competing Brazilian aircraft, the Embraer 190/195-E2, have also been subject to the engine checks but are reported to use a different version of the software from the larger A220.
(Additional reporting by David Shepardson Editing by Alexandra Hudson)
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