A Boeing 747-8 in Daxing International Airport in Beijing Photo: IC

Boeing Co is aiming to conduct a key certification test flight on its grounded 737 MAX jet in late June, two people briefed on the matter said on Wednesday.

The 737 MAX has been grounded since March 2019 after two fatal crashes killed 346 people. Boeing told airlines it hopes to conduct the flight in late June, the sources said.

Boeing did not comment on the certification flight timing, but said late on Wednesday it had won approval from the Federal Aviation Administration (FAA) for a service bulletin that details the modifications required for 737 MAX wiring.

"Boeing has already begun modifying airplanes that have not yet been delivered and is coordinating modification efforts with the airlines," the company said. 

"New airplanes being built will include this update," it added.

In March, Reuters first reported Boeing would separate 737 MAX wiring bundles, flagged by regulators as potentially dangerous, before the jet returns to service.

Boeing also said it had shared with airlines "draft pilot training materials and related information to help operators facilitate and plan the development of their training programs."

Reuters has previously reported that the FAA does not plan to clear Boeing for a resumption of 737 MAX flights until at least August.

The sources warned the date could slip into July for the certification test flight, as the dates for many milestones for returning the plane to service have been repeatedly pushed back.

The FAA said on Wednesday it was "in regular contact with Boeing as the company continues its work on the 737 MAX... The aircraft will be cleared for return to passenger service only after the FAA is satisfied that all safety-related issues are addressed."

In early April, Boeing confirmed it would make two new software updates to the 737 MAX's flight control computer. But Boeing said software issues that prompted the updates were unrelated to a key anti-software system known as MCAS faulted in both fatal crashes.

Boeing resumed production of the 737 MAX on May 27 after halting it in January.

Bloomberg News reported the communication to airlines earlier on Wednesday.