• The FAA launched an audit of Boeing's aircraft production after a door panel blew out during an Alaska Airlines flight.
• The agency has dispatched a "couple of dozen" inspectors to Boeing's production lines, FAA Administrator Mike Whitaker told CNBC.
• "We're shifting from more of an audit approach to a direct inspection approach," he said.

WASHINGTON — The Federal Aviation Administration has "boots on the ground" at Boeing's 737 Max factory — and will keep them there until the agency is convinced the manufacturer's quality control system is working, FAA Administrator Mike Whitaker told CNBC.

The FAA earlier in January said it will audit Boeing's Max production line, after an almost brand-new Boeing 737 Max 9's door plug blew out on an Alaska Airlines flight at 16,000 feet, exposing passengers to a force so powerful it sucked out seatbacks and headrests, according to federal investigators.

No one was seriously injured on the flight, and no one had been seated next to the gaping hole left by the blowout. The FAA grounded that model of Boeing's best-selling 737 Max a day after the accident and later said it will increase oversight of the company's production lines.

"We've got a lot of inspectors on the ground, visually inspecting the aircraft as it comes through," Whitaker said Tuesday in an interview at FAA headquarters. "We're shifting from more of an audit approach to a direct inspection approach."

The scale of such a review is a challenge, Whitaker said, citing the manpower required to conduct that many inspections. The FAA has dispatched a "couple of dozen" inspectors, he said.

"Until we're comfortable that the [quality assurance] system is working properly ... we're going to have boots on the ground," he said.

Both Alaska and United Airlines said they found loose bolts on several Max 9 planes during preliminary inspections.

The National Transportation Safety Board, which is leading the Flight 1282 accident investigation, said the the agency is returning to Boeing's 737 factory in Renton, Washington on Friday so investigators can review documentation related to the aircraft's production, manufacturing and maintenance.

"As part of the investigation, NTSB investigators will build a timeline from the early stages of production of the door plug to the accident flight," an NTSB spokeswoman said in a statement.

Return to service

The FAA is working with Boeing and airlines on inspection instructions that would pave the way for the 737 Max 9 to return to service. Whitaker, who is three months into the FAA's top job, declined to comment on when he expected the planes to return to service.

"It's been difficult to predict, so we've sort of stopped trying," he said. "But as soon as we get it sorted out it'll be up again."

Though safety inspections were initially estimated to take between four and eight hours per plane, Whitaker said they've "been longer than that."

"We've required a lot of measurements," he said. "Once the area's exposed, we want to understand bolt tensions and gaps and things of that nature. So we've required more data than would normally be the case because we really wanted to understand the issue."

United, which has 79 Boeing 737 Max 9 planes in its fleet, more than any other carrier, said Monday it's assuming the planes will remain grounded through the end of January. The carrier is forecasting an adjusted loss of as much as 85 cents per share this quarter as a result.

United CEO Scott Kirby on Tuesday expressed frustration at Boeing and its repeated production issues and delays. He said United is taking the larger variant, the 737 Max 10, out of its fleet plans, because of lengthy delivery delays. The FAA hasn't yet certified that plane, nor has it certified the 737 Max 7, a smaller model that Southwest Airlines is awaiting.

Boeing scrutiny

The accident on Alaska Airlines Flight 1282 is the latest and most serious in a string of apparent production flaws at Boeing, which has been trying to clean up a reputation for quality that was tarnished by two deadly crashes about five months apart. Those accidents involved the 737 Max 8, a smaller variant of the same aircraft family. A worldwide grounding of both the Max 8 and Max 9 began to lift about four years ago.

Alaska Airlines CEO Ben Minicucci told NBC News on Tuesday that the door-plug blowout was "unacceptable out of the factory" and that the carrier is adding additional staff for oversight on the production line to make sure there is "a second set of eyes to look at those critical areas."

Minicucci said the airline found "many" of the carrier's Max 9 aircraft had loose bolts.

On Tuesday, Stan Deal, CEO of Boeing's commercial airplane unit, its largest, apologized for the delays in getting its aircraft to customers.

"We have let down our airline customers and are deeply sorry for the significant disruption to them, their employees and their passengers," he said in a written statement. "We are taking action on a comprehensive plan to bring these airplanes safely back to service and to improve our quality and delivery performance."

Boeing is planning to pause work at several production lines for safety sessions for factory workers to "evaluate what we're doing, how we're doing it and make recommendations for improvement," Deal told staff Tuesday. The sessions start Thursday at the 737 factory in Renton, Washington.

Boeing CEO Dave Calhoun sought out meetings with senators on Capitol Hill this week, "to answer all their questions, because they have a lot of them," he said.

Boeing announced Jan. 16 the appointment of an independent advisor to lead a review of the Max 9 problem.

When asked whether the Max 9 crisis will mean more of a permanent change in how the FAA, which certifies Boeing's planes, oversees the company, Whitaker said the agency is "looking at all options."

"If there are functions that Boeing has not done appropriately, I think we'll look at whether we should take over some of those functions or whether there's an opportunity for a third party, a nonprofit technical organization, to provide a fresh set of eyes," he said.

"There's no reason to think that they can't get back to a point where they're meeting their quality standards and an increasing production," Whitaker said. "But right now, we need to be assured of that."