Pilots test Boeing 737 Max 8 software update as more flights grounded

March 25 (UPI) -- Major U.S. airlines are preparing for more canceled flights as the Boeing 737 Max 8 remains grounded and the aircraft manufacturer launches a software update.

Southwest, American and United airlines pilots tested new software patches for the planes while executives met to talk about additional pilot training.

The pilots using the flight simulator landed safely with the new software update. Two foreign airlines were also flying the simulator.

All 737 Max 8 aircraft were grounded on March 13, three days after Ethiopian Airlines Flight 302 crashed after takeoff, killing 157 people. The United States was one of the last countries to ground its flights. Investigators believe the Ethiopian crash had similar issues to the Lion Air crash that killed 189 people in October, prompting concerns about the aircraft's safety.

At issue is the aircraft's anti-stall system that can give false readings, forcing the aircraft into a dive to avoid a stall.

Ethiopian Airlines' CEO said the stall prevention system was activated as Flight 302 was crashing.

Some pilots have said they didn't know the aircraft would do that until after the Lion Air crash.

Southwest Airlines flew all 34 Boeing 737 Max 8s to the Mojave desert in California. The airline is canceling 130 flights a day. American Airlines has 24 of the planes and said it's canceling 90 flights a day.