PARIS (Reuters) - A U.S.-led investigation into a series of engine failures on Airbus’s smallest jet, the A220, is studying whether a software change allowed unexpected vibrations that tore parts and forced three emergency landings, several people familiar with the case said.

FILE PHOTO: The Airbus logo is pictured at Airbus headquarters in Blagnac near Toulouse, France, March 20, 2019. REUTERS/Regis Duvignau/File Photo

The airline Swiss halted its fleet of A220 jets for more than a day on Oct. 15 after a third flight in as many months was forced to divert or return to base with engine damage. Engine maker Pratt & Whitney has urged checks on similar engines worldwide.

Investigators are focusing their attention on a recently revised version of engine software that may have allowed parts inside the engine to be set in a way that caused mechanical resonance or destructive vibrations, two of the people said.

Neither the A220 plane nor the engine has been grounded but Airbus and Pratt & Whitney have told pilots not to push engines above 95% of their maximum thrust when flying above 29,000 feet - a demanding configuration currently only required by Swiss.

The larger and more widely used A320 partly relies on the same family of Pratt engines but the version used for those jets is not affected by the software review.

On modern aircraft, engine settings are controlled by engine manufacturer software that interprets pilot commands and tells the engines what to do. The Swiss problems first arose following a recent update of the software, two of the people said.

A new version of the software should allow the flight restrictions to be lifted but may not be ready until the first quarter, the people said.

However, the underlying cause of the problem has not yet been confirmed and other scenarios have not been ruled out. Other previous causes of engine failures include defects in materials.

Airbus, which was starting a Pacific tour to promote the A220 on Thursday, declined comment on the investigation.

Officials at the U.S. National Transportation Safety Board (NTSB), which is leading an official investigation into the 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.

The company said on Thursday it was supporting the investigation and referred further queries to the NTSB.

Nobody was hurt in the three incidents, which all took place on the 750 km (470 miles) route between London and Geneva.

But in one of the incidents, compressor debris punched a hole in the engine casing, and such “uncontained” engine failures are always investigated thoroughly. In the most recent event, parts escaped from the back of the engine when the diverted A220 landed in Paris, but the engine housing was left intact.

France’s BEA air accident agency made a rare appeal for 150 volunteers to help look for a titanium part from one of those engines in the woods 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.

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 program to Airbus last year because of heavy losses.