During the later stages of the approach the aircraft developed a higher rate of descent than desired. The right main gear subsequently struck a sand filled drum which was located just short of the threshold lip. The impact resulted in the right wing being buckled and one of the right engine mounts fracturing. The pilot reported that he did not believe that the landing was heavy and as a consequence did not discover the damage on a subsequent superficial inspection before continuing the flight. The approach was flatter than normal and into a 20 to 30 knot wind. On late final the aircraft encountered a downdraught and the pilot did not arrest the rate of descent, resulting in the touchdown occurring prior to the threshold.
