Accident Overview
On June 6, 2013, at 12:18:52 local time, a mid-air collision occurred between a Ventus 2b glider (registration HB-3373) and a Mooney M20J (registration HB-DFP) near Gisliflue, in the municipality of Auenstein, Canton Aargau, Switzerland. The glider pilot suffered minor injuries, while the two occupants of the Mooney were uninjured. The glider was destroyed; the Mooney sustained substantial damage.
Aircraft and Pilots
The glider, a Ventus 2b, was owned and operated by the Segelfluggruppe Birrfeld. Its pilot, a Swiss citizen born in 1952, held a sailplane pilot license and a valid medical certificate with restrictions requiring multifocal lenses and a hearing aid. He had 2,439.55 total flight hours, with 867.59 hours on the accident type.
The Mooney M20J was privately owned and operated by the Lommair-Verein. The flight crew comprised two pilots: Pilot 1 (born 1944) held a PPL(A) and a Class Rating Instructor certificate, with 2,967 total hours (408.58 on type). Pilot 2 (born 1949) held a PPL(A) and had 495 total hours (3.45 on type). Both pilots had valid medical certificates with corrective lens requirements.
Flight History
Glider Flight
On the morning of June 6, the glider pilot prepared for a flight along the Jura mountains. He arrived at Birrfeld airfield around 09:30, assembled the glider with assistance, and performed preflight checks. At 11:55, he departed in an aerotow on runway 08. Approximately three minutes later, he released at about 1,100 m (3,906 ft) AMSL near Chestenberg. After circling without gaining altitude, he flew westward towards the Jura, then returned to Chestenberg. At 12:10, he found a thermal and climbed to about 1,400 m (4,593 ft) AMSL, then followed two other gliders toward Gisliflue, flying 150–200 meters below them.
Mooney Flight
Pilot 1 and Pilot 2 planned a familiarization flight from Lommis (LSZT) to Ecuvillens (LSGE). They met at Lommis around 09:00, conducted flight planning, refueled, and performed preflight inspections. At 11:55, they departed from runway 06, with Pilot 2 in the left seat and Pilot 1 in the right. They flew via reporting point WHISKEY at 2,900 ft AMSL toward Zurich East VOR, then toward Trasadingen VOR. South of Schaffhausen, Pilot 1 engaged the autopilot to demonstrate it. After passing Trasadingen VOR, they set a heading of 220 degrees to bypass Birrfeld. At 12:16, near Villigen, they initiated a climb at approximately 400 ft/min. One minute later, near Linn, the aircraft turned south.
Collision Sequence
After crossing the Aare River westward in straight flight, the glider pilot looked north to assess weather conditions. At that time, the glider's ground speed was about 160 km/h and the Mooney's about 220 km/h. Approximately one minute later, the glider pilot perceived movement in his peripheral vision and then saw an aircraft he identified as a Mooney with retracted landing gear, approximately 50 meters away, approaching from the right. At 12:18:52, the two aircraft collided. The glider's FLARM collision warning system and the Mooney's GPS recorded the event.
Post-Collision
Following the collision, the glider became uncontrollable and crashed. The glider pilot sustained minor injuries. The Mooney pilots remained uninjured and likely landed safely. The glider was destroyed, and the Mooney suffered heavy damage. Minor forest damage occurred.
Investigation Findings
The Swiss Transportation Safety Investigation Board (STSB) determined that the accident resulted from insufficient active monitoring of the airspace by both crews, leading to the collision. The glider became uncontrollable and crashed. Two systemic factors were identified: the motor aircraft was not equipped with a collision warning system, and the transponder signals from the motor aircraft could not be received by the glider's collision warning system.
Probable Cause
The accident was due to the collision between a glider and a motor aircraft because both crews monitored the airspace too passively. Consequently, the glider became uncontrollable and crashed. Systemic factors: the motor aircraft was not equipped with a collision warning system; the transponder signals from the motor aircraft could not be received by the glider's collision warning system.