American Airlines said Sunday that it would extend flight cancellations for Boeing 737 Max aircraft by four months while it waits for Boeing and the Federal Aviation Administration to fix and recertify the planes’ flight-control systems, which have been implicated in two deadly plane crashes in recent months. The cancellations will affect about 115 flights through Aug. 19.

The move reflects a growing realization among Boeing’s airline customers that a worldwide grounding of Boeing jets, now in its fourth week, is unlikely to wrap up soon. In early April, the company took the extraordinary step of cutting its 737 Max production rate from 52 per month to 42. And Southwest Airlines, the other U.S. airline that has 737 Max 8 jets, also has cancelled flights involving the aircraft through August.

In a letter to pilots and employees, American Airlines President Robert Isom and CEO Doug Parker said the airline is confident that Boeing and the FAA will recertify the Max jet before mid-August, at which time the airline would bring the planes back into service as spares to supplement its operations throughout the summer.

“By extending our cancellations through the summer, we can plan more reliably for the peak travel season and provide confidence to our customers and team members when it comes to their travel plans,” Parker and Isom wrote.

They added that the airline has been in close contact with the FAA, the Department of Transportation, the National Transportation Safety Board and international regulators and are “pleased with the progress so far.”

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“We remain confident that the impending software updates, along with the new training elements Boeing is developing for the MAX, will lead to recertification of the aircraft soon,” Parker and Isom wrote.

Boeing chief executive Dennis Muilenburg said at the George W. Bush Presidential Center in Texas last week that the manufacturer is working closely with its airline customers in its effort to return the Max jets to flight.

“My team and I are working closely with our customers to answer their questions, get their feedback and ensure those who operate the Max are prepared when the grounding is lifted and the fleet returns to flight,” Muilenburg said.

Boeing 737 Max 8 and 9 jets have been grounded in the United States since March 13, when the FAA concluded that it needed to investigate “the possibility of a shared cause” between the two crashes. Investigators have concluded that an automatic antistalling feature known as the Manoeuvring Characteristics Augmentation System ― which can point the plane’s nose automatically downward in certain rare but dangerous situations ― played a role in both crashes.

Boeing and the FAA announced March 12 that they would develop and recertify a set of modifications to the MCAS system and its related flight-control software. The software fix is to include changes to prevent the system from overreacting to bad data, a new set of cockpit alerts designed to make pilots aware of potentially dangerous situations, and a new pilot training course.

The FAA initially said Boeing would complete the software fix “no later than April.” A Boeing executive speaking on the condition of anonymity because he was not authorized to discuss the issue publicly said late last month that the training component of that update had been “provisionally approved” by regulators. The company had initially planned to submit its full package of software and training changes to the FAA for review by the first week of April, but the submission was delayed.

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In remarks on Thursday, Muilenburg said the company had completed 96 test flights totalling a little over 159 hours of flight time on 737 Max jets with the updated software. He also noted that about two-thirds of the company’s 50 737 Max operators worldwide had participated in a simulator session that included the new software. For one test flight in early April, Muilenburg joined the pilots on one of the planes.

“We’re taking a comprehensive, disciplined approach, and taking the time to make sure we get it right,” he said.

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