On March 10, 2011, about 1841 Pacific standard time, a Fairchild SA227-AC, N671AV, sustained substantial damage during a gear-up landing at the Boeing Field King County International Airport (BFI), Seattle, Washington. The commercial pilot, the sole occupant of the airplane, was not injured. The airplane was registered to UAS Transervices Inc., Pasadena, California, and operated by Ameriflight Inc., Burbank, California, under the provisions of Title 14 Code of Federal Regulations Part 135. Visual meteorological conditions prevailed and an instrument flight rules flight plan was filed for the on-demand cargo flight. The cross-country flight originated from Lewiston (LWS), Idaho, at 1715 with an intended destination of BFI. In a written statement, the pilot reported to the National Transportation Safety Board investigator-in-charge that he began his duty day at 0530 for his scheduled flight from BFI to LWS. Upon arriving at LWS, the pilot was notified of an unscheduled flight back to BFI and subsequent return to LWS. The pilot stated that the unscheduled trip during that day "disrupted my normal sleep pattern.” Upon returning to LWS the afternoon of the day of the accident, he began his normal scheduled afternoon flight from LWS to BFI. The pilot said that the flight was uneventful except for moderate turbulence over the Cascade mountain range. While en route to BFI, the pilot obtained weather at BFI, noting wind from 210 degrees at 16 knots, gusting to 35 knots. The pilot further reported that his first two landing attempts at BFI resulted in aborted landing attempts due to the crosswinds and airspeed deviations of 30 knots. He said that during his third attempt to land, he was instructed by the air traffic control tower (ATCT) controller to follow a Cessna Citation that was on final approach for runway 13R. Upon reporting the traffic in sight, the pilot was given his landing clearance and subsequently told to "turn base early" due to another airplane on approach to landing. The pilot stated that the Citation "missed their runway turnoff" to the taxiway and that he was told by the air traffic controller to make "S" turns for spacing. As the airplane descended to about 200 feet above ground level on final approach to the runway, the pilot was instructed by the controller to "go around, maintain altitude.” The pilot said that he executed the go-around procedure, which included flaps to one-half and landing gear up. About 10 seconds later, the pilot was issued a landing clearance by the controller. The pilot stated that he "...proceeded with my landing clearance, knowing that all check-lists have been completed, and I was in a normal position to land. There is no gear warning system with flaps 1/2 and no call outs or check-lists to indicate my gear had been retracted." Subsequently, the airplane landed gear up on runway 13R. The airplane came to rest upright in a safety area between runway 13R and 13L just south of alpha 7 taxiway. Review of the pilot training manual for the Fairchild SA227-AC revealed that a landing gear warning horn will sound if either power lever is retarded to flight idle or the flaps are lowered slightly beyond 1/2 when the landing gear is not in the down and locked position. Examination of the airplane by company maintenance personnel and a Federal Aviation Administration (FAA) inspector revealed structural damage to both engine firewalls and aft fuselage bulkhead. Following recovery of the wreckage, the airplane was placed on jack stands. Power was applied to the airplane and the landing gear system was actuated to the up and down positions with no mechanical anomalies noted.