U.K. C-17 Near Miss with Glider Flying with Transponder Switched Off

A U.K. Royal Air Force (RAF) C-17 Globemaster III military transport aircraft narrowly avoided colliding with a civilian glider on July 17, 2024, during landing at RAF Mildenhall in Suffolk County, England.

The incident occurred over Bury St Edmunds when the C-17 pilot, while descending, spotted the ASH 31 glider and made a sharp left turn after rapidly disengaging the autopilot. According to the U.K. Air Accident Board, the aircraft were within 150 meters (500 feet) of each other, posing a serious collision risk. The incident was classified as Category A, the highest risk level in aviation.

Investigators found that the glider pilot had turned off his transponder to conserve battery power after flying for more than six hours, making the aircraft undetectable by the C-17 pilot and air traffic control. The board noted that the glider’s electronic equipment was "incompatible," preventing the ASH 31 pilot from detecting the approaching C-17.

The U.K. Air Close Crossing Board stated that both pilots share equal responsibility to avoid mid-air collisions. Officials described the airspace as "one of the busiest military zones" in the U.K. and said the incident "reduced safety to a minimum."

Military and civil aviation authorities have launched an investigation into the near miss.

The Boeing C-17 Globemaster III has a maximum take-off weight of 265,350 kg (585,000 lb). The ASH 31 glider is a privately operated, single-seat aircraft with a wingspan of up to 21 meters and a maximum take-off weight of 700 kg (1,543 lb).