Boeing has flown test flights of its 737 MAX to evaluate a fix for the system targeted as a potential cause of two deadly plane crashes, two sources familiar with the matter said.
The aviation giant, which has been under fire and its flagship narrow-body planes grounded since March 13, tested the system upgrade on Monday, two days after pilots from American Airlines did simulation flights in Renton, Washington, the sources said.
Boeing needs authorizsation from the Federal Aviation Administration before the MAX can return to service. But the company still has not submitted its proposed software patch to the FAA.
The aircraft was grounded following two deadly accidents involving Ethiopian Airlines earlier this month and Lion Air in October which killed 346 people.
Engineers have been focusing on addressing problems with the MCAS system, a stall prevention system designed to point the nose of the 737 MAX 8 downward if it is in danger of stalling, or losing lift.
The system has been criticised since it can malfunction, and make it difficult for pilots to control the aircraft. Both of the recent crashes occurred moments after takeoff.