A United Airlines Boeing 737 lost an external panel during a flight on Friday, the airline said. The damage to the Boeing 737-800 was discovered after it had landed safely with 139 passengers and six crew members on board in the southern Oregon city of Medford, United said. The plane is reportedly 25 years old. “After the aircraft was parked at the gate, it was discovered to be missing an external panel. We’ll conduct a thorough examination of the plane and perform all the needed repairs before it returns to service,” a statement from United said. “We’ll also conduct an investigation to better…