Boeing Co has scaled back use of automation to make fuselage sections for its 777 jetliners amid reports of reliability issues and returned to having mechanics do some of the work, the planemaker said on Thursday.

The company began to build 777 fuselages in 2015 in an upright orientation, with robots drilling holes and installing fasteners, an initiative known as fuselage automated upright build (FAUB).

The world's biggest planemaker said it had stopped using robots for the forward and rear sections of the fuselage. According to industry sources, the system had caused problems with reliability and rework issues.

Boeing had faced delays last year when the FAUB machine was tried out on existing versions of the 777.

Instead, the company is switching to "flex tracks" - an automated method to drill the holes along the circumference of the airplane that are then fastened manually.

The system was developed by Boeing Commercial Airplanes and has been tested in commercial and defense programs, company spokesman Paul Bergman said in an emailed statement.

Implementation of flex tracks for the 777 fuselage began in the second quarter and Boeing expects the transition to be complete by the year-end, Bergman said.

There are no planned changes in total staffing and the company continues to implement robotic systems on areas such as wing manufacturing for the 777X.

For example, Boeing was still using robots for the middle section of the 777 fuselage, which it was still assembling in an upright orientation, Bergman said.

The setback highlights continuing pressure on the company's management at a time it is trying to obtain approval to return its grounded 737 MAX jetliner to service before the end of this year.

The change in production strategy for the 777 was reported earlier by Bloomberg.