Despite two crashes within six months, a growing number of grounding orders worldwide for the Boeing 737 MAX, and a number of recent complaints from US pilots over problems with the aircraft's automatic trim controls, the Federal Aviation Administration continues to allow the 737 MAX to fly. "The United States Federal Aviation Administration is not mandating any further action at this time, and based on the information currently available, we do not have any basis to issue new guidance to operators," a Boeing spokesperson said in a March 12 statement.
But government inaction may have been at least partially to blame for the crash of an Ethiopian Airlines 737 MAX on March 10—the US government shutdown reportedly pushed back a fix to the aircraft's software for more than a month.
On March 11, Boeing announced that the company "has been developing a flight control software enhancement for the 737 MAX, designed to make an already safe aircraft even safer." The shutdown of non-essential operations at the FAA caused work on the fix to be suspended for five weeks, according to unnamed US officials cited by the Wall Street Journal. The fix is expected to be mandated for installation by the FAA by the end of April.
The update seeks to correct what may have been the root cause of the crash of Lion Air Flight 610 in Indonesia last October—the Maneuvering Characteristics Augmentation System's (MCAS') reliance on a single sensor to determine whether the aircraft is entering a stall. But according to a WSJ report, that fix was delayed because the FAA shutdown interrupted the approval process.
"We don’t have any comment on claims in the WSJ’s story," a Boeing spokesperson told Ars.
A stall occurs when an aircraft's angle of attack (AOA)—the relative angle of the aircraft's wing surfaces to the flow of air across them—reaches the point where the wing can no longer generate enough lift to sustain flight. Usually, this happens in a climb with insufficient air speed. Automatic control systems such as MCAS try to solve this problem by pushing the nose of the aircraft down—putting the aircraft into a descent and increasing airspeed and relative airflow across the wings. MCAS relies on an AOA sensor to determine whether this is required. If the AOA sensor is faulty, it could create a false signal of a stall—which is what happened in the case of Lion Air Flight 610 and may have been the issue with the Ethiopian Airlines flight.
The MCAS software update includes a new "enhanced flight control law," a Boeing spokesperson said, which "incorporates [AOA] inputs, limits stabilizer trim commands in response to an erroneous angle of attack reading, and provides a limit to the stabilizer command in order to retain elevator authority."
In other words, it uses multiple sensor inputs to determine whether adjustments to the flight controls are necessary, giving the pilot direct control over the tail control surfaces to override any automatic adjustments. Currently, the pilot would have to entirely disable automatic stabilizer trim to counteract "stabilizer trim runaway" in the event of a sensor error.
Go home, 737 MAX…
On March 13, the European Union's civil aviation authorities joined China, Australia, Singapore, Ethiopia, Malaysia, the United Kingdom, and a number of Latin American air carriers in grounding Boeing 737 MAX aircraft after the crash of the Ethiopian Airlines flight just after take-off on March 10. As the EU imposed its grounding, a number of 737 MAX aircraft were forced to turn around in flight because they were no longer considered airworthy at their destinations.
This poor guy got turned around. pic.twitter.com/M7MZ5bK32n
— Outdated Ineffective Wall (@darthbender) March 12, 2019
Today, Canada's transportation minister announced a grounding of all 737 MAX aircraft in Canada and a ban on the incursion of 737 MAX aircraft from other countries into Canadian airspace. The decision, Minister Marc Garneau said, was based on new satellite tracking data reviewed by Canadian aviation authorities.
Meanwhile, Boeing continues to stand behind the safety of the aircraft. "We understand that regulatory agencies and customers have made decisions that they believe are most appropriate for their home markets," a Boeing spokesperson said. "We’ll continue to engage with them to ensure they have the information needed to have confidence in operating their fleets."
Those concerns may not be addressed until the software patch for MCAS is pushed out.