At a heated Senate hearing Tuesday, Boeing CEO Dennis Muilenburg said the grounded 737 MAX jet will fly again only when "everyone is convinced it's safe".

Muilenburg was questioned by the US Senate Committee on Commerce, Science and Transportation in Washington over what the company knew about its MCAS stall-prevention system linked to two deadly plane crashes and about delays in turning over internal 2016 messages that described erratic behavior of the software in a simulator.

The hearing put pressure on a revamped Boeing senior management team fighting to repair trust with airline customers and passengers shaken by an eight-month safety ban on its 737 MAX following the crashes. The crashes on Oct 29, 2018, in Indonesia and March 30 in Ethiopia killed 346 people.

"This is not going to be timeline driven," Muilenburg testified. "We are committed to answering every question regulators have."

Investigators have focused on the jet's anti-stall device, called the Maneuvering Characteristics Augmentation System (MCAS), which may have erroneously pointed the nose of the planes down to gain speed to prevent a mid-air stall, and into a fatal plunge.

"You have told me half-truths over and over again," Senator Tammy Duckworth of Illinois told Muilenburg, questioning why the manufacturer did not disclose more details about MCAS' lack of safeguards. "You have not told us the whole truth and these families are suffering because of it."

US Senator Maria Cantwell, a Democrat from Washington state, asked if the Federal Aviation Administration (FAA) and Boeing had "rushed to certify the MAX". She noted, "To date, we haven't gotten all those answers."

Senator Roger Wicker, a Republican from Mississippi and chairman of the committee, said: "Both of these accidents were entirely preventable." He invited family members to stand and hold up large photos of relatives killed in the crashes.

Wicker questioned the CEO over the company's delay in releasing the messages in which a former test pilot described erratic behavior of a simulator version of the MCAS software. He said the messages revealed a "disturbing level of casualness and flippancy".

"One of the things we've learned ... is we need to provide additional information on MCAS to pilots," Muilenburg said.

Senator Jon Tester of Montana noted Boeing had won approval from the FAA to avoid having to add new crew alerts because it would have been expensive.

"It wouldn't have happened if FAA would have been doing their job, and it also wouldn't have happened if you had known what the hell was going on," he said. "I would walk before I would get on a 737 MAX. I would walk. There is no way ... You shouldn't be cutting corners and I see corners being cut."

US airlines flying the MAX have canceled flights through January and are taking a financial hit. Returning the jet to commercial service is vital to Boeing's future because the MAX, priced at about US$135 million each, is Boeing's top seller.

Muilenburg testified that the company is in the "final stages" of testing the revised software and seeking recertification for the jet. His testimony before the committee was his first appearance before Congress since the crashes. He is scheduled to appear Wednesday before the US House Committee on Transportation and Infrastructure.

"We have learned and are still learning from these accidents," the CEO said Tuesday. "We know we made mistakes and got some things wrong. We own that, and we are fixing them. We have developed improvements to the 737 MAX to ensure that accidents like these never happen again. We also are learning deeper lessons that will result in improvements in the design of future airplanes."