Boeing ‘s safety and quality were under fire again in two Senate hearings Wednesday as the manufacturer faces mounting scrutiny after a midair door blowout and near catastrophe on one of its planes in January.

A Boeing engineer-turned- whistleblower testified before a Senate panel, reiterating his allegations that the plane maker cut corners to move wide-body jets through the production line, despite flaws. Sam Salehpour alleged that the company failed to adequately shim tiny gaps at meeting points on the 787 Dreamliner’s fuselage, and that that could “ultimately cause a premature fatigue failure without any warning,” according to his testimony. A shim is a thin piece of material used to fill tiny gaps.

“I believe that Boeing can do better and that the public’s trust in Boeing can be restored,” he said in prepared remarks to a subcommittee of the Senate Homeland Security Committee ahead of the hearing “Examining Boeing’s Broken Safety Culture: Firsthand Accounts.”

Boeing has denied the allegations, calling them inaccurate, and has defended the aircraft and its testing. On Monday, it gave reporters a roughly two-hour briefing about what it described as exhaustive fatigue testing on the 787 and 777 aircraft, saying it did not find safety risks.

Scott Kirby, CEO of United Airlines , a major Dreamliner operator, brushed off concerns about the plane on Wednesday.

“I am totally confident that the 787 is a safe airplane,” he told CNBC’s “Squawk Box .”

Still, the Jan. 5 blowout of a door panel on a Boeing 737 Max 9 plane when an Alaska Airlines flight was at 16,000 feet has again thrust Boeing’s safety culture into the spotlight and caused a crisis at the manufacturer. New plane deliveries from Boeing have slowed as the Federal Aviation Administration ramps up its scrutiny of the company’s production lines.

Boeing’s CEO, Dave Calhoun , last month said he would step down by year’s end, while the company replaced its head of its commercial airplane unit and its board chair.

A separate hearing, by the Senate Commerce Committee on Wednesday, addressed Boeing’s safety culture after a report issued earlier this year from an expert panel ordered by Congress found a “disconnect” between Boeing’s senior management and other members of the organization on safety culture.