During simulator testing of the Boeing 737 Max’s new flight control system software, an issue arose that may further delay re-certification of the airplane. 

Max operator Southwest Airlines released a statement June 27 acknowledging the further delays and pushing back its schedule for returning the Max to service, a cut of about 150 daily flights. Earlier the airline had planned to resume flying the Max after September 2. “With the timing of the Max’s return-to-service still uncertain, we are again revising our plans to remove the Max from our schedule through October 1,” the airline said.

The FAA issued a statement that identified a new problem with the Max’s flight control system. “On the most recent issue, the FAA’s process is designed to discover and highlight potential risks,” said the FAA. “The FAA recently found a potential risk that Boeing must mitigate.”

Boeing said it agrees with the FAA's decision and request, and is working on the required software. “Addressing this condition will reduce pilot workload by accounting for a potential source of uncommanded stabilizer motion,” said the manufacturer. 

“During the FAA’s review of the 737 Max software update and recent simulator sessions, the [FAA] identified an additional requirement that it has asked the company to address through the software changes that the company has been developing for the past eight months,” it added. “The FAA review and process for returning the 737 Max to passenger service are designed to result in a thorough and comprehensive assessment.

“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.”