(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.