The Federal Aviation Administration has found a potential risk in the Boeing 737 Max software update that was supposed to improve safety after the jets were grounded worldwide following two fatal crashes since October.
Investigators found a problem with the updated Maneuvering Characteristics Augmentation System, or MCAS, automated flight control system, during a flight simulation. If the MCAS software senses the plane is going into a stall, the system pushes the plane's nose down repeatedly, which is a standard procedure to avoid a stall. Investigators believe the MCAS system may have been responsible for the two deadly crashes.
Pilots testing the MCAS update in a simulator, however, found that it took them too long to recover the airplane if the software was trying to avoid a stall, a source close to the investigation told NBC News.
The FAA said in a statement to NBC News that its job is to find potential problems and that it found "a potential risk that Boeing must mitigate."
"The FAA will lift the aircraft's prohibition order when we deem it is safe to do so," the statement said. "We continue to evaluate Boeing's software modification to the MCAS and we are still developing necessary training requirements."
Boeing has traced the issue to a microprocessor and how the chip handles data, but believes it can address the issue with a software code update.
"The safety of our airplanes is Boeing's highest priority," it said. "We are working closely with the FAA to safely return the MAX to service."
The jets were grounded in March in the wake of that month's Ethiopian Airlines crash and a Lion Air accident in October that together killed 346 people. Boeing said in May that it had tested the 737 Max with the updated software for 360 hours on 207 flights and planned to provide updated training materials for 737 Max pilots.
United Airlines and American Airlines said that it plans to ground 737 Max planes until at least Sept. 3 of this year. United said in a statement Wednesday it expects to cancel approximately 40-45 flights a day for July, which results in roughly 1,290 flights for the month.
"We've used spare aircraft and other creative solutions to help our customers, who had been scheduled to travel on one of our MAX aircraft, get where they are going," United said. "But, it's harder to make those changes at the peak of the busy summer travel season."