A319 IFR avoids Ventus-2cM at FL 100; transponder off, TCAS blind

An Airbus A319 on IFR descent to Stuttgart (Germany) came close to a Ventus-2cM at FL 100, about 3,000 m AMSL, near Heilbronn at the boundary between Class E and C; both aircraft landed safely. The A319 copilot saw the glider ahead at the same altitude, disengaged the autopilot, and turned left from heading 213 to about 200, while the Ventus pilot turned right. Minimum separation was about 0.9 km. The Ventus carried a transponder but it was off, ATC primary radar showed only an intermittent echo, and the A319's TCAS received no data. The captain stated that without the turn a collision would have occurred.

1. A319 IFR descent cleared to FL 100: At 16:03 LT, Langen Radar cleared an Airbus A319 (Berlin-Tegel → Stuttgart, 5 crew + 131 passengers) to descend to FL 100 at ~250 KCAS. The flight level was simultaneously the highest usable altitude in Class E and the lowest in Class C.
2. Ventus transponder OFF: The Ventus-2cM was VFR with engine retracted and carried a transponder, but the transponder was not switched on. Above 5,000 ft AMSL a transponder is mandatory for powered aircraft. As a result ATC primary radar showed only an intermittent echo and the A319's TCAS received no data on the Ventus.
3. FL 100 = Class E/C boundary: In Class E, IFR traffic is separated from other IFR traffic but not from VFR traffic; the controller could only have offered the A319 traffic information about VFR traffic, and only if visible. With no transponder return the controller could give no warning.
4. A319 copilot spots Ventus ahead: At ~16:05:12 the A319 crew reported '[…] at twelve o'clock, same altitude a glider in eh level one-hundred.' The copilot had seen the glider on a similar heading right ahead; the captain only acquired it visually after the avoidance turn was already underway.
5. A319 turns left 213° → ~200°: At 16:05:05 the copilot disengaged the autopilot and initiated a left turn from heading ~213° to ~200° while maintaining FL 100 at ~250 KCAS. The controller responded 'any turn approved, you may climb as well, no traffic above.'
6. Ventus pilot turns right: The Ventus pilot also saw the airliner heading toward Stuttgart-Echterdingen and turned right; he estimated the closest distance at 300–500 m and did not perceive a danger. The A319 crew reported passing the Ventus on the right side.
7. Min sep ~0.9 km; both land safely: The A319 crew gave a minimum separation of about 0.5 NM (~0.9 km). The A319 landed at Stuttgart and the Ventus continued to Hahnweide airfield and landed there. No injuries, no damage to either aircraft. The A319 captain stated that without the avoidance turn a collision would have occurred.