The Federal Aviation Administration is weighing possible temporary action against United Airlines beyond what was described in a letter the company sent to employees Friday, two sources familiar tell CBS News. 

This comes in response to a series of concerning incidents involving United aircraft over the last month that included a wheel coming off a Boeing 777, and a panel flying off an aging Boeing 737. 

Among the possible temporary measures discussed include barring United from launching new routes that it has not yet begun selling tickets for. Another being considered would be to allow the carrier to continue taking possession of new aircraft — but pausing its ability to introduce the new planes into revenue service, which refers to commercial flights that carry paying passengers. 

A third possibility would be temporarily not allowing United check airmen to certify new captains. Airlines typically do those sign-offs internally. 

Sources stress that discussions inside the FAA may not result in action, so some or all of these measures may not come to bare at all. United says it has not been notified of a final decision by the FAA, and those internal FAA discussions may be ongoing. 

"Due to recent safety events, the FAA is increasing oversight of United Airlines to ensure that it is complying with safety regulations; identifying hazards and mitigating risk; and effectively managing safety," the FAA said in a statement provided to CBS News Saturday. "Certification activities in process may be allowed to continue, but future projects may be delayed based on findings from oversight. The FAA will also initiate an evaluation of United Airlines under the provisions of the Certificate Holder Evaluation Process."

In an interview with NBC News this week, FAA Administrator Michael Whitaker acknowledged that he spoke last weekend to United CEO Scott Kirby about the recent incidents.

"I know that they're taking some heightened measures, and looking at these issues," Whitaker told NBC News. "We're going to look at each one of these incidents and see if we see a pattern…He's concerned, I'm concerned, no one likes to see this spike of incidents. So we're both doing our jobs to look at where those risks might be at."

In a Friday letter to employees, Sasha Johnson, United vice president of corporate safety, seemed to acknowledge some temporary action was coming. 

"Over the next several weeks, we will begin to see more of an FAA presence in our operation as they begin to review some of our work processes, manuals and facilities," Johnson wrote. "As part of this effort, the FAA will also pause a variety of certification activities for a period of time. Those activities will differ depending on the work group and we will learn more from the FAA about that soon."

The FAA's potential temporary action was first reported by Bloomberg. 

"Safety is our highest priority and is at the center of everything we do," wrote Kirby in a letter to customers sent March 18. "Our team is reviewing the details of each case to understand what happened and using those insights to inform our safety training and procedures across all employee groups."

United has aggressive growth plans, which includes hundreds of new planes on order, and has been rapidly growing its international route map. Earlier this month, United announced plans to launch service to Marrakesh, Morocco, Cebu, Philippines, and Medellin, Colombia. 

In that same March 7th announcement, the airline said it plans to increase flying to Hong Kong, Seoul, South Korea, Porto, Portugal, and Shanghai, China. 

Pausing route expansion and introducing new aircraft has the potential to have a significant impact on United's bottom line already impacted by ongoing delivery delays from Boeing. 

Sources at the airline were unable to say when that "pause" would begin, or what precisely would be paused. 

In:
• Boeing
• United Airlines
• Airlines

Kris Van Cleave

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

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