The National Transportation Safety Board released the results of a monthslong review of potential lapses in the design and approval of Boeing’s 737 Max on Thursday, faulting the company for making erroneous assumptions during the development of the jet and calling for broader changes in the way airplanes are certified.
The agency said Boeing had underestimated the effect that a failure of new automated software in the aircraft could have on the environment in the cockpit. When activated, the system, known as MCAS, automatically moves the Max’s tail and pushes its nose down. The system contributed to two crashes in less than five months that killed 346 people and caused regulators around the world to ground the plane.
Results of an internal review by Boeing, which its board made public on Wednesday, recommended changes to the design of cockpits and the company’s organizational structure to improve safety.
Also on Wednesday, in a hearing on Capitol Hill, the deputy director of the Federal Aviation Administration faced questions from senators about allegations that it misled Congress about the qualifications of inspectors who helped determine the training that pilots received. A task force composed of several international regulators is expected to submit a report this month regarding how the plane was certified.
Two investigators handled the bulk of the work for the National Transportation Safety Board, reviewing thousands of pages of documents, interviewing officials at Boeing and the F.A.A., and studying black-box data from the two crashes, in Indonesia and Ethiopia. They focused on MCAS, which sent both planes into nose dives.
When Boeing developed the Max, it assumed that if MCAS activated erroneously, pilots would immediately react by performing a standard emergency procedure. But the company had tested the possibility of an MCAS failure only in isolation, failing to account for just how chaotic the cockpit would become when the activation caused other malfunctions.
On the doomed Lion Air and Ethiopian Airlines flights, a faulty sensor triggered MCAS, which produced a cascading number of warnings that may have overwhelmed the pilots.
“They did not look at all the potential flight deck alerts and indications the pilots might face,” said Dana Schulze, the director of the Office of Aviation Safety at the safety board. “Multiple alerts and indications have been shown through years of research to have potentially an impact where pilots will not respond as perhaps you might have intended.”
Ms. Schulze said the agency would like the F.A.A. to review Boeing’s safety assessment of MCAS before allowing the plane to fly again. The plane is grounded while Boeing works on a software update and other changes intended to make it safer.
The safety board also recommended that the F.A.A. require Boeing and other manufacturers to consider the effect of multiple cockpit warnings when assessing how quickly pilots will respond to a malfunction. It also suggested that the agency direct plane makers to develop technology that could diagnose a problem during flight and tell pilots what procedure to follow.
“We are committed to working with the F.A.A. in reviewing the N.T.S.B. recommendations,” a Boeing spokesman, Gordon Johndroe, said. An F.A.A. spokesman, Lynn Lunsford, said the agency “will carefully review these and all other recommendations as we continue our review of the proposed changes to the Boeing 737 Max.”
The F.A.A. continues to face criticism for its handling of the Max certification.
This week, the United States Office of Special Counsel sent a letter to President Trump and Congress saying the F.A.A. provided incomplete information about a complaint made by a whistle-blower who claimed that inspectors at the agency weren’t fully qualified to determine pilot training on the Max.
In response to a request this year by Senator Roger Wicker of Mississippi, Republican chairman of the Senate Commerce Committee, the F.A.A. said that the complaint concerned officials working on a Gulfstream aircraft, not the Max, and that it had determined that inspectors evaluating pilot training on both planes were competent.
At the Wednesday hearing, top F.A.A. officials defended their comments. “Any insinuation that the F.A.A. misled Chairman Wicker in our reply to his inquiry is not what happened,” said Daniel K. Elwell, the deputy director of the agency. He called the special counsel’s allegation “simply inaccurate.”