(Reuters) - The new head of Boeing's ( BA ) troubled commercial airplane unit said the U.S. planemaker faces a "pivotal moment" as it works to boost quality and address significant concerns from regulators and airline customers after a panel flew off a 737 MAX 9 jet in January.

"This is a pivotal moment for us, and we have serious work ahead to build trust and improve our operations," said Stephanie Pope, who was named president and CEO of Boeing Commercial Airplanes on Monday, in an email to employees seen by Reuters.

Pope was named chief operating officer in December and retains the title after holding a wide range of prior jobs at Boeing ( BA ).

On Monday, Boeing ( BA ) CEO Dave Calhoun announced he would leave by the end of the year, while the company's long-time head of commercial airplanes, Stan Deal, retired effectively immediately and the board chair Larry Kellner stepped down and was replaced as chair by director Steve Mollenkopf.

Boeing ( BA ) has come under intense criticism since a door plug panel tore off a new Alaska Airlines 737 MAX 9 jet at 16,000 feet.

In the aftermath of the incident, the FAA grounded the MAX 9 for several weeks, barred Boeing ( BA ) from increasing the MAX production rate and ordered it to develop a comprehensive plan to address "systemic quality-control issues" within 90 days.

Boeing ( BA ) production has fallen below the maximum 38 MAX planes per month the FAA is allowing. The Justice Department has opened a criminal investigation into the MAX 9 incident.

"Our path forward is clear. We will put safety and quality above all else in order to meet and exceed the expectations of our regulators, customers, flying public and each other," Pope said.

She said over next couple of weeks, she "will be spending my time meeting and engaging with our team as we enhance and implement our safety and quality improvement plan." FAA Administrator Mike Whitaker said earlier the agency and Boeing ( BA ) hope by the end of March to define the milestones the manufacturer must meet.