It happened before my time at SA, We were briefed that the aircraft was in a high hover over the Housatonic River. The Stabilizer failed in the full trailing edge down position when the pilots pitched over to accelerate into forward flight. The pilots were not able regain manual control in time to prevent contact with the water. The functionality of Stab manual control switch was brough into question and ultimately secondary control switches were installed in front of each pilots cyclic grips.