American Airlines won’t use Boeing 737 Max aircraft until at least September, pushing back the expected reintroduction of the troubled plane by at least two more weeks. The airline announced Sunday that, while it “remains confident” that a forthcoming software update and new training efforts from Boeing will be enough to get the 737 Max recertified, it wants to give “customers and team members” ample time to “reliably plan their upcoming travel.”

Boeing said in May that it had completed the software update meant to fix the anti-stall system that contributed to two deadly crashes in five months, but that the Federal Aviation Administration (FAA) was holding up the process. Boeing has completed its own test flights using the new software, but still needs to perform a certification flight as part of the FAA’s formal review process.

American Airlines expects to lose $350 million as a result during what could be the busiest summer travel season ever

In the meantime, American Airlines has to cancel 115 flights per day between now and September 3rd, when it expects to be able to fly the 737 Max again. American is one of three US airlines that flies the 737 Max, along with United and Southwest. The company said in April that it expects to lose $350 million while the 737 Max is grounded, and Sunday’s announcement means those losses could deepen. United and Southwest had previously canceled thousands of 737 Max flights through August, during what is expected to be the busiest summer travel season ever.

Countries around the world grounded the 737 Max in March after one of the planes (operated by Ethiopian Airlines) crashed shortly after takeoff, killing all 157 people on board. It was the second fatal 737 Max crash in five months, the first being a doomed Lion Air flight where all 189 people on board were killed last October.

Both crashes had to do with a change Boeing made to how the 737 Max operates — one the company did not properly disclose to airlines and their pilots, according to the FAA.

Boeing designed the 737 Max to use bigger engines than its predecessors in order to improve fuel efficiency, as it fought for market share with France’s Airbus. These bigger engines had to be mounted higher up and more forward compared to older 737s, which affected how the Max handled in certain situations.

In particular, the 737 Max’s nose would sometimes pitch so high on takeoff it risked stalling the engines. Boeing’s fix for this was an automated system called Maneuvering Characteristics Augmentation System, or MCAS. MCAS was supposed to recognize when the nose pitched too high by taking data from so-called “angle-of-attack” sensors (essentially small weather vanes that measure the angle a plane is pointing). It would then automatically point the nose down if it pitched too far up.

While commercial aircraft are often outfitted with multiple angle-of-attack sensors, Boeing designed MCAS to pull data from just one. And in both fatal crashes, it appears those sensors were damaged — a somewhat common problem. As a result, the MCAS system on each flight repeatedly tried to pitch the plane down in order to avoid a stall that wasn’t about to happen. The pilots were not able to shut off MCAS in large part because they probably didn’t know how; Boeing made the change in such a way that it did not require new training, which would have cost the company millions of dollars.