The fuselage panel of a United Airlines flight — a Boeing 737-800 — was discovered missing Friday afternoon after the plane landed at an airport in Medford, Oregon, after having departed San Francisco, officials said. 

United Flight 433 from San Francisco International Airport to Rogue Valley International Medford Airport landed safely as scheduled in Medford, the Federal Aviation Administration reported. 

It was only during a “post-landing airline inspection” that the missing panel was discovered, the FAA said. CBS News learned the missing panel is from the underside of the aircraft, just adjacent to where the main landing gear deploys.   

“After the aircraft was parked at the gate, it was discovered to be missing an external panel,” United said in a statement, adding that it would conduct a “thorough examination of the plane and perform all the needed repairs before it returns to service.”

The plane was carrying 139 passengers and six crew members, United said. Airline monitoring website Flight Aware indicated the flight departed San Francisco at 10:20 a.m. and landed ahead of schedule in Medford at 11:36 a.m.  

Amber Judd, Rogue Valley airport director, also confirmed in a statement to CBS News that the missing panel was discovered during a “routine post-flight inspection.”

Airport operations were briefly paused while a search of the runway was conducted, but “no debris on the airfield” was found, Judd added. 

The FAA said it would be launching an investigation. The plane is approximately 25-years-old.

United noted in its statement that no emergency was declared during the flight “as there was no indication of the damage during flight.” The airline did not indicate if it knew when the plane may have lost the external panel, or if the panel was missing prior to taking off from San Francisco.   

On Jan. 5, the door plug of an Alaska Airlines flight bound for Ontario, California, blew out just minutes after the Boeing 737 Max 9 had taken off from Portland, Oregon. The plane was able to safely return to Portland International Airport, and there were no serious injuries.

The lost door plug was later found in the backyard of a home in the Portland metro area. 

The Boeing plane in Friday’s incident is from a generation manufactured prior to Boeing’s Max aircraft. 

This marks the latest United flight to report an issue amid a series of recent problems for travelers on the airline. A seemingly disproportionate number of those flights have either originated from, or been bound for, San Francisco, including three since Monday. 

United Flight 1816 from Dallas to San Francisco landed without major problems after it “encountered a hydraulic leak as it neared its destination” Thursday morning.  

That flight landed safely after its trip from Texas, and the jet’s passengers were able to deplane normally at the gate. However, United noted that emergency vehicles were standing by as a precaution during the landing after smoke was seen around the landing gear. 

On Monday, another United flight bound for San Francisco from Australia had a “maintenance issue” and was forced to return to Sydney.

The rash of incidents on United flights stretch back to March 4, with multiple United aircrafts facing problems that forced flights to return to their point of departure or make emergency landings.

On March 4, United Flight 1118 bound for Fort Myers, Florida, had to return to Houston’s George Bush Intercontinental Airport after the engine burst into flames shortly after takeoff. An airline investigation later determined that the fire was caused by bubble wrap that was sucked into the engine. 

Two other flights were diverted to LAX after leaving SFO. On March 7, a jet lost a wheel during takeoff, with the wheel crushing one car and damaging several others in an SFO employee parking lot. 

The following day, March 8, problems with a jet’s hydraulics system forced a plane bound for Mexico City to make an emergency landing in Los Angeles.

Also on March 8, United Flight 2477 from Memphis to George Bush Intercontinental Airport in Houston veered off the runway just after landing. 160 passengers and six crew members on the flight were forced to depart the plane using air stars on the open runway. There were no injuries in any of these recent incidents.

United has maintained that the rash of safety incidents were unrelated, with aviation experts backing up the airline’s statement. 

— Kris Van Cleave contributed to this report.