A United Airlines passenger plane lost a cover on its fuselage in flight on Friday. The damage to the Boeing 737-800 was only discovered on the ground after it landed without any problems in the city of Medford, Oregon, United said. The local newspaper “Rogue Valley Times” had previously published an eyewitness photo showing exposed mechanics.

According to United, the plane took off from San Francisco with 139 passengers and six crew members. Judging by the onboard number from another eyewitness photo, the aircraft was put into service in 1998.

Boeing is currently increasingly in the spotlight after a virtually new 737-9 Max aircraft lost a part of its fuselage while climbing after take-off at the beginning of January. According to initial investigations, the US accident investigation authority NTSB assumes that four fastening bolts were missing from the torn out fuselage fragment.

However, for a 25-year-old aircraft like United’s, maintenance or material fatigue are likely to play a larger role than production. United announced an investigation into how the incident occurred.