Extract from report into AS355 TR Control Failure over Cardiff G-SAEW, Flown by an ex student of mine:
The helicopter had been hovering in the area for about 10 to 15 minutes, facing in a south-westerly direction, when it suddenly made an uncommanded yaw to the left through some 180 degrees. The pilot immediately applied full right yaw pedal to counter this yaw. However, although the helicopter stabilised for a moment, it then yawed more rapidly to the left. At this time he called out to the two observers on board to warn them of a problem with the helicopter. He partially lowered the collective lever in an attempt to regain control and applied some forward cyclic to gain forward motion and airspeed, but the helicopter then entered a steeply spiralling/yawing descent to the left. The pilot realised that he would not be able to recover full control of the helicopter and abandoned his attempt to fly out of the situation. He concentrated on keeping the helicopter as level as possible whilst looking out through the right side window for visual reference, since he found the forward view too confusing due to the rapid yawing motion. He adjusted collective to achieve what he judged to be the best combination of rate of descent against yaw, and when he caught sight of the surface in his peripheral vision he pulled the collective lever fully up to cushion the impact.
The helicopter came to rest embedded in the roof of a house, as shown in Figure 1, having broken through the rafters and settled in a right side low attitude. After the impact, the pilot was unable to reach the engine fuel controls on the overhead panel until he had unstrapped himself from his seat. However, when he was able to reach the speed select and emergency fuel shut off levers he could not move them due to impact induced distortion of the overhead panel. He was able to activate both fire extinguishers and to turn the battery switches OFF. The observer in the front left seat escaped through his door on the left side of the helicopter and the rear seat observer climbed past the pilot and exited through the same door; the pilot then followed. The three occupants, all of whom were wearing protective helmets, were uninjured and later used a ladder to climb down from the roof.
A considerable amount of skill and quick thinking with Lady Luck for the impact. That boy did well!