Southwest and United Airlines have extended the cancellation of flights using Boeing 737 Max aircraft, following the news that the Federal Aviation Administration (FAA) has discovered a new flaw in the plane’s flight computer.

Southwest Airlines had previously announced it would begin using the 737 Max again starting September 2nd, as long as the plane was re-certified by the FAA. Now, the airline says it won’t start using the plane again until at least October 1st. Some 150 flights will be removed from Southwest’s schedule of 4,000 per day.

United Airlines had previously canceled 737 Max flights through August 3rd. But the company announced Wednesday that it was extending that cancellation through at least September 3rd. United will have to drop between 40 to 45 flights per day in July, and 60 per day in August, according to a statement.

The airlines are canceling more than 1,000 flights per month with the 737 Max grounded

American Airlines extended its own cancellation of 737 Max flights to at least September 3rd earlier this month. But the company is not moving that date again (yet, at least) following the FAA’s discovery. “Our team continues to work collaboratively with the FAA, Boeing and the Allied Pilots Association,” Leslie Scott, a spokesperson for American Airlines, said in an email to The Verge. “At this point, I don’t have anything additional to share.”

The 737 Max was grounded across the globe in March after two of the planes crashed within five months, killing 346 people total. The crashes both had to do with a piece of software that Boeing had installed on the 737 Max known as the Maneuvering Characteristics Augmentation System, or MCAS.

Boeing designed the 737 Max with bigger engines to improve the plane’s fuel efficiency — a metric that affects the ultimate cost of operation, one that is crucial in the fight with rival Airbus over market share. But the 737 Max’s bigger engines needed to be mounted higher up and further forward than on older 737s, making the plane susceptible to stalls in some situations.

MCAS was supposed to help compensate for these differences. It was designed to recognize when the plane’s nose pitched too high, which it measured by taking readings from “angle-of-attack” sensors — small, weather vane-like features that measure the angle a plane is pointing at any given time. MCAS would automatically tilt the nose down if it pitched too far up.

Boeing did not properly disclose MCAS to airlines or pilots, according to the FAA, because doing so would have required new training, which would have cost the company millions of dollars.

It was already taking longer than Boeing expected to get the 737 Max back in the air

A software fix has been ready for months, though the FAA had not yet flight-tested Boeing’s solution. But this week, the FAA discovered another potential flaw in the 737 Max’s computer system. The agency found 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,” according to Bloomberg.

The FAA asked Boeing to fix the problem, which quickly acknowledged the flaw. One Boeing official told Reuters Thursday that a software update fix won’t be ready until September, which would mean the 737 Max might not return to flight until October.

If that happens, United and American Airlines would have to adjust their cancellations again. Both airlines said Thursday that they aren’t making another change just yet, though.

“If we need to further adjust the schedule, we will do so in advance in order to provide advanced notice to our customers and team members,” Scott said of American Airlines.

“We are still keeping our Sept 03 date at this time,” United Airlines spokesperson Frank Benenati said.