April 29 (UPI) -- Boeing did not initially tell Southwest Airlines, the largest U.S. operator of the 737 Max 8, that a standard safety feature warning pilots about faulty sensors had been deactivated, the carrier said.
The airline said it was believed pilots would get an alert if the angle of attack sensor produced false readings, allowing them to prevent the aircraft from automatically going into a dive -- a malfunction believed to have caused plane crashes in Indonesia and Ethiopia that grounded the global Max 8 fleet.
"It was presented that it worked," said Jon Weaks, president of the Southwest pilots union.
Southwest told CNBC Boeing's manual for the Max 8 indicated the warnings -- called AOA disagree lights -- were functional. It was only after the Indonesia crash in October, the carrier said, when Boeing informed Southwest the indicators were not active. Southwest said it subsequently activated the cockpit warnings.
"After the Lion Air event, Boeing notified us that the AOA Disagree Lights were inoperable," Southwest said. "At that time, Southwest installed the AOA Indicators."
Angle of attack sensors measure tilt and onrushing wind while preventing the aircraft from entering a stall, which occurs when a plane is moving too slowly. A malfunction with the sensor is suspected in both crashes in Ethiopia and Indonesia that killed a combined 346 people.
Boeing told Southwest after the Indonesia crash in October the safety feature wasn't included in the 737 Max unless the airline bought an additional indicator display, the airline said. Boeing has since made it a standard feature.
"As we return to service, all customers will have the AOA disagree alert as standard and have the option to include the AOA indicator at no cost," a Boeing spokesperson said. "This change will be made to all Max aircraft -- production and retrofit."
Southwest has 34 Max 8 jets in its fleet, all of which are grounded through at least Aug. 5. Southwest's first-quarter revenue took a $200 million hit and lost $150 million in net income, in large part because the fleet wasn't able to fly.
"We're not happy with the situation," Southwest President Tom Nealon said last week. "Who would be? Boeing has already conceded that there are things that they need to address. And obviously, we totally agree with that."
Boeing CEO Denis Muilenburg will face questioning Monday from investors at the company's shareholder meeting in Chicago. Protesters are also expected outside from demonstrators weary of the plane-maker's commitment to safety.