April 30 (UPI) -- Boeing has clarified why a safety feature on its 737 Max airplanes was switched off on some models -- an explanation that came after some U.S. pilots said they weren't told until one of the planes crashed last fall.
Monday, Southwest Airlines said the feature -- a warning light to indicate a faulty Angle of Attack sensor -- apparently hadn't been working, and Boeing didn't say anything about it until a Max 8 plane crashed off Indonesia in October. The carrier and pilots said they were led to believe the alert was always functional.
Boeing said late Monday the warning, called a disagree alert, was an optional feature switched on only for carriers that ordered the plane with a separate indicator display. Boeing said it didn't intentionally deactivate the alert.
"Unless an airline opted for the angle of attack indicator, the disagree alert was not operable," the company said.
The AOA sensor is believed to have played a role in the crashes of two 737 Max 8s in Indonesia and Ethiopia. The models have been grounded worldwide until Boeing fixes the fleet. The company says it's working on a software update, but the planes aren't expected to return to flight for months. Southwest and American Airlines said they won't fly them again until at least August.
Boeing has also said it will switch on the AOA sensor warning at no cost, and that all future Max aircraft will have a functional disagree alert.
In an address to shareholders later Monday, Boeing CEO Dennis Muilenburg said the disagree alert isn't a necessary piece of equipment for pilots to safely fly the plane.
"We don't make safety features optional," he said. "Safety has been and always will be our top priority, and every one of our airplanes includes all of the safety features necessary for safe flight.
"We feel the immense gravity of these events and recognize the devastation of the families and friends of the loved ones who perished. We also regret the impact the grounding has had on our airline customers and their passengers."
Muilenburg said Boeing has finished its engineering test flight with the new software, the final step before the certification flight.
"With the certified software update implemented, the 737 MAX will be one of the safest airplanes ever to fly," he added.
Shareholders on Monday rejected a proposal to split Muilenberg's roles as chairman and CEO, which would have stripped him of one of the titles.
Outside the Chicago office building where Muilenburg spoke, Protesters demanded safety and accountability.
"Their response has been a farce," activist Tarek Milleron, whose 24-year-old niece died in the Ethiopian Airlines crash, told The Washington Post. "It's a hollow denial."