Boeing Co. (BA.N) 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 planemaker said it didn’t deactivate a warning that was supposed to show conflicting readings between two angle-of-attack vanes. But the alert only functioned 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 best-selling jet, which has been grounded since shortly after an Ethiopian Airlines 737 Max 8 crashed March 10 -- the second disaster in five months. Boeing is working to convince airlines and regulators that the Max will be safe once an update is installed to so-called MCAS software, which played a role in both accidents after being activated by erroneous angle-of-attack data from a single sensor.

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.

Flawed Manual

“After the Lion Air event, we were notified that the AOA disagree lights were inoperable without the optional AOA indicators on the Max aircraft,” Southwest spokeswoman Brandy King said in an email Sunday, referring to the equipment by its acronym.

“The manual documentation presented by Boeing at Max entry into service indicated the AOA 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 AOA vanes measure the tilt of an aircraft against onrushing wind.

In both 737 Max crashes, the jet’s Maneuvering Characteristics Augmentation System misfired and repeatedly pressed the nose of the planes down until flight crews lost control of both doomed aircraft. The accidents killed 346 people.