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The investigation into the crash of an Ethiopian Airlines 737 MAX 8, which killed 157 people on Sunday, is still in its earliest stages, but already regulators around the world have grounded the Boeing jet. The American FAA issued its own grounding order today, noting that, based on the wreckage and satellite-based tracking of the jet’s route, it found similarities between this crash and that of the Lion Air 737 MAX 8, which crashed in Indonesia in October, killing 189. Those similarities “warrant further investigation of the possibility of a shared cause for the two incidents that need to be better understood and addressed,” the FAA wrote.
The thing is, Boeing and the FAA had already settled on a way to address the likely cause of the Lion Air crash. And they were well on their way to implementing it when the Ethiopian plane went down.
Indonesia’s civil aviation authority hasn’t published any findings as to the cause of the Lion Air crash, but in its preliminary report, the agency examined the MAX’s Maneuver Characteristics Augmentation System, or MCAS. Boeing designed the system after discovering during flight testing that the 737 MAX engine placement—higher and farther out on the wing than on the previous generation—could pitch the plane upward in certain conditions, increasing the likelihood of a stall.
When the MCAS detects the plane climbing too steeply without enough speed—a recipe for a stall—it moves the yoke forward, using the horizontal stabilizer on the tail to bring the nose of the plane down. “It’s a fancy name for what we used to call ‘a stick pusher,’” says Ross Aimer, the CEO of Aero Consulting Experts, who is rated to fly every type of Boeing jet. It’s distinct from an autopilot system, and only kicks in when the plane is being manually flown, the way a car’s traction control activates to keep a driver from skidding out.
The problem with the Lion Air flight was the MCAS went to work when it shouldn’t have. The 737 MAX was climbing normally, but due to a faulty sensor, the digital flight data recorder detected a hard-to-believe 20-degree difference in the angle of attack between the left and right sides. Over the next 10 minutes, the pilots repeatedly tried to pull the plane’s nose back up, but the MCAS kept forcing the yoke forward, pushing the plane down. Ultimately, the plane crashed into the Java Sea, killing everyone aboard.
If the pilots had known the MCAS was at fault, they could have shut down the plane’s ability to automatically adjust its trim (which determines its position in the air), so they could manually do it themselves. But they ended up “behind the airplane,” confused and trying to figure out what the computer was up to. “When you’re behind the airplane, you’re almost dead,” Aimer says.
That may be because they didn’t know the MCAS existed: In the jet’s manual, Boeing had not noted its presence. In November, The Wall Street Journal reported that the company had worried about “inundating average pilots with too much information—and significantly more technical data—than they needed or could digest.” Meanwhile, Boeing was pitching the 737 MAX to airlines as being so similar to the previous generation, pilots flying the old 737 would barely need any new training.
Soon after the Lion Air crash, the FAA issued an Emergency Airworthiness Directive, telling Boeing to update its manual and provide instructions for how to handle what happened to the Lion Air pilots. “If an erroneously high single angle of attack (AOA) sensor input is received by the flight control system, there is a potential for repeated nose-down trim commands,” it wrote.
Like most modern plane crashes, the Lion Air one was born of multiple problems. The angle of attack sensor’s incorrect reading triggered a system the pilots didn’t know existed and didn’t know how to manage. “We have this strange failure of this sensor signal, and behind it unfortunately we have a weak system implementation,” says Bjorn Fehrm, a Swedish Air Force veteran and aviation analyst with Leeham News and Analysis.
The solution, then, is two-fold: Boeing started by warning airlines that the MAX’s angle of attack sensors had malfunctioned before, that such a failure could lead the MCAS to push the plane’s nose down, and that pilots could safely defuse the problem by cutting off the trim system and working the plane manually.
After making sure pilots knew about the problem and how to resolve it, Boeing would work on a long term solution. Essentially, it would rejigger the software governing MCAS so that it wouldn’t be as prone to jumping into action based on one scary sensor reading, instead considering more data. And it would limit how many times it can engage.
Boeing said it would have it done within a few months. Then, the Ethiopian Airlines jet crashed. We don’t yet know if the jet’s MCAS system is what brought the plane down, or what other factors may have been at work. We do know that what seemed a straightforward fix to an unforeseen problem is now muddied—and that the 737 MAX won’t take off again until it’s been cleared up.
