Boeing faces more turbulence even though flight landed safely as it was unclear when or how the panel went missing.

Flight 433 from San Francisco landed safely at the Rogue Valley International Medford Airport in Oregon at about 20:45 GMT on Friday, but a post-flight inspection revealed a missing panel, the FAA said.

Medford airport paused operations to check the runway and airfield for debris but none was found, according to the facility’s director, Amber Judd.

The missing panel was on the underside of the plane, where the wing meets the body and just next to the landing gear, United said.

There were 139 passengers and six crew members on board the 25-year-old aircraft. No injuries were reported.

Boeing has come under intense scrutiny since January when a panel that plugged a space left for an extra emergency door blew out midair on an Alaska Airlines Boeing 737 Max 9 flight shortly after takeoff from Portland, Oregon.

Pilots were forced to make an emergency landing. There were 174 passengers and six crew on board and some people reported minor injuries.

US regulators immediately launched investigations into the company’s safety and quality standards in its production process. The FAA grounded Boeing 737 Max 9 jetliners, affecting 171 planes worldwide and said it had identified “non-compliance issues in Boeing’s manufacturing process control, parts handling and storage, and product control” following an audit of the company.

The National Transportation Safety Board and the US Department of Justice also launched separate investigations into Boeing.

A United Airlines spokesperson said no emergency was declared because there was no indication of the damage during the flight.

“We’ll conduct a thorough examination of the plane and perform all the needed repairs before it returns to service,” the airline said. “We’ll also conduct an investigation to better understand how this damage occurred.”

The plane made its first flight in April 1998, according to the FAA. It is a 737-824, part of the 737-800 series that was a precursor to the Max.

There was no immediate comment from Boeing.

The aircraft manufacturer shuffled its leadership following the Alaska Airlines incident but faces a widening gap in market share with rival Airbus following fewer orders and deliveries per year in the last decade.

The 737 Max aircraft was grounded for 20 months between 2019 and 2020 after two fatal crashes killed nearly 350 people.

On March 4, an engine fire forced a United Airlines-operated Boeing 737 bound for Florida to make an emergency landing in Houston, Texas, shortly after takeoff. United said the engine ingested some plastic bubble wrap that was on the airfield prior to departure.

Two days later, fumes in the cabin of a Boeing 737-800 forced an emergency landing in Portland.

Last week, a tyre fell off a Boeing 777-200 after takeoff in San Francisco, destroying a car. The plane was bound for Japan but diverted to Los Angeles, where it landed safely.

A day later, a United Airlines-operated Boeing 737 Max rolled onto the grass and off the runway in Houston.

On Friday, Boeing told airlines to inspect switches on pilots’ seats in its 787 Dreamliner jets after a published report said an accidental cockpit seat movement likely caused the sudden plunge of a Latam Airlines plane last Monday.

About 50 people were injured on the plane that was flying to New Zealand from Australia.