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, 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.