WASHINGTON • The United States Federal Aviation Administration (FAA) has identified a new potential risk that Boeing must address on its B-737 Max before the grounded jet can return to service, the agency said on Wednesday.

The 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.

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 B-737 Max jets could happen as soon as late this month.

The world's largest planemaker has been working on the upgrade for a stall-prevention system known as MCAS since a Lion Air crash in Indonesia last October, when pilots were believed to have lost a tug of war with software that repeatedly pushed the plane's nose down.

A second deadly crash in March in Ethiopia also involved MCAS. Both crashes together killed 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 e-mailed to Reuters. "The FAA will lift the aircraft's prohibition order when we deem it is safe to do so."

Boeing said in a securities filing late on Wednesday that the FAA had asked it to address through software changes a specific flight condition not covered in the company's already-unveiled software changes.

The US planemaker also said it agreed with the FAA's decision and request, and was working on a fix to address the problem. "Boeing will not offer the 737 Max for certification by the FAA until we have satisfied all requirements for certification of the Max and its safe return to service," the plane-maker wrote in the filing.

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 that 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.

Boeing yet to fly out of Max cloud

CHICAGO • Boeing continues to be hobbled by the grounding of its best-selling B-737 Max jetliner despite receiving a surprise US$24 billion (S$32.5 billion) commitment from British Airways owner IAG to buy 200 of the planes at the recently concluded Paris Air Show.

The world's largest plane-maker reported on its website that it received no aircraft orders last month. It has cut production of the B-737 Max, which was grounded in March following two deadly crashes involving Indonesia's Lion Air last October and Ethiopian Airlines in March that killed 346 people in total.

Deliveries of the plane have also been suspended, leading to a 1.4 per cent drop in aircraft shipments last month. Still, the IAG order injected some optimism into a Boeing campaign that had looked increasingly bleak as regulators worldwide looked at proposals to make the plane safer.

Nearly 500 B-737 Max jets have remained grounded across the world since March.

Boeing has been working on an upgrade for a stall-prevention system known as MCAS since the two crashes, and hopes to get the jet flying again by year's end.

The 737 Max is the fastest-selling plane in Boeing's history, racking up more than 5,000 orders worth in excess of US$600 billion - including those that have been delivered.

Orders came from over 100 customers worldwide, including Air Canada, Air China, American Airlines, SilkAir and Virgin Australia.