July 13 (Reuters) - Boeing ( BA ) formally began flight

testing of the 777-9 with U.S. aviation regulators onboard, The

Air Current reported late on Friday, marking a step forward in

the certification process.

The start of flight testing with the Federal Aviation

Administration (FAA) accompanied a key certification milestone,

known as Type Inspection Authorization. This means that aspects

of the design are sufficiently ready for government evaluations

to ensure the aircraft meets regulatory standards, the aviation

news service said, citing two people familiar with the program's

process.

Boeing ( BA ) and FAA did not immediately respond to a Reuters

request for comment out of office hours.

The 777X, known as the 777-9 and a larger and more

fuel-efficient version of the 777 wide-body jet, has been in

development since 2013 and faced multiple delays.

The chairman of Emirates, the plane's biggest customer, said

in May he does not expect the certification of the plane before

the first quarter of 2025.

Boeing ( BA ) has been under increased scrutiny from the FAA over

potential safety issues since a Jan. 5 mid-air panel blowout on

a 737 MAX plane.

In April, the FAA also began investigating a whistleblower's

claims that the company dismissed safety and quality concerns in

the production of the planemaker's 787 and 777 jets.