The Federal Aviation Administration said Wednesday that it had recently discovered a new problem with the 737 Max jet that Boeing must correct before the plane is returned to service.
In a flight simulator last week, FAA pilots tested erroneous activations of anti-stall software that pushes down the nose of the Max, two people with knowledge of the matter said. The software, known as MCAS, was involved in two crashes that killed 346 people.
In at least one instance, an FAA pilot was unable to quickly and easily follow Boeing's emergency procedures to regain control of the plane. The pilot rated that failure as catastrophic, meaning it could lead to the loss of an aircraft mid flight, the people said. The situation that was tested is highly unlikely to occur during a typical passenger flight, but the regulator is still requiring Boeing to make a fix, one of the people said.
The FAA, in a statement, referred to the issue as "a potential risk that Boeing must mitigate."
The discovery may erode confidence in Boeing's assertions, in conversations with regulators, airlines and aviation unions, that well-trained pilots can easily handle a software malfunction based on their understanding of standard emergency procedures.
It also adds to the roadblocks that have kept pushing back the Max's return to flight. Nearly 500 Max jets have remained grounded across the globe since March, as the company faces a barrage of questions from the FAA and international regulators.
In recent weeks, the FAA has been testing a broad array of potential failures involving the anti-stall software, partly to ensure that the fix Boeing has developed does not introduce new problems.
The issue discovered last week is linked to the data-processing speed of a specific flight control computer chip, according to the two people with knowledge of the matter. In the test, the FAA pilot encountered delays in executing a crucial step required to stabilise an aircraft.
Test pilots in previous simulations found that they had less than 40 seconds to override MCAS in a crisis and prevent a nose-dive.
In a statement on Wednesday, Gordon Johndroe, a Boeing spokesman, said the company was working on software to address the issue, which he said would "reduce pilot workload by accounting for a potential source of uncommanded stabiliser motion."
Boeing has been developing a software update for the Max for eight months, he said. It is unclear whether the new flaw can be resolved by reprogramming the software or requires a hardware fix, which would be costlier and could take months longer.
"The safety of our airplanes is Boeing's highest priority," Johndroe said. "Boeing will not offer the 737 Max for certification by the FAA until we have satisfied all requirements for certification of the Max and its safe return to service."
For now, Boeing is still trying to patch up its reputation. Its credibility took a hit after the two accidents — a Lion Air flight in Indonesia in October and then an Ethiopian Airlines flight in March — as it faced scrutiny over the Max's aggressive production schedule and possible flaws in the certification of the jet.
MCAS, originally intended to activate only in limited circumstances, was overhauled late in its development, leading to a deadly domino effect of assumptions that affected design, certification and training decisions related to the Max. When the plane debuted, pilots were not informed of MCAS's existence. A key warning light, assumed to be standard, worked only if airlines had bought a separate indicator.
Boeing has begun a charm offensive that has gotten a chilly reception from flight attendants, pilots and regulators. During a congressional hearing last week, aviation leaders criticized Boeing's conduct.
After repeated delays, Boeing hopes to have the Max back in the air soon. But the new MCAS issue may push the schedule back even further, complicating the company's efforts to reassure customers.
Several airlines have canceled flights on the Max, which was Boeing's bestselling plane, through the summer. The grounding of the jets contributed to a decline of more than $1 billion in revenue for Boeing's commercial airplane division in its most recent fiscal quarter. The company's financial future is hazy.
But recently, during the Paris Air Show, Boeing received a signal of support from International Airlines Group, the parent of British Airways, which said it planned to buy 200 Max jets, albeit at what it said was a significant discount.
Written by: Natalie Kitroeff and Tiffany Hsu
Photographs by: Ruth Fremson
© 2019 THE NEW YORK TIMES