On June 2, 2014, about 0930, mountain daylight time, an Aero Commander, Callair A-9B, N7292V, sustained substantial damage during a forced landing shortly after departure from the Gardiner Airport (29S) Gardiner, Montana. The commercial pilot was seriously injured. The airplane was registered to Nardin and Nardin Co., and operated by the pilot under the provisions of Title 14 Code of Federal Regulations Part 137 as an aerial application flight. Visual meteorological conditions prevailed and no flight plan was filed for the local flight. The pilot reported that the airplane's climb rate was not sufficient after takeoff and he decided to perform an emergency landing on a grass field. During the landing sequence, the airplane's wing struck terrain and subsequently the airplane came to rest inverted. Examination of the airplane by a Federal Aviation Administration (FAA) inspector revealed substantial damage to the airplane's fuselage and wings. The airplane was recovered to a secure storage facility for further examination. Additional examination revealed that the engine was separated from the airframe by the engine mount. The engine case was breeched on top of the area where the front cylinders attach. All the rocker covers were removed and the cylinder overhead areas were lubricated and unremarkable. The cylinders were removed and each combustion chamber was examined. The inspection of the internal areas of the cylinders revealed normal operational conditions. All engine accessories were examined and no anomalies were observed. The top sparkplugs were removed and all the sparkplug electrodes exhibited normal wear signatures when compared to the Champion Check-A-Plug comparison chart The two-bladed propeller remained attached to the crankshaft flange. Both blades were bent aft about mid-span and twisted. Marks and abrasions were observed on the blades that were chord-wise in direction. The postaccident examination of the airframe and engine revealed no evidence of a mechanical malfunction that would have precluded normal operation. According to an FAA sponsored performance study of the airplane, the maximum gross weight limitation for flight was 3,000 pounds. The take-off gross weight was calculated by the National Transportation Safety Board investigator-in-charge (IIC), using the empty weight of the airplane, the reported weight of the occupant, 40 gallons of fuel, 12 quarts of oil, and 1600 pounds of chemical. The calculated gross weight at the time of departure was about 3,881 pounds, which exceeded the maximum gross weight limitation. Utilizing the weather conditions at the nearing reporting station, the density altitude was calculated by the IIC to be about 5,661 feet mean seal level, for the departure time of the accident flight.