Boeing Misinformed Southwest Airlines About MAX AoA Warnings (Revised)

Southwest Airlines, Boeing's biggest customer for the troubled 737 MAX, said this week that the airplane maker's documentation incorrectly claimed that its aircraft had operable angle-of-attack disagree warning lights. But Boeing informed Southwest that the AoA function was actually inoperative only after the Lion Air crash in Indonesia last October.

Even as Boeing finishes software revisions on its 737 MAX airplanes and Boeing CEO Dennis Muilenburg insisted the original design met certification standards and was safe, Southwest's statement suggests the airline didn't get what it paid for. In a statement, Southwest said: "Upon delivery (prior to the Lion Air event), the AOA Disagree Lights were depicted to us by Boeing as operable on all MAX aircraft, regardless of the selection of optional AOA Indicators on the Primary Flight Display (PFD). The manual documentation presented by Boeing at Southwest's MAX entry into service indicated the AOA Disagree Light functioned on the aircraft, similar to the Lights on our NG series. After the Lion Air event, Boeing notified us that the AOA Disagree Lights were inoperable without the optional AOA Indicators on the MAX aircraft. At that time, Southwest installed the AOA Indicators on the PFD, resulting in the activation of the AOA Disagree lights - both items now serve as an additional crosscheck on all MAX aircraft."

In its own statement, Boeing said the company "included the disagree alert as a standard feature on the MAX, although this alert has not been considered a safety feature on airplanes and is not necessary for the safe operation of the airplane. Boeing did not intentionally or otherwise deactivate the disagree alert on its MAX airplanes."

But Boeing added that "the disagree alert was not operable on all airplanes because the feature was not activated as intended.The disagree alert was tied or linked into the angle of attack indicator, which is an optional feature on the MAX. Unless an airline opted for the angle of attack indicator, the disagree alert was not operable."

The disagree lights are of interest because the investigations into two 737 MAX hull losses—Lion Air in Indonesia and Ethiopian Airlines near Addis Ababa last month—focus on the MAX's MCAS stability augmentation system. MCAS—for Maneuvering Characteristics Augmentation System—is active when the aircraft is hand flown at high angles of attack with flaps and slats up. It rolls in nose-down stabilizer trim to lower the aircraft angle of attack.

MCAS derives data from a single AoA sensor and faulty sensors are implicated in both crashes. If the aircraft had had operable AoA disagree lights, the pilots might have known MCAS activation was being caused by faulty data. In fact, according to the summarized CVR data from the Ethiopian crash, the pilots did mention faulty AoA indications.

Shortly after the Lion Air crash, Southwest added a software update that displays the actual AoA on the cockpit primary flight displays. But, as the airline's statement suggests, Boeing documentation led Southwest to believe the AoA disagree warning would function whether the PFDs had the angle values displayed or not.

The revised MCAS software, according to an Aviation Week report, uses data from both AoA sensors and includes filters to detect anomalies that would indicate one or both sensors are unreliable. The flight computer would then inhibit MCAS.

Although media outlets—including AVweb—have described MCAS as a stall-protection feature, Boeing now insists that it never was. MCAS was designed to address stability issues at the very corner of the flight envelope with slats and flaps retracted and at light weights with full aft center of gravity. MCAS is inhibited when the airplane is flown on autopilot.

Boeing is finishing testing on the revised software, but no firm date has been set for returning more than 350 grounded MAX aircraft to service.