Why it matters: Boeing’s grounded 737 Max jets are going to take even longer to get back in the air after the US Federal Aviation Administration (FAA) discovered a new potential safety risk.

The aircraft were grounded following two crashes in Indonesia and Ethiopia that saw a total of 346 people lose their lives. Both incidents are believed to have been caused by the 737 Max’s new Manoeuvering Characteristics Augmentation System (MCAS), an anti-stall measure. Following the March crash, the 737 Max was grounded worldwide while it awaited a software fix and approval from aviation authorities to restart flights. The situation has forced Boeing to use its employee car parks to store some of the jetliners.

During a simulator test last week, the FAA identified another, unrelated safety flaw, which the regulator said Boeing “must mitigate.” While it didn’t elaborate on the problem—it could be a software or hardware issue—Bloomberg’s sources say that “data processing by a flight computer on the jetliner could cause the plane to dive in a way that pilots had difficulty recovering from in simulator tests.”

The FAA last month said that the 737 Max jets could receive approval to fly again as soon as late June. But the newly discovered issue means Boeing will not conduct a certification test flight until July 8 at the earliest, after which the FAA will spend at least two to three weeks examining the results before deciding whether the planes can return to service.

“On the most recent issue, the FAA’s process is designed to discover and highlight potential risks. The FAA recently found a potential risk that Boeing must mitigate,” the FAA said in the statement emailed to Reuters. “The FAA will lift the aircraft’s prohibition order when we deem it is safe to do so.”

Boeing said it was “working closely with the FAA to safely return the MAX to service” when asked about the new issue.

American Airlines and Southwest Airlines already canceled flights up until early September due to the grounding. Yesterday, United Airlines said it would not use the 737 Max until September 3.