The U.S. FAA is pressing on with planned changes to Boeing 737 MAX aircraft stemming from the October 2018 Lion Air Flight JT610 accident, but has not seen evidence from the recent Ethiopian Airlines MAX 8 accident to take additional steps, such as the precautionary groundings around the globe.
The changes, in the works for months, are being fast-tracked and rolled out on Boeing MAX family aircraft in the coming weeks. Certification flights are thought to be currently underway, using the company's initial 737 MAX 7 model, and FAA plans to have an airworthiness directive mandating the upgrades out by April.
Boeing says the changes include updates to its Maneuvering Characteristics Augmentation System (MCAS) flight control law that is under scrutiny in the investigation of the Lion Air Flight 610 (JT610) accident. But the changes go beyond MCAS into the flight control system, and will also affect pilot displays, operations manuals and crew training.
"The enhanced flight control law incorporates angle of attack (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," Boeing said.
FAA viewed the changes as sufficient to correct the issues uncovered in the JT610 probe, and has seen no evidence from the March 10 Ethiopian accident to sway its thinking. There is little evidence to go on, as readouts of the digital flight data and cockpit voice recorders was still in process.
"Following the accident of an Ethiopian Airlines Boeing Model 737-8 airplane on March 10, 2019, the National Transportation Safety Board (NTSB) as the accredited representative, and the FAA as Technical Advisors, are supporting the Ethiopian Accident Investigation Bureau," FAA said in a March 11 Continued Airworthiness Notification to the International Community. "The FAA has dispatched personnel to support the investigative authorities in determining the circumstances of this event. All data will be closely examined during this investigation, and the FAA will take appropriate action if the data indicates the need to do so."
FAA's notification emphasized actions taken after JT610, which went down in the Java Sea shortly after takeoff as its crew battled with, among other issues, erroneous data from angle-of-attack (AOA) sensors. Among them: Boeing's plans to modify the 737 MAX FCS, which incorporates MCAS.
MCAS provides automatic nose-down inputs to assist pilots in certain manual, flaps-up flying scenarios, "especially at slow airspeeds and high AOA," Boeing explained in an operators' bulletin issued last November. The MAX’s larger CFM Leap-1B engines create more lift at high angles of attack than the CFM56-7B used on the 737 Next Generation. MCAS was added as a certification requirement to help mitigate this.
MCAS is fed by a single AOA sensor—something that is expected to change as part of Boeing's update. Erroneous data can trigger it, and pilots are supposed to follow a memorized checklist to override it. But the system's reaction in certain error modes may be confusing pilots, and its being looked at in the JT610 investigation.
"In the event of erroneous AOA data, the pitch trim system can trim the stabilizer nose down in increments lasting up to 10 seconds," Boeing explained to operators in a bulletin issued last November, following the JT610 accident. "The nose down stabilizer trim movement can be stopped and reversed with the use of the electric stabilizer trim switches but may restart 5 sec. after the electric stabilizer trim switches are released. Repetitive cycles of uncommanded nose down stabilizer continue to occur unless the stabilizer trim system is deactivated."
Among the theories being considered by JT610 investigators: the aircraft's crew did not fully understand what was happening to its aircraft, and attempted to counteract MCAS with the trim switches, instead of deactivating it with a cutout switch.
Operators and regulators around the world, concerned that MCAS is not well enough understood and is too susceptible to faulty sensor data, have taken the extraordinary step of grounding MAX aircraft without direct evidence of a safety issue. FAA has so far relied on that lack of evidence to justify its decision to keep them flying.
Even if MCAS is considered too risky to be used in service, regulators have at least one option beyond grounding aircraft, a former regulatory official told Aviation Daily. "If MCAS is the problem, disconnect it," the official said. "Establish revised operational limitations that reflect MCAS's unavailability, such as not flying below certain airspeeds, and mandate them until the issue is resolved. There's no need to ground the aircraft"