Indonesia 737 crash caused by “safety” feature change pilots weren’t told of

By Sean Gallagher

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On November 6, Boeing issued an update to Boeing 737 MAX aircrews. The change, directed by the Federal Aviation Administration (FAA), came because Boeing had never provided guidance to pilots on what to do when part of an updated safety system malfunctioned—the very scenario that the pilots of Indonesia's Lion Air Flight 610 faced on October 29. Not knowing how to correct for the malfunction, the aircrew and their passengers were doomed. All aboard were lost as the aircraft crashed into the Java Sea.

First approved for commercial operation by the FAA on March 8, 2017, the MAX is just beginning to be delivered in large volumes. Lion Air was one of Boeing's primary foreign customers for the MAX, which is also flown by Southwest Airlines, American Airlines, and Air Canada. The Lion Air aircraft lost in the accident was virtually brand new, delivered by Boeing in August; this was the first accident involving an aircraft touted for its safety.

But Boeing never told pilots about one key new safety feature—an automated anti-stall system—or how to troubleshoot its failure. The manual update raised an outcry from pilots in the US.

Allied Pilots Association spokesperson and 737 captain Dennis Tajer told Reuters that his union members were only informed of a new anti-stall system that had been installed by Boeing on 737 MAX aircraft after the Lion Air crash. “It is information that we were not privy to in training or in any other manuals or materials,” Tajer told Reuters.

Jon Weaks, president of the Southwest Airlines Pilots Association, told Bloomberg, “We don’t like that we weren’t notified.” Southwest has ordered 257 737 MAX aircraft; American has orders for 85 still pending.

A stall is a dangerous situation where the lift from an aircraft's wings no longer is enough to counter the weight of the plane, causing it to lose control and fall. Stalls usually happen when an aircraft's angle of attack (AOA)—the angle relative to the horizon that the aircraft's nose is pointing—reaches an angle that reduces the lift the wings can produce at its current air speed; pulling the nose of the aircraft up at lower air speed can result in a stall. If the airplane's tail is in the wake of the wings during a stall, the aircraft can enter what is called a "deep stall," and the pilots may lose the ability to control the aircraft.

Top: normal flight air flows. Bottom: a "deep stall," in which the tail is in the turbulence wake of the wings.
Top: normal flight air flows. Bottom: a "deep stall," in which the tail is in the turbulence wake of the wings.

In the past, anti-stall systems have either issued audible warnings to pilots or, as in newer systems, used a sensory feedback system that warns the pilot by putting more resistance on the aircraft's control stick or yoke when the pilot is approaching the critical angle. But the new system in the 737 uses data from the aircraft's AOA and airspeed sensors to proactively counter pilot error, adjusting the aircraft's controls to push the nose down if the sensors indicate the aircraft could stall.

Initial data from the investigation of the crash of Lion Air Flight 610 indicates that the AOA sensor was providing "erroneous input," according to a Boeing statement. The Operational Manual Bulletin sent out by Boeing on November 6 provides procedures for dealing with that sort of situation, but no prior training provided by Boeing ever mentioned the automated system.

The failure of an air safety system is exceedingly rare but not unheard of for new aircraft. This accident shows the inherent risk of taking control of critical systems away from humans without their knowledge.