FAA and EASA are conducting separate reviews of the flight-control software update intended to address the cause of two fatal jet crashes.
Boeing has said it hoped to return the twin-engine aircraft to service during the fourth quarter of 2019.
In addition to Boeing, the delay is causing disruptions to schedules for multiple carriers, including Southwest Airlines, American Airlines, Southwest Airlines, United Airlines, and RyanAir and Norwegian Air in the European market.
Boeing halted deliveries of the 737 series and reduced the production rate to 42 jets/month. Boeing’s software redesign reportedly will incorporate data from two flight-control computers rather than one.
Beyond the return to service, Boeing also aims to resume production and deliveries of the 737 series – which it had hoped to initiate this month.
Collected and summarized from the source below by Minh Pham https://www.americanmachinist.com/news/safety-reviews-keeping-737-max-return-uncertain