During the take-off roll the right mainwheel entered an area of soft sand and sank to a depth of 230 mm. The resultant loads applied to the scissor link caused it to fail and the wheel swivelled through 180 degrees. This in turn produced flexing of the gear leg, the downlock disengaged, and the leg collapsed. The aircraft swerved to the right and came to rest just outside the flight strip. The pilot and the operating company were aware that soft areas were present on the strip. The pilot had landed and departed again during the morning without undue problems, and had made an uneventful landing shortly before this particular take-off attempt. However, on this occasion wind conditions dictated a take-off in the opposite direction to that employed earlier in the day and a soft area was encountered at a speed of about 50 knots.