Airlines are preparing for more flight cancellations as Boeing readies a software fix for its best-selling 737 Max planes, following two fatal crashes of the aircraft that prompted regulators around the world to ground the plane.

Pilots from U.S. carriers on Saturday tested Boeing’s software changes to the automatic anti-stall system in Renton, Washington, where Boeing assembles the 737 Max planes. Representatives from Southwest Airlines, American Airlines, and United Airlines — the U.S. airlines that fly the 737 Max — also met with Boeing officials about the software changes and additional pilot training.

The government has ordered airlines to suspend flights using the Boeing 737 Max plane, joining dozens of other countries in taking that step amid concerns about the similarities between the March 10 crash of an Ethiopian Airlines 737 Max and a Lion Air crash in October, which together killed 346 people.

Boeing late Sunday said it invited more than 200 airline pilots and regulators to Renton last Wednesday to “share more details about our plan for supporting the safe return of the 737 MAX to commercial service.”

The Federal Aviation Administration expects to get a look at the software early in the week, according to a person familiar with the matter. The agency needs to certify Boeing’s changes before it can be added to the aircraft.

French and Ethiopian investigators said last week there were “clear similarities” between the Ethiopian Airlines and Lion Air crashes.

Scrutiny is mounting on the FAA’s certification of the planes and the anti-stall system Boeing added to the 737 Max planes before they debuted in 2017. That program, the Maneuvering Characteristics Augmentation System, or MCAS, can automatically push the nose of the planes downward in order to avoid a stall in flight. Some pilots complained that they didn’t know the system existed until after the Lion Air crash.

Investigators are still probing what brought down the Ethiopian Airlines jetliner earlier this month. The airline’s Chief Executive Officer Tewolde Gebremariam told the Wall Street Journal that “to the best of our knowledge” MCAS was engaged during the flight and that it would be “very difficult” for Boeing to restore trust in the 737 Max. He said Boeing should have done more to explain the MCAS system both before and after the Lion Air crash in Indonesia.

Earlier Monday, the CEO said in a statement that he “still believes” in Boeing after the crash.

A Senate aviation subcommittee has called FAA officials and the chairman of the National Transportation Safety Board to testify at a hearing Wednesday.

Investigators in the Lion Air crash have indicated that the pilots may have been battling the system that repeatedly pushed the plane’s nose downward. Erroneous data from the plane’s sensor that reads the aircraft’s so-called angle of attack — can be catastrophic.

Among Boeing’s changes include feeding the MCAS system with two angle-of-attack sensors instead of the current one. It would also limit the number of times the nose would automatically tilt downward if inaccurate data is received from the sensors, Boeing said.

While Boeing 737 Max planes make up a small part of their fleets, some airlines are preparing for more flight cancellations as the aircraft remain grounded. Southwest has flown its Boeing 737 Max planes to a facility in the Mojave desert in California. The airline has 34 of the planes in its fleet of about 750 Boeing 737s, more than any other U.S. airline.

The carrier is canceling about 130 flights per day out of a daily schedule of around 4,000 flights and is calling off flights about five days ahead.

American Airlines, which has 24 Max planes in its fleet, on Sunday said it’s canceling 90 flights a day due to the grounding and has canceled flights through April 24, which encompasses the busy Easter and Passover traveling period. American, which operates about 6,700 flights a day, noted that even passengers whose flights were not assigned a 737 Max plane may see cancellations as the carrier deploys planes to other flights.

Compliance with the FAA directive “have caused, and are expected to continue to cause, significant disruption to our customers and financial costs to us,” American said in a filing on Monday. It said it could not currently forecast those costs.

Some airlines are preparing for even longer disruptions. Air Canada said last week it plans to remove its Boeing 737 Max through July.