U.S. air-safety regulators have tentatively approved sweeping software and pilot-training changes for Boeing Co.’s grounded 737 MAX jets, aimed at fixing problems with a suspect flight-control system, according to internal government documents and people familiar with the details.
The extensive revisions, these industry and government officials said, will make the automated stall-prevention feature, called MCAS, less aggressive and more controllable by pilots.
They also said the enhanced training, relying on self-guided interactive instruction on laptops, highlights information about when the system engages and how pilots can shut it off.?
The changes amount to a reversal from major design and engineering principles Boeing relied on when it developed the stall-prevention system, which is suspected of causing the fatal dive that killed 189 people on board a Lion Air 737 MAX in Indonesia last October. A team of international crash investigators also is looking into whether a similar problem led to the crash of an Ethiopian Airlines plane less than five months later.
The Federal Aviation Administration has said it was working with Boeing to develop and install a revised MCAS system based on lessons learned from the Lion Air tragedy, but the extent of the changes goes beyond what some industry officials expected. An FAA spokesman declined to comment on specifics of the pending changes.
Accident investigators have said the Lion Air plane got erroneous information from one sensor that caused the stall-prevention system to misfire, repeatedly pushing the nose of the plane and ending at the maximum downward angle even though the pilots were resisting. Authorities have said they see clear similarities between that accident and the Ethiopian crash on March 10.
The modifications, officials said, create a gentler stall-prevention feature, redesigned so it won’t overpower other cockpit commands or misfire based on faulty readings from a single sensor. It is devised to automatically push the nose down only once—for no longer than 10 seconds—if the aircraft is in danger of stalling and losing lift.
The changes have been tentatively approved by FAA officials, the people familiar with the details said, subject to final ground-simulator checks and flight tests. They could be rolled out to airlines’ 737 MAX jets in the next few weeks.
A Boeing official said the new MAX software could still go through revisions, and the timing of formal approval from the FAA and foreign regulators remains fluid.
Even after the changes are fully implemented in the U.S., air-safety regulators in Canada and the EU are poised to conduct their own evaluation of the new software as well as how the FAA initially certified the plane to carry passengers. Those reviews could take months, according to safety experts.
Among other changes, the revised software would rely on two “angle of attack” sensors, rather than one, to measure the upward or downward angle of the wings and nose in flight. If two sensors send data differing by five degrees or more, MCAS wouldn’t activate at all, according to the officials briefed on the tentative changes.
The cockpit crew on the Lion Air flight struggled against MCAS—using manual nose-up commands some two dozen times—before losing control and plunging into the Java Sea at more than 500 miles an hour. The interim accident report revealed a constant 20-degree difference between signals from the sensor on the captain’s side and those from the co-pilot’s-side sensor.
On Saturday, Boeing said it has been “working diligently and in close cooperation with the FAA on the software update,” adding that the company is “taking a comprehensive and careful approach to design, develop and test the software that will ultimately lead to certification” by regulators.
During the investigations of the two crashes, Boeing and the FAA have faced criticism from pilot groups, airlines, politicians and airlines for alleged lapses in the original MCAS design—and for failing to adequately inform aviators.
About a dozen pilots from U.S. and international carriers are getting previews this weekend of the changes in the works, as well as related manuals and training, according to the Boeing official. "We want their feedback,” this official said. “It’s a dialogue.”
The group engaging in this weekend’s preview of the changes includes pilots from U.S. MAX operators: Southwest Airlines Co., American Airlines Group Inc. and United Continental Holdings Inc., a person familiar with the matter said. On Wednesday, this person added, a larger group of more than 100 pilots from a broad cross section of MAX operators are due at Boeing’s 737 factory in Renton, Wash., for a similar session.
Investigators in the Lion Air crash said faulty data transmitted from a single sensor caused the MCAS system to assume the plane was in danger of stalling. The warnings began during takeoff and continued for much of the roughly 11-minute flight, apparently confusing the pilots and creating a cascade of related warning signals.
Under the new design, warning devices will alert crews if there is a problem with sensors before takeoff or in flight, people familiar with the redesign said. They said automated commands to move a flight-control surface on the tail, called a horizontal stabilizer, can be counteracted by pilot commands.
The changes will be standard on all 737 MAX aircraft, for which Boeing has roughly 5,000 orders.
A draft FAA document spelling out the training revisions shows pilots now will be specifically informed about “MCAS activation thresholds,” “flight crew alerts” and how to turn off the system by flipping a single switch. Such details weren’t highlighted in earlier manuals or training materials circulated by Boeing.
FAA officials have determined the handing qualities of 737 MAX jets will be close enough to earlier 737 models that pilots won’t need additional training in ground-based simulators, which is expensive for airlines and disruptive to their schedules.