Incident Overview
On September 8, 2021, at approximately 12:19 Japan Standard Time, a serious incident occurred over the west traffic pattern of Menuma Gliding Field in Kumagaya City, Saitama Prefecture. An Alexander Schleicher ASK21 glider, registered JA2379 (Aircraft A), operated by Chuo University, and a Cessna U206G, registered JA3904 (Aircraft B), operated by Suisan Aviation Co., Ltd., came into close proximity. The pilot in command (PIC) of Aircraft A executed an evasive maneuver and subsequently filed a Near Collision Report, leading to the incident being classified as a serious incident by the Japan Transport Safety Board.
Flight Histories
Aircraft B departed Chofu Airfield at 12:01 under visual flight rules (VFR) for an aerial photography mission over Gyoda City, Saitama Prefecture. At 12:13, the PIC of Aircraft B communicated with Tokyo Information, reporting an altitude of 1,500 ft. At 12:17, he informed Tokyo Information of the start of photography and attempted to contact Menuma Flight Service to confirm glider activity but received no response. Aircraft B continued toward the photography location at approximately 1,400 ft (about 430 m) and 80 kt.
Aircraft A was launched by winch from Menuma Gliding Field's Runway 14 at about 12:18 for a training flight, with the PIC in the aft seat and a trainee in the front. After reaching about 370 m, the cable was released. Due to a planned runway change to Runway 32, Aircraft A was to conduct a 180-degree turn over the downwind leg on the west traffic pattern.
Encounter and Evasive Action
While flying at 330 m on a heading of 320° at 50 kt, Aircraft A was turning from the crosswind leg to the downwind leg. At that moment, the PIC of Aircraft A sighted Aircraft B at the 2 o'clock position, slightly above, flying in the same direction. The horizontal distance was estimated at about 100 m, with an altitude difference of 50 to 60 m. The PIC took over control from the trainee and made a gentle left turn to avoid the other aircraft.
Meanwhile, the PIC of Aircraft B had also observed Aircraft A but assessed it as slightly descending and at a lower altitude, judging no collision risk. He continued toward the photography location, passing to the right of Aircraft A. The PIC of Aircraft B was occupied with coordinating with the cameraperson and was unable to call Menuma Flight Service again.
The trainee on Aircraft A reported being able to read the registration mark on Aircraft B's lower wings, confirming the close proximity.
Aftermath
On September 9, 2021, the PIC of Aircraft A submitted a Near Collision Report to the Ministry of Land, Infrastructure, Transport and Tourism. The Japan Transport Safety Board initiated an investigation on September 10, 2021. The PIC of Aircraft B did not submit a report.
Meteorological Conditions
Visual meteorological conditions prevailed with a flight visibility of 10 km or more.