The head of the Federal Aviation Administration told a U.S. House subcommittee on Tuesday that safety culture improvements at Boeing may take three to five years to complete.

"It is not a six-month program - it is a three-year to five-year program," FAA Administrator Mike Whitaker said at a two-hour hearing, adding he has spoken to Boeing CEO Kelly Ortberg and the company's board of directors about the need for safety culture reforms.

He said Boeing has made significant improvement in the short term. "On culture it is a long-term project .... There is progress but they are not where they need to be."

Congress is holding two days of hearings on Boeing and the company's safety turnaround efforts with a Senate panel taking up the issue on Wednesday.

In June, Whitaker said the agency was "too hands off" in oversight of Boeing before the January mid-air emergency in a new Alaska Airlines 737 MAX 9 and faulted its prior audits. Boeing faces Justice Department and FAA probes into the Alaska incident.

Boeing has no choice but to make improvements, Whitaker said.

"There has to be culture change or they won't be able to go back to producing aircraft at the level they want," Whitaker said, adding the planemaker plans to hold a new safety training program.

Boeing declined to comment on Whitaker's remarks. Boeing shares, which are down 38% this year, fell 0.3% on Tuesday.

Whitaker, who has said the agency has permanently boosted use of in-person inspectors at Boeing, in January barred the planemaker from raising production of its best-selling 737 MAX until it makes quality and safety improvements.

That cap, Whitaker said on Tuesday, "really gives us the leverage we need to make sure these changes happen."

In July, Boeing agreed to plead guilty to a criminal fraud conspiracy charge and pay at least $243.6 million after breaching a 2021 Justice Department agreement.

The planemaker also agreed to spend at least $455 million to boost safety and compliance programs, overseen by an independent monitor for three years.

U.S. lawmakers expressed frustration with Boeing after hundreds died in fatal crashes on 737 MAX planes in 2018 and 2019.

"We don't want Airbus to get all the planes (sales) but Boeing keeps messing up," Representative Steve Cohen said. "For America's interests, Boeing needs to get its act together."