SEATTLE, Jan 16 (Reuters) - Boeing BA.N resumed testing for its long-delayed 777X widebody jet on Thursday, with the first flight since the U.S. planemaker grounded the test fleet in August due to the failure of a key engine mounting structure.

The grounding came just five weeks after it had started certification flights for the 777-9 with officials from the U.S. aviation regulator onboard.

Federal Aviation Administration staff were not on board for Thursday's flight, according to the company.

The 777X is the successor to Boeing's 777, one of the most commercially successful long-haul airliners. The company initially planned to deliver the first 777X to launch customer Qatar Airways in 2020.

First delivery of the 777-9 has since been pushed back to 2026, followed by the smaller 777-8 and a freighter version later in the decade.

Boeing's other 777X airline customers include Emirates, Lufthansa LHAG.DE, Singapore Airlines SIAL.SI and Cathay Pacific Airways 0293.HK. The planemaker has 481 777X orders, including 170 from Emirates and 60 from Qatar, according to Cirium, an aviation industry analytics company.

Boeing's 777-9 test plane made a return flight from Boeing Field in Seattle to Moses Lake, Washington on Thursday.

"We continue to execute a rigorous test program to demonstrate the safety, performance and reliability of the 777-9," Boeing said after it landed in the afternoon.

A company spokesperson declined to comment on how the airplane performed during the flight.

(Reporting by Dan Catchpole; Editing by Jamie Freed)

((dan.catchpole@thomsonreuters.com))