Pilots at Southwest Airlines, the largest operator of the Boeing 737 MAX jet in the US, believe the plane won’t return to service until February 2020, as much as two months later than the aircraft manufacturer’s target, China Daily reported

Like competitors American and United, Southwest Airlines, which has a MAX fleet of 34, has said it expects the MAX to resume commercial flights in January. Boeing believes the aircraft will return to service in the fourth quarter of this year.

The Southwest Airlines Pilots Association (SWAPA) has sued Boeing, alleging the manufacturer “deliberately misled” the airline about MAX, forcing the cancellation of about 30,000 flights and costing the pilots about US$100 million in lost wages. The airline has said it will share any payment from Boeing over the MAX grounding with employees.

SWAPA’s lawsuit, filed in Dallas, alleges that Boeing “abandoned sound design and engineering practices, withheld safety critical information from regulators and deliberately misled its customers, pilots and the public about the true scope of design changes to the 737 MAX.”

In a statement, Boeing said the lawsuit is “meritless” and pledged to “vigorously defend against it.” Boeing said it will continue to work with the airline and its pilots to guarantee the safe return of the MAX to service, the report said.

“We continue to support the regulators and our customers as we work to safely return the MAX to service,” Boeing said.

American Airlines has canceled MAX flights through December and its pilots also seek compensation for lost pay stemming from the plane’s grounding, but they have not filed a lawsuit.

Boeing has yet to submit updated software for the plane’s anti-stall device to the Federal Aviation Administration for review. Other regulators then must approve the plane, and that could mean it will return to service at different times in different jurisdictions.

In April, the FAA asked regulators from nine countries, including China, to participate in the review of the US regulator’s oversight and approval of the Boeing 737 MAX’s Maneuvering Characteristics Augmentation System (MCAS), an automated anti-stall device. Investigators believe MCAS may have erroneously pointed the nose of the plane down to gain speed to prevent a mid-air stall and into a fatal plunge.

The crashes Oct 29, 2018, in Indonesia and March 30 in Ethiopia killed 346 passengers and crew. China was the first to ground the MAX, and the US was the last.

Meanwhile, Boeing may have to book billions of dollars in additional charges, two brokerages said on Monday, following latest developments around the planemaker’s grounded 737 Max jet that calls into question the timing of the aircraft’s return to service, the CBC reported.

Credit Suisse and UBS downgraded the stock after reports  showed internal messages between two Boeing employees stating that the plane’s anti-stall system behaved erratically during testing before the aircraft entered service, the report said.

In newly released instant messages from 2016, a top Boeing 737 Max test pilot tells a colleague that the jet’s MCAS flight control system — the same one linked to two fatal crashes — was “running rampant in the (simulator) on me.”

The new revelations pose fresh challenges for Boeing, which is reeling under pressure after two fatal crashes forced the company to ground the planes and book billions of dollars in losses.

The company on Sunday expressed regret over the messages, and said it was still investigating what they meant.