A United Airlines jet grounded in Medford since Friday after a post-landing inspection revealed a missing external panel, apparently lost in flight, returned to San Francisco without incident on Sunday morning.

The no-passenger ferry flight lasted just under an hour.

Online flight tracking and data platform company FlightAware reported the 25-year-old Boeing 737-824 jet took off from Medford Airport at approximately 10:53 a.m. local time Sunday and landed at 11:48 a.m. at San Francisco International Airport, a United Airlines hub and maintenance base.

United Flight 433 initially arrived in Medford from San Francisco late Friday morning after the aircraft had flown from Nashville to San Francisco earlier in the day. The flight landed safely, and the Medford airport was the scheduled destination, according to an email Friday from United Airlines’ media relations team. There were 139 passengers and six crew on board.

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

“We’ll also conduct an investigation to better understand how this damage occurred,” the statement added.

According to the aircraft’s registration, the plane was delivered to Continental Airlines on Dec. 22, 1998. It has been operated by United Airlines since Nov. 30, 2011.

Jackson County Airport Director Amber Judd said Friday no injuries were reported.

The plane was not a new aircraft. Judd said that newer model aircraft — such as the Alaska Airlines Boeing 737-MAX 9 that lost a passenger compartment side panel mid-flight near Portland in January — do not fly in Medford.

“It was not a MAX aircraft,” Judd said.

According to the aircraft’s N-number, the plane was delivered to Continental Airlines on Dec. 22, 1998. It has been operated by United Airlines since Nov. 30, 2011.

The whereabouts of the missing metal piece is unknown. “They don’t know where they lost it,” Judd said.

The search for the missing part prompted airport officials to temporarily close two runways Friday. Judd said it lasted “just a matter of a few minutes, maybe 15 minutes” while crews ensured there was no debris.

“Then the other planes started landing and taking off,” Judd said.

Four Boeing planes operated by United have suffered incidents over the past two weeks, the Los Angeles Times reported.

On Monday, a San Francisco-bound United Airlines flight turned around two hours after leaving Sydney. The Boeing 777-300 aircraft returned due to a maintenance issue, according to the L.A. Times.

Prior to that, a Boeing 777-200 operated by United Airlines made an emergency landing in Los Angeles after a tire fell off on March 7, the newspaper reported.

There was also an emergency landing in Houston on March 4 after flames were spotted coming from a United Airlines Boeing 737-900ER. United confirmed the engine ingested bubble wrap, according to the L.A. Times.

Four days later, a Boeing 737-8 Max rolled onto the grass near a runway in Houston upon landing, though no passengers were injured, the report said.