Proteus Airlines Flight 706

Proteus Airlines Flight 706 was a scheduled local flight from Lyon, France to Lorient, France. On July 30, 1998, Flight 706 collided in mid-air with a Cessna 177 light aircraft over Quiberon Bay. This disaster was known as the Quiberon Bay mid-air collision. 
Investigation.
The investigation revealed that the Cessna's transponder had not been switched on.
In documentation published by the Aeronautical Information Service in 1997 and 1998 and probably used by the pilot of the Cessna, the use of a transponder while operating under visual flight rules could be interpreted as optional. As a result of the transponder being off, the Cessna was not depicted on the radar screen of the Lorient approach controller and its traffic information could not be relayed to the crew of the Beechcraft.
Following this accident, the BEA recommended that pilots should only cancel instrument flight rules flight plans in cases of necessity.