On September 7, 2013, at 1330 mountain daylight time, a Cessna model 180 airplane, N561AT, was substantially damaged while landing at Gallup Municipal Airport (KGUP), Gallup, New Mexico. The commercial pilot and his passenger were not injured. The airplane was registered to and operated by the pilot, under the provisions of 14 Code of Federal Regulations Part 91, without a flight plan. Day visual meteorological conditions prevailed for the personal flight, which departed Portales Municipal Airport (KPRZ), Portales, New Mexico, about 1100. The pilot reported that the accident occurred as he was landing the tail wheel-equipped airplane on runway 06 (7,316 feet by 100 feet, asphalt). He stated that the landing roll was uneventful until he began retracting the wing flaps at a ground speed comparable to a "brisk walk." He reported that the airplane ground-looped after he inadvertently released left brake pressure while his attention was diverted with the mechanical flap handle. The left wing was substantially damaged when it impacted the runway during the ground-loop. The pilot reported that although the airport's weather radio broadcast indicated a calm wind condition, there was a slight right crosswind during the landing. The pilot stated that the crosswind condition did not contribute to his loss of directional control during landing roll, but rather his failure to follow his own standard practice of bringing the airplane to a complete stop before completing any post-landing checklist items (including retracting the wing flaps). The pilot reported no preaccident malfunctions or failures with the airplane that would have prevented normal operation. At 1253, the KGUP automated surface observing system reported: variable wind direction at 3 knots, visibility 10 miles, few clouds at 8,500 feet above ground level, temperature 28 degrees Celsius, dew point 6 degrees Celsius, and an altimeter setting of 30.25 inches of mercury. The airport elevation was 6,472 feet mean sea level. The calculated density altitude was 9,145 feet.