IMMEDIATELY AFTER TAKEOFF, THE PILOT BECAME UNSURE OF CLEARING THE MOUNTAINOUS TERRAIN. A 180 DEG TURN WAS INITIATED DURING WHICH THE PILOT ENCOUNTERED A STRONG DOWNDRAFT. PASSENGERS AND WITNESSES STATED THAT THE AIRPLANE MAINTAINED AN UNUSUAL NOSE-HIGH ATTITUDE AND LOW AIRSPEED FROM LIFT-OFF TO JUST BEFORE IMPACT. THE PILOT HAD TAKEN OFF FROM A ROUGH AREA ON THE FROZEN LAKE, DIFFERENT FROM THE USUAL TAKEOFF AREA.