Boeing's CEO acknowledged to a US congressional panel on Tuesday that the company's culture was imperfect, but insisted the aviation giant was making progress and committed to improving safety.

"Our culture is far from perfect, but we are taking action and making progress," David Calhoun said early in the hearing.

Calhoun opened his remarks by standing to apologise to family members of victims from two Boeing 737 Max crashes in 2018 and 2019.

The hearing, an examination of "Boeing's Broken Safety Culture," follows an April session of the Senate Permanent Subcommittee on Investigations featuring a Boeing engineer who testified that he was punished for raising safety questions about the top-selling 787 Dreamliner and 777.

Calhoun's appearance marks his first testimony before a congressional panel since an alarming mid-flight incident in January on a 737 Max plunged the company back into crisis mode. US investigators are still probing the incident with the Alaska Airlines plane, which made an emergency landing after a fuselage panel blew out.

On Tuesday morning, the Senate committee detailed additional complaints from Boeing workers, including an official filing from a whistleblower who worried that Boeing's lax policies on the use of damaged or inadequate parts could "lead to a catastrophic event," according to a subcommittee memo.

Boeing stands "at a moment of reckoning and an opportunity to change a broken safety culture," said Democratic Senator Richard Blumenthal, who is chairing the hearing.

The session comes ahead of a Department of Justice determination on next steps after concluding in May that the company could be prosecuted for violating a criminal settlement following two fatal 737 Max crashes in 2018 and 2019, off Indonesia and in Ethiopia.

Blumenthal said there "is near overwhelming evidence... that prosecution should be pursued."

The DOJ has said it will notify a federal court of its plans by July 7.

At the April 17 hearing, witnesses painted a disturbing picture of a company that dismissed safety questions and sidelined critics as it chased faster production and bigger profits.

The star witness was engineer Sam Salehpour, who went public with allegations that, because of flawed manufacturing processes, the Dreamliner could suffer a potentially catastrophic accident because of excessively large gaps in the plane's assembly.

Boeing has pointed to extensive testing that it says proves the 787 is safe.

Calhoun emphasised that he welcomes when workers speak out about problems, viewing such expressions as a critical component of safety.

He also pledged to get back to the committee on questions about workers who may have been fired for expressing concerns about safety.

Calhoun also said he would provide details on instances on how the company responds to workers who engage in retaliation against workers who speak out.

In connection with the probe, Blumenthal and Senator Ron Johnson, a Republican, sent a letter to Calhoun seeking records that would shed light on Salehpour's allegations about the 787 and 777, as well as records relating to Boeing's whistleblower policies and protocols.

The same two senators also sent a letter to FAA Administrator Michael Whitaker related to the allegations, as well as other ongoing Boeing-related matters, such as a six-week FAA audit of the company following the Alaska Airlines incident.

The Senate panel on Tuesday released details on additional workers who have come forward with problems.

This includes a new whistleblower, Sam Mohawk, who said that Boeing ordered improperly stored parts to be hidden from federal aviation inspectors, who would have demanded Boeing increase storage capacity and hire additional staff, raising costs.

The hearing was attended by a small group of family members who lost relatives in the 2018 and 2019 Max crashes, which together claimed 346 lives. The group held signs with their loved ones at the outset of the session.

They also held a press conference just ahead of the hearing.

Calhoun is a "mass killer," said Adnaan Stumo, brother to Nadia Milleron, who died in the Ethiopian Airlines crash. "There needs to be criminal charges for the people at the top. The people in the driver's seat were responsible for 346 deaths, including my sister." (AFP)