TL;DR:

Boeing upgraded the 737 to the 737 MAX 800. Part of the MAX is MCAS, a system that prevents stalls. That system relies on an angle-of-attack sensor, that prior to the MAX, had a warning system that would say "your angle of attack vane is fucked." For the MAX, however, that warning system was disabled and could only be enabled if the customer opted to pay for it. Meanwhile, Boeing didn't tell Southwest (at least) that they'd disabled this system and all the flight manuals indicate (still!) that the system is in place.

So there's this vane on the front of the nose. In a normal 737, you hear alarm bells when that vane malfunctions. In a 737 MAX, the plane flies itself into the ground when that vane malfunctions. And it's stronger than you. And you're used to hearing alarm bells. And you don't anymore because Boeing has decided your boss should pay extra for that.

Some people better go to jail for this one.

Full text follows.

Plane maker Boeing Co. BA -0.53% didn’t tell Southwest Airlines Co. LUV -0.69% when the carrier began flying 737 MAX jets in 2017 that a standard safety feature, found on earlier models and designed to warn pilots about malfunctioning sensors, had been deactivated.

Federal Aviation Administration safety inspectors and supervisors responsible for monitoring Southwest, the largest MAX customer, were also unaware of the change, according to government and industry officials.

Boeing had turned off the alerts which, in previous versions of the 737, informed pilots if a sensor known as an “angle-of-attack vane” was transmitting errant data about the pitch of a plane’s nose. In the MAX, which featured a new automated stall-prevention system called MCAS, Boeing made those alerts optional; they would be operative only if carriers bought additional safety features.

Southwest’s cockpit crews and management didn’t know about the change for more than a year after the planes went into service. They and most other airlines operating the MAX globally learned about it only after the fatal Lion Air crash last year led to scrutiny of the plane’s revised design. The FAA office’s lack of knowledge about Boeing’s move hasn’t been previously reported.

“Southwest’s own manuals were wrong” about the status of the alerts, said Southwest pilots union president, Jon Weaks. Since Boeing hadn’t communicated the modification to the carrier, the manuals still reflected incorrect information.

Following the Lion Air crash, Southwest asked Boeing to reactivate the alerts on planes already in its fleet. This move, along with questions about why they had been turned off, prompted FAA inspectors overseeing Southwest to consider recommending that the airline’s MAX fleet be grounded while they assessed whether pilots needed additional training about the alerts. Those internal FAA discussions, however, were brief and didn’t go up the chain, according to documents reviewed by The Wall Street Journal.

Boeing hasn’t specifically addressed why it turned off the feature, called “AOA disagree alerts,” without informing customers. Questions surrounding that move have remained unanswered since October, when the Lion Air accident killed 189 people, followed by an Ethiopian Airlines crash in March of the same model that took 157 lives. MAX planes remain grounded. Boeing recently said it would book $1 billion in expenses tied to the groundings and related business disruptions.

In previous 737 models, the computer-generated alerts appear as colored lights in the cockpit when a plane’s twin angle-of-attack sensors provide significantly different data from each other. In the MAX, they serve the same purpose but additionally are intended to warn pilots that MCAS, the new automated system implicated in both accidents, could misfire because of faulty sensor data.

MCAS commands that automatically push down the nose of a plane can overpower a pilot’s efforts to get out of a dive. In the Ethiopian jet, which lacked the disagree alerts, it took four minutes for the pilots to realize that incorrect data was coming from one of the sensors, according to investigators’ preliminary report.

A Boeing spokesman said that from now on, “customers will have the AOA disagree alerts as standard” on all MAX aircraft, including those coming out of the factory and already delivered to airlines. Boeing is currently devising a new software package that aims to fix MCAS by making it less powerful, while also restoring the alerts. The moves are among the safeguards the plane maker and FAA have embraced to make MCAS less hazardous if it misfires, and to get the fleet back in the air.

A Southwest spokeswoman said that before the loss of Lion Air Flight 610, the carrier had assumed the alerts “as operable on all MAX aircraft.” Boeing “did not indicate an intentional deactivation,” she said. Today, the reinstated feature offers “an added cross-check on all MAX aircraft,” even though none are flying.

Although the alerts were reactivated, some midlevel FAA officials who oversaw Southwest briefly considered the possibility of grounding its fleet of roughly 30 of the 737 MAX aircraft until the agency established whether pilots needed to receive new training, according to documents reviewed by the Journal.

Less than a month after the Lion Air jet went down, one FAA official wrote that AOA-related issues on 737 MAX jetliners “may be masking a larger systems problem that could recreate a Lion Air-type scenario.”

Roughly two weeks later, other internal emails referred to a “hypothetical question” of restricting MAX operations with one message explicitly stating: “It would be irresponsible to have MAX aircraft operating with the AOA Disagree Warning system inoperative.” The same message alluded to the FAA’s power: “We need to discuss grounding [Southwest’s] MAX fleet until the AOA Warning System is fixed and pilots have been trained” on it and related displays.

The email discussions, previously unreported, were fleeting red flags raised by a small group of front-line FAA inspectors months before the Ethiopian jet nose-dived last month. The concerns raised by the FAA inspectors never progressed up the agency. Within days, they were dismissed by some involved in the discussions who concluded that the alerts provided supplemental pilot aids rather than primary safety information, and therefore no additional training was necessary. During that stretch and beyond, Boeing and the FAA continued to publicly vouch for the aircraft’s safety.

These very concerns, however—ranging from potential training lapses to confusion by many aviators about the specifics of angle-of-attack alerts—have now emerged as high-priority items as Boeing’s design decisions face scrutiny. The issues are among those being pursued by various congressional, criminal and Transportation Department investigators, say people with knowledge of their lines of inquiry.

On Wednesday, a Boeing spokesman said that while the internal FAA discussions were under way last year, “there was no data that indicated the fleet should be grounded.”

An FAA spokesman said the agency expects to mandate that all 737 MAX aircraft include working disagree alerts. But government and industry officials said questions about why the alerts were turned off in the first place remain central to uncovering the history and safety problems surrounding the MAX fleet.

Testifying before a Senate panel last month, acting FAA chief Daniel Elwell said one important factor is prioritizing data pilots receive. “Every piece of real estate in a cockpit is precious,” he said. “You put one gauge up there, you are sacrificing another.”

At American Airlines Group Inc., one of the few carriers that initially had working angle-of-attack alerts as part of a broader array of optional MAX safety features for which it paid extra, pilots are still anxious to see Boeing and the FAA get all the steps right to end the grounding.

In a meeting about a month after the first crash, a Boeing executive told American Airlines pilot union officials that American’s MAX cockpit warning lights would have helped them avoid problems like those encountered by the Lion Air pilots, union officials who attended the meeting said. A Boeing spokesman has previously said the executive didn’t recall making that statement.

“Our minds are not at ease on this,” said Dennis Tajer, the pilot union spokesman.

—Andrew Tangel, Robert Wall and Alison Sider contributed to this article.