Boeing Co. will step up inspections of aircraft during production and open its factories to airline customers as the planemaker bolsters its response to a near-disaster on a 737 Max 9 jet earlier this month.

A Boeing team has been sent to its biggest supplier, Spirit AeroSystems Holdings Inc., to inspect and approve mid-exit door plugs before fuselage sections can be shipped to the planemaker, commercial aircraft chief Stan Deal said in a letter to employees on Monday.

Boeing will also bring in outside experts to review its quality controls and suggest improvements, he said, adding another layer of scrutiny as Boeing seeks to improve manufacturing and rebuild confidence in its 737 Max following the midair scare on an Alaska Airlines flight.

“Everything we do must conform to the requirements in our quality management system. Anything less is unacceptable,” Deal said. “Let each one of us take personal accountability and recommit ourselves to this important work.”

The planemaker was thrown into crisis on Jan. 5, when a door-sized fuselage panel tore off of a 737 Max 9 on Alaska Airlines Flight 1282 shortly after takeoff. While the aircraft returned safely to Portland, Oregon, the Federal Aviation Administration has grounded most of the single-aisle variant as it investigates the accident.

The FAA has reacted quickly to increase oversight of Boeing, after being criticized for acting too slowly in response to two deadly 737 Max crashes nearly five years ago that cost 346 lives.

The agency last week initiated a probe of Boeing manufacturing, said it will increase monitoring of so-called “in-service events” on the Max 9, and plans to audit Boeing’s production line as well as those of its suppliers.

Monday’s actions by Boeing are separate from the FAA’s investigation and oversight plans, Deal said. “We will cooperate fully and transparently with both as we work to restore trust with our regulator and our customers,” he said.

Spirit, a former Boeing unit, makes about 70% of the 737’s frameset at its base in Wichita, Kansas, where it installs the so-called plug on the Max 9’s fuselage. The units are then shipped by rail to Boeing’s factory in Renton, Washington.

The National Transportation Safety Board is investigating the root cause of the accident. It has said that four locking bolts that secure the plug door to the frame are missing, and it wasn’t clear whether they were ever installed.

The door plug that ejected has been recovered and is currently undergoing inspection at an NTSB site.

In addition to the door plugs, Boeing is also inspecting more than 50 other points in Spirit’s manufacturing process and assessing the contractor’s build plans against engineering specifications, Deal said.

Spirit, a former Boeing unit, makes about 70% of the 737’s frameset at its base in Wichita, Kansas, where it installs the so-called plug on the Max 9’s fuselage.

The stakes are high for Boeing to repair the reputation of its cash-cow 737 Max, get about 171 grounded Max 9 aircraft back in the sky and preserve customer relationships.

The move to open its factories to customers follows the discovery of more loose bolts by Alaska Air Group Inc. and by the Max 9’s biggest operator, United Airlines Holdings Inc.

— Anthony Palazzo, Ryan Beene / Bloomberg News