Boeing officials “inappropriately coached” test pilots during recertification efforts after two fatal 737 MAX crashes killed 346 people, according to a lengthy new US congressional report.

The report from the Senate Commerce Committee raised questions about whether this year’s testing of a key safety system known as MCAS — tied to both fatal crashes — was contrary to proper protocol.

The committee concluded Federal Aviation Administration (FAA) and Boeing officials “had established a pre-determined outcome to reaffirm a long-held human factor assumption related to pilot reaction time”.

“It appears, in this instance, FAA and Boeing were attempting to cover up important information that may have contributed to the 737 MAX tragedies,” the report said.

The report cited a whistleblower who alleged Boeing officials encouraged test pilots to “remember, get right on that pickle switch” prior to the exercise that resulted in pilot reaction in approximately four seconds, while another pilot in a separate test reacted in approximately 16 seconds.

The account was corroborated during an FAA staff interview, the committee added.

Numerous reports have found Boeing failed to adequately consider how pilots respond to cockpit emergencies in its development of the 737 MAX.

Boeing said it takes “seriously the committee’s findings and will continue to review the report in full”.

In a statement, the FAA said the report “contains a number of unsubstantiated allegations” and defended its review of the MAX, calling it thorough and deliberate.

The agency added it was “confident that the safety issues that played a role in the tragic [737 MAX] accidents involving Lion Air Flight 610 and Ethiopian Airlines Flight 302 have been addressed through the design changes required and independently approved by the FAA and its partners.”

Senator says report highlights ‘failed leadership’

Senate Commerce Committee chairman, Republican Roger Wicker, said the report “details a number of significant examples of lapses in aviation safety oversight and failed leadership in the FAA.”

Following a lengthy review of Boeing changes, the FAA last month approved the plane to fly again if airlines update a key flight-control system and make other changes.

The Senate report, however, criticized a key part of the FAA review. It said that Boeing “inappropriately influenced” FAA testing of pilot-reaction time to a nose-down pitch of the plane.

The committee also said “multiple independent whistleblowers contacted the committee to allege FAA senior management was complicit in determining the 737 MAX training certification level prior to any evaluation.”

Boeing resisted requiring simulator training for pilots before operating the 737 MAX but reversed course in January.

Boeing still faces an ongoing criminal probe into the MAX.

The committee said its review was “constrained due to the continued criminal investigation”

Flights resumed in Brazil last month after the FAA review. The first US 737 MAX commercial flight with paying passengers is set for December 29.

Last month, the Senate committee unanimously a bill to reform how FAA certifies new airplanes and grant new protections for whistleblowers, among other reforms, while the US House of Representatives unanimously passed a similar bill.

Wires/ABC