The growing number focused further attention on issues with the revolutionary carbon-fibre and plastic composite plane, which has been rolled out to customers over the past year.

Spokeswomen for United and Boeing said the problems on United's planes occurred in an electrical distribution panel, not a generator, as had been previously believed.

On December 4, a United flight from Houston to Newark, New Jersey, made an emergency landing after it appeared that one of its power generators failed.

Last Thursday, Qatar Airways said it had grounded one of its three 787 jets because of the same problem United had experienced.

On Friday, the Seattle Times reported that the second United jet had electrical problems, bringing the total to three.

The newspaper quoted United as saying that on the December 4 flight, "a power-distribution panel caused a nuisance generator fault indication", prompting the emergency landing. That meant a warning light came on when it shouldn't have. United said it "replaced the panel and returned the aircraft to service".

The Boeing spokeswoman said the power-distribution panel that developed the latest problem was different from one that caught fire during 787 testing in 2010.

The United spokeswoman declined to give further details because the problems are still being investigated and details about them may change.

On Friday, Boeing CEO Jim McNerney said in a television interview that the problems were "normal squawks" that occur when new planes are brought into service.

The 787 uses an electrical system with generators and power distribution panels to perform many functions that on other jet models are accomplished with compressed air from the plane's engines.

Problems with the 787, the first Boeing jet to be globally outsourced to suppliers around the world, caused it to enter service 3-1/2 years behind schedule. The electrical problems aren't the only issues arising with the new jet.

Earlier this month, the U.S. Federal Aviation Administration told airlines to make immediate checks of fuel line couplings on the 787 to ensure that they were not leaking. The order came after the agency received notice that two 787s had developed leaks that could cause them to run out of fuel or catch fire.

Through November, Boeing had delivered 38 of its 787s. United is the only U.S. carrier that has put the jet into service. Other carriers flying the jet include launch customer All Nippon Airways Co Ltd (9202.T), Japan Airlines Co Ltd (9201.T), LOT Polish Airlines and LAN Airlines of Chile. (Reporting by Alwyn Scott; Editing by Edmund Klamann)

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