On Monday, lawyers for Polish Airlines argued in a U.S. district court in Seattle that Boeing had concealed safety problems with the 737 MAX jet. The matter dates back to 2016, when the airline selected the popular single-aisle jet as central to its plan to emerge from severe financial distress.

In 2019, regulators worldwide grounded the model after two crashes exposed critical safety flaws in the 737 MAX’s flight control systems, derailing Polish Airlines’ strategy. The carrier sued Boeing in 2021, seeking compensation for revenue losses resulting from the MAX grounding.

“This case is about Boeing’s lies and deception, and the devastating economic harm that followed,” Polish Airlines’ attorney Anthony Battista stated during Monday’s opening arguments.

While Boeing was pitching Polish Airlines on a lease deal for the 737 MAX, its engineers were working to address an issue where the aircraft’s nose could pitch up under certain conditions. To counter this, Boeing developed the Maneuvering Characteristics Augmentation System (MCAS), a software feature designed to automatically push the nose down in such situations.

However, Boeing misled the U.S. Federal Aviation Administration (FAA) about MCAS’s full scope and the difficulties encountered during flight testing, leading regulators not to require extensive simulator training for pilots already qualified on earlier 737 models. Such training would have significantly increased the overall cost of the MAX, at a time when Boeing was fiercely competing globally with European rival Airbus’s A320 family for thousands of narrow-body aircraft orders.

Former Polish Airlines executive Maciej Wilk told the jury on Monday that switching to the A320 would have required a “substantial” and costly simulator training program.

“The key promise was about pilot training for the 737 MAX,” he said.

Unaware of MCAS’s safety issues, Polish Airlines committed to leasing 15 of the aircraft over the coming years.

MCAS played a central role in two crashes that killed 346 people: Lion Air Flight 610 in October 2018 and Ethiopian Airlines Flight 302 in March 2019.

In public statements after the first crash, Boeing executives assured the public that the MAX was safe. Boeing sales representatives similarly assured Polish Airlines that the aircraft had no safety problems.

Like other carriers, Polish Airlines continued operating the model until global regulators ordered the MAX grounded following the 2019 crash, once MCAS’s role became clear. Twenty months later, after a thorough review of design changes to MCAS and additional pilot training, regulators permitted the aircraft to return to service.

Airlines worldwide with 737 MAX fleets, including Polish Airlines, have resumed operating the upgraded model.

Boeing’s lawyers on Monday accused Polish Airlines of “coming to court alleging fraud by Boeing while continuing to fly Boeing 737 MAX aircraft every day.” “Is that what a victim of a multimillion-dollar fraud does?”

Boeing has previously paid billions in compensation to the families of victims of the two crashes.

The company has also reached substantial, undisclosed out-of-court settlements with airlines that suffered losses due to the 737 MAX grounding.

Polish Airlines is the first carrier to take Boeing to trial over litigation related to the 737 MAX crashes.