A new "potential risk" has been discovered that will keep the US plane-maker's model grounded. The 737 Max has crashed twice since October last year.
Boeing was hoping the troubled jet would be back in use by the end of June following encouraging signs from authorities, but this latest setback means a new certification is unlikely before the end of July.
Out of control
Boeing has been working on its stall-prevention system known as MCAS since a Lion Air crash in Indonesia in October 2018. The pilots were believed to have lost control of the plane's nose due to a fault in the software.
A second crash in March of this year, in Ethiopia, killing all 157 on board, led to the model being grounded after similarities were found between the two crashes.
Workers pick up wreckage from the plane crash in Ethiopia that killed 157 people. The crash led to the 737 MAX being grounded until further notice
New defect discovered
"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 news agency Reuters. "The FAA will lift the aircraft's prohibition order when we deem it is safe to do so."
Boeing was in complete agreement with the authorities on the matter and would continue to seek a solution.
"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 company said.
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Meanwhile, American Airlines, United Airlines and Southwest Airlines will not be using the Max as part of their fleets until at least September.
jsi/se (AP, Reuters, dpa)