Boeing Co. acknowledged that a cockpit alert notifying pilots of a sensor malfunction linked to two fatal accidents wasn’t working as represented on every 737 Max.

The plane maker said it didn’t deactivate a warning that was supposed to show conflicting readings between two angle-of-attack vanes. But the alert functioned only on jets that had a separate indicator display — available for a fee — with readings from the sensors, Boeing said in a statement Monday.’

“The disagree alert was intended to be a standard, stand-alone feature on Max airplanes,” the company said. “However, the disagree alert was not operable on all airplanes because the feature was not activated as intended.”

The disclosure adds a new mystery about the design of Boeing’s bestselling jet, which has been grounded since shortly after an Ethiopian Airlines 737 Max 8 crashed March 10 — the second deadly Max disaster in five months. Boeing is working to convince airlines and regulators that the Max will be safe once an update is installed to software that played a role in both accidents after being activated by erroneous data from a single sensor.


In both crashes, the jet’s Maneuvering Characteristics Augmentation System, or MCAS software, misfired and repeatedly pressed the nose of the planes down until flight crews lost control of the doomed crafts. The accidents killed everyone aboard the two planes, a total of 346 people.

Southwest Airlines Co., the largest Max operator, said it first learned from Boeing about the problem with the disagree alert after a 737 Max crash in Indonesia in October, when a Lion Air jet slammed into the Java Sea. The Max debuted in May 2017.

“After the Lion Air event, we were notified that the [angle of attack] disagree lights were inoperable without the optional [angle of attack] indicators on the Max aircraft,” Southwest spokeswoman Brandy King said in an email Sunday.

“The manual documentation presented by Boeing at Max entry into service indicated the … disagree light functioned on the aircraft, similar to how it works on” the previous 737 model, she said.


All information needed to safely operate the plane is provided in the flight deck and flight-deck display, Boeing said. That includes air speed, aircraft attitude and altitude displays, as well as an audible and physical alarm known as a stick-shaker.

At Southwest, pilots aren’t trained to use angle-of-attack information routinely because they don’t use it to fly, said Jon Weaks, the head of the carrier’s aviators union. The vanes measure the tilt of an aircraft against onrushing wind.