US Federal Aviation Administration has identified a new potential risk with Boeing's 737 MAX aircraft that it says must be fixed before the grounded jet can return to service.
The new risk was discovered during a simulator test last week and it is not yet clear if the issue can be addressed with a software upgrade, or will require a more complex hardware fix, sources with knowledge of the matter told Reuters.
The FAA did not elaborate on the latest setback for Boeing, which has been working to get its best-selling airplane back in the air following a worldwide grounding in March in the wake of two deadly crashes within five months.
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The new issue means Boeing will not conduct a certification test flight until July 8 at the earliest, the sources said, and the FAA will spend at least two to three weeks reviewing the results before deciding whether to return the plane to service.
Last month, FAA representatives told members of the aviation industry that approval of the 737 MAX jets could happen as early as late June.
The world's largest planemaker has been working on an upgrade for the stall-prevention system known as MCAS since a Lion Air crash in Indonesia in October, when pilots were believed to have lost a tug of war with software that repeatedly pushed the nose down.
A second deadly crash in March in Ethiopia also involved MCAS, together killing 346 people.
"On the most recent issue, the FAA's process is designed to discover and highlight potential risks. The FAA recently found a potential risk that Boeing must mitigate," the FAA said in the statement emailed to Reuters. "The FAA will lift the aircraft's prohibition order when we deem it is safe to do so."
Asked about the new potential risk, Boeing said it is "working closely with the FAA to safely return the MAX to service."
Boeing's aircraft are being subjected to intense scrutiny and testing designed to catch flaws even after a years-long certification process.
Two people briefed on the matter told Reuters that an FAA test pilot during a simulator test last week was running scenarios seeking to intentionally activate the MCAS stall-prevention system. During one activation it took an extended period to recover the stabiliser trim system that is used to control the aircraft, the people said.
It was not clear if the situation that resulted in an uncommanded dive can be addressed with a software update, or if it is a microprocessor issue that will require a hardware replacement, but Boeing has told the FAA it believes the issue can be addressed with a software upgrade.
A hardware fix could add new delays to the plane's return to service.
The FAA also said on Wednesday that it continues "to evaluate Boeing's software modification to the MCAS" and is developing necessary training requirements following any changes.