A BELL 206B WENT OUT OF CONTROL AND CRASHED WHILE LNDG AT A 5,720 FT HIGH HELIPAD. THE PLT INDICATED THAT HE TERMINATED THE APRCH AT A 8-10 FT HOVER AND BEGAN TO SPIN TO THE RIGHT WITH FULL LEFT PEDAL APPLIED. AFTER 1 1/2 TO 2 TURNS THE PLT ATMPTD TO FLY OUT OF THE SPIN BY LOWERING THE NOSE AND REDUCING POWER. THE ACFT CONTINUED TO SPIN AND COLLIDED WITH THE TERRAIN ROLLING ONTO IT'S LEFT SIDE. THE APPROX DENSITY ALTITUDE (DA) FOR THE HELIPAD WAS 6,800 FT MSL. AC 61-13B STATES, 'IF THE DESTINATION IS AT A HIGHER ALTITUDE AND/OR HIGHER TEMPERATURE AND MOISTURE CONTENT PREVAIL, SUFFICIENT POWER MAY NOT BE AVAILABLE TO HOVER AT THE DESTINATION WITH THE EXISTING GROSS WEIGHT'. THE MANUAL ALSO STATES THAT A NORMAL APRCH TO THE SURFACE SHOULD BE USED WHEN HIGH DA OR HEAVILY LOADED CONDITIONS EXIST. THE FAA DEFINED 'UNANTICIPATED RIGHT YAW' AS AN OCCURRENCE OF AN UNCOMMANDED RIGHT YAW RATE WHICH DOES NOT SUBSIDE OF ITS OWN ACCORD AND WHICH, IF NOT CORRECTED CAN RESULT IN LOSS OF ACFT CONTROL.