A North Korean Air Koryo airliner was forced to make an emergency landing in Pyongyang in May after a flap on the plane’s wing fell off mid-air, multiple sources confirmed to NK News on Tuesday.

The incident, which took place on Thursday May 25 onboard a flight from Pyongyang to Beijing, happened after the aircraft crossed into Chinese airspace, one source familiar with the incident told NK News.

“The whole plane started vibrating and people noticed the flap coming loose,” the source said on condition of anonymity.

“I heard it fell off in the end (and then) they returned to Pyongyang one hour later on