Here’s why Boeing 737 Max hasn’t returned to service yet

The Federal Aviation Administration has found a new problem in Boeing's troubled 737 Max that the company must address before the regulatory agency will allow the airplanes to fly passengers again. The discovery further delays the airliner's return to service.

Boeing's popular narrow-body aircraft has been grounded since March after an Ethiopian Airlines 737 Max crashed shortly after taking off from the airport in Ethiopia's capital, Addis Ababa, killing all 157 people on board. It was the second crash of a Max plane in five months; as a Lion Air jet crashed in Indonesia last October, killing 189 people.

Investigators link both crashes, in part, to an automated flight control system that acted on erroneous information from malfunctioning sensors and put the planes into nose dives the pilots could not pull the planes out of.

Boeing has developed a software fix for that flight control system, called MCAS, but sources familiar with the situation tell NPR that in simulator testing last week, that FAA test pilots discovered a separate issue that affected their ability to quickly and easily follow recovery procedures for runaway stabilizer trim and stabilize the aircraft.

A statement from the regulatory agency says as part of a process designed to discover and highlight potential risks, "the FAA found a potential risk that Boeing must mitigate."

Boeing says in a statement that the company is working on the required software fix to address the FAA's request. A spokesman told NPR the company is committed to working closely with the FAA to safely return the 737 Max to service.