Washington — Boeing CEO Dave Calhoun was on Capitol Hill Wednesday trying to reassure key senators that the company’s planes are safe, after an incident earlier this month in which the door panel of a 737 Max 9 blew off during an Alaska Airlines flight over Portland, Oregon.  

“We fly safe planes,” Calhoun told reporters Wednesday. “We don’t put airplanes in the air that we don’t have 100% confidence in.  I’m here today in the spirit of transparency.”

The Federal Aviation Administration had grounded all 171 Boeing 737 Max 9s indefinitely since the incident. On Wednesday evening, the FAA announced that it had approved a “thorough inspection and maintenance process that must be performed on each of the grounded” aircraft.  

Once those inspections are complete, the aircraft “will be eligible to return to service.”

In a letter to employees Wednesday, United Airlines Chief Operating Officer Toby Enqvist said that its 737 Max 9s are slated to begin returning to service on Sunday. Enqvist said that 26 of the airline’s aircraft have already undergone full inspections “under the FAA’s supervision.”

The FAA also said Wednesday that it would not issue a production expansion to Boeing for any Max aircraft, meaning that the aerospace giant can continue production at its current monthly rate, but cannot increase that rate. 

“We will not agree to any request from Boeing for an expansion in production or approve additional production lines for the 737 MAX until we are satisfied that the quality control issues uncovered during this process are resolved,” FAA Administrator Mike Whitaker said in a statement. 

“We will continue to cooperate fully and transparently with the FAA and follow their direction as we take action to strengthen safety and quality at Boeing,” a spokesperson for Boeing said in a statement in response to the FAA’s announcement. “We will also work closely with our airline customers as they complete the required inspection procedures to safely return their 737-9 airplanes to service.”

United Airlines and Alaska Airlines are the only two U.S. carriers who operate the 737 Max 9, and as a result have been forced to cancel thousands of flights.

Both airlines previously reported that door plugs on grounded 737 Max 9s revealed “loose hardware” such as “bolts that needed additional tightening.”

The FAA Sunday also informed airlines to inspect door panels on another type of Boeing 737 jet, the 737-900ER.    

The National Transportation Safety Board investigators have focused on the four bolts that should have held that blown-out door panel in place. An anonymous Jan. 16 post to the comment section of a website that appeared to be from a current Boeing employee alleges that Boeing’s own records show “these 4 bolts were not installed,” raising questions about quality control.

The anonymous poster alleged that the panel arrived to Boeing’s plant in Renton, Washington, as part of a fuselage built by Spirit AeroSystems, but that it had an issue that needed to be repaired. The poster said the bolts were not installed when the repairs were complete.

CBS News has confirmed that 737 fuselages arrived at the Boeing plant with so many problems that Spirit AeroSystems assigned a team to be on site to make repairs.

The NTSB told CBS News it is aware of the anonymous post. And when reached by CBS News, Boeing declined to comment, referring CBS News to the NTSB.

“As the air safety agency responsible for investigating this accident, only the U.S. National Transportation Safety Board can release information about the investigation,” Boeing’s statement read.

A spokesperson for Spirit AeroSystems told CBS News in a statement that it was “precluded from providing information regarding the ongoing investigation, to which it is an active party. As a company, we remain focused on the quality of each aircraft structure that leaves our facilities.”  

Ed Pierson, a former Boeing senior manager turned Max whistleblower, told CBS News that the systems and processes described in the post were accurate and phrased in a manner consistent with a Boeing employee. He said he wasn’t surprised by the revelation that the panel may not have had bolts.

“In fact, the only thing surprising to us is we’re so thankful that it wasn’t a fatal crash,” Pierson said.

On the night of Jan. 5, Alaska Airlines flight 1282 was carrying 174 passengers and six crew members bound for Ontario, California, when a door panel blew out just minutes after takeoff from Portland.

The plane was able to safely return to Portland International Airport. Officials said several people sustained minor injuries, but no one was seriously hurt. 

The lost door panel was later found in the backyard of a high school physics teacher’s home in the Portland metropolitan area. Two phones that were sucked out of the plane were also found.

In a letter obtained exclusively by CBS News, Democratic Sen. Tammy Duckworth of Illinois demanded that the Federal Aviation Administration deny Boeing’s request for safety waivers on the future 737 Max 7, which is not yet in service.

“The exemption Boeing seeks involves an anti-ice system that can overheat and cause the engine nacelle to break apart and fall off,” Duckworth wrote in her letter. “This could generate fuselage-penetrating debris, which could endanger passengers in window seats behind the wing and/or result in a loss of control of the aircraft.”

She noted that Boeing has indicated it won’t have a fix for this issue until 2026. 

“It is such a bold-faced attempt to put profits over the safety of the flying public,” Duckworth told CBS News. “It astonishes me that they would do this.”

Boeing Thursday will pause production at its Renton factory for a safety stand-down focused on improving quality.

“Boeing is better than this, Flight 1282 should never have happened,” Alaska Airlines CEO Ben Minicucci told NBC News Tuesday. 

In a Jan. 9 meeting, Calhoun admitted to employees that Boeing is “acknowledging our mistake” in the wake of the event. Multiple passengers who were aboard Flight 1282 have filed a class-action lawsuit against Boeing.

— Aimee Picchi contributed to this report. 

Kris Van Cleave

Kris Van Cleave is CBS News’ senior transportation and national correspondent based in Phoenix.