On July 11, 2004, at 1925 mountain daylight time, a PZL-Mielec M-18T, N7077N, operating as a public use aircraft under contract with the Bureau of Indian Affairs, sustained substantial damage during a forced landing in a wheat field near the Pine Ridge Airport (IEN), Pine Ridge, South Dakota. The pilot was not injured. The public use fire fighting flight departed IEN at 1838 and was returning to IEN when a loss of power occurred and a forced landing was executed. Visual meteorological conditions prevailed at the time of the accident. A flight plan was filed. The pilot reported that the flight was uneventful until he was returning to IEN and was descending through 4,500 feet mean sea level (msl) to enter the right downwind for runway 30. As he reached 4,000 feet msl he reapplied power, but there was no noticeable increase in thrust and the N2 (propeller) warning light illuminated, indicating an overspeed condition. The N2 gauge read 1,800 rpm. (The normal maximum N2 reading is 1,720 rpm) The N1 (engine speed) was at 91 percent and torque was at 17 psi. The pilot reported that a N1 setting of 91 percent should produce a 22 psi of torque. The pilot reported that he immediately reduced power and brought the propeller to high pitch to try regain control of the propeller. He attempted to land on runway 6, but when he reapplied power, the N2 overspeed light illuminated again and the N1 indicated 91 percent. It became obvious to him that he would not make the runway so he executed a forced landing in a rolling wheat field. He reported that the "glidepath was steep and the three point landing was firm with a slight drift to the right from a northerly wind gust" resulting in damage to the right wing spar. As a result of the on-site inspection of the airplane and pilot testimony, the propeller control assembly, a Mohawk control, part number 557996, serial number SE 15579, was shipped to Pacific Propellers, Inc. (PPI), Kent, Washington, for a teardown inspection. PPI bench tested the propeller control assembly. The technician reported that when he started to run the test with the unit at 600 - 700 rpm, the unit went to either high or low pitch with high torque. He reported there was an "extreme pressure build-up which erupted out the rear-seal flange." The teardown of the propeller control assembly revealed contamination of the 10 pressure relief valve filter disks, the bottom of the pressure sump, the main pump intake filter, and the scavenge pump filter. There were no other anomalies found in the propeller control assembly. PPI reported, "With the scavenge pump plugged, you were not moving any oil to the pressurized pump and, therefore, you had no oil to control because that's where your oil is coming from to control pitch change. So it would be equivalent to a loss of control in terms of what's going to happen." PPI further stated, "With no oil pressure you get pitch lock. ... Without the oil to control it from the pressurized sump, you're going to get an over-speed and it'll pitch lock." The operator of the airplane was contacted to determine what type of maintenance was performed on the propeller control assembly. The operator reported that he had purchased the airplane about three years prior. The airplane had been modified with a Lycoming T53 turbine engine and propeller assembly from an Army OV-1 Mohawk aircraft. The Army maintenance manual (Technical Manual (TM) 55-1510-217-23) stated that the propeller control unit shall be serviced with hydraulic fluid, Spec MIL-H-5606. The manual also contains a WARNING that states that synthetic oil shall not be used in the propeller control unit as it "would be harmful to the propeller and control preformed packings." The manual also had a CAUTION that stated, "If a container is used to fill the sump it must be thoroughly clean and must not have the slightest trace of synthetic base oil. Just a few drops can be very harmful to propeller and control preformed packings." The airplane operator reported that he used 20-weight (20W) or 30-weight (30W), non-detergent automotive engine oil in the propeller control assembly of N7077N. He reported that PPI had recommended using the automotive oil instead of the hydraulic fluid required by the Army maintenance manual. PPI was contacted and they reported that no one in their organization would recommend using anything but the proper hydraulic fluid in the propeller control assembly. The pilot of the accident aircraft, who was also an Aircraft and Powerplant (A&P) mechanic with Inspection Authorization (IA), was contacted to determine what maintenance actions he performed on the propeller control assembly. The pilot reported that 30W, non-detergent automotive engine oil was used in the propeller control assembly. He reported that after a flight from the Standing Rock Indian Reservation to Pine Ridge, South Dakota, the propeller control assembly needed servicing, so he added 4 ounces of 50W detergent oil. He reported that the Federal Aviation Administration had not approved the use of automotive engine oil in the propeller control assembly instead of the required hydraulic fluid. FAR 43.13 states, "Each person performing maintenance, alteration, or preventive maintenance on an aircraft, engine, propeller, or appliance shall use the methods, techniques, and practices prescribed in the current manufacturer's maintenance manual or instructions for Continued Airworthiness prepared by its manufacturer, or other methods, techniques, and practices acceptable to the Administrator."