The DC-3 departed Zurich at 08:57 on a training flight. Purpose of the flight was to train airline pilot candidates (VFR flight exercises in cutting of one engine and feathering and unfeathering the prop in cruise) and to perform some Swissair planning department tests for a revision of the DC-3 performance table. Eyewitnesses saw the aircraft flying in various directions at 1000-3000 m in the Lake Constance/Bodensee area. At 10:20 when flying between Romanshorn and Arbon, the aircraft was seen flying in level flight when it suddenly stalled and went into a spin. The aircraft struck the water in a very steep dive. PROBABLE CAUSE: “The stalling of the aircraft following a loss of airspeed, whereupon it unintentionally went into a spin. In view of insufficient altitude, it was not possible to level off the aircraft.”