FAA Probes Potential 737 MAX Design Flaw


Following the Lion Air crash in Indonesia last month, the FAA is focusing new scrutiny on the control architecture of the Boeing 737 MAX and may require the company to engineer a fix for an auto trim system installed on the airplane. The system wasn’t well documented and reporting has revealed that pilots and airlines were unaware of it.

Meanwhile, a Florida-based law firm has filed a lawsuit against Boeing, claiming that the trim system was an unsafe design and contributed to the crash. “It is particularly surprising to hear from safety experts and the heads of pilots’ unions that Boeing failed to warn its customers and the pilots of its new 737 MAX aircraft about this significant change in the flight-control systems,” the suit claims. It was filed on behalf of the parents of Dr. Rio Nanda Putrama, who was killed along with 188 others when the Lion Air 737 crashed into the Java Sea near Jakarta on Oct. 29.

One focus of the investigation is an automated stabilizer trim system Boeing added to the 737 MAX aircraft. It’s called Maneuvering Characteristics Augmentation System or MCAS and is designed to automatically trim the aircraft nose down when it’s being flown manually at high angles of attack and high load factors with flaps up. It’s disabled when the aircraft is flown on autopilot or with the flaps down.

Pilots transitioning to the MAX series weren’t trained on the system because Boeing provided little or no specific documentation on it, according to two pilot unions. Unions for Southwest and American Airlines, both of which operate the MAX, have complained about the oversight.

Following the crash, Boeing produced more detailed documentation on MCAS and the FAA followed up with an emergency AD (PDF) requiring operators to add MCAS failure indications to approved flight manuals. Boeing says it’s cooperating with the accident probe but insists the MAX is a safe aircraft.

The FAA’s emergency AD frames where investigators are focusing one aspect of the accident probe, specifically how MCAS functions in the event of sensor failures. MCAS depends, at least in part, on data from the aircraft’s vane-type angle-of-attack sensor. The Lion Air 737 had a recent history of both unreliable airspeed indications and AoA sensor anomalies. The sensor had been replaced just prior to the accident flight, according to Lion Air.

The FAA said Thursday that the AoA values as well as data from other sources that MCAS uses are being reviewed. It’s not yet known if the AoA sensor represented a single-point failure that would cause the auto trim to continue trimming nose down because it inaccurately sensed stall angle of attack. The Seattle Times on Thursday quoted a former Boeing flight control engineer who said the system could have a design flaw.

“To contemplate commanding the nose down clearly is a major concern. For it to have been triggered by something as small as a sensor error is staggering,” Peter Lemme told the Times. “There’s going to be hell to pay for that,” he added.

About 200 MAX-series airplanes are in service, some with three U.S. airlines including Southwest, American and United. Airlines continue to operate the aircraft, but pilots have been notified and briefed on what’s known about both MCAS and the Lion Air accident. Indonesian authorities say they’ll release a preliminary report before the end of November.