Accident Narrative
The accident involved a PZL-Mielec M-18T that had been modified with a Lycoming T53 turbine engine and a propeller assembly from an Army OV-1 Mohawk aircraft. According to the pilot, the flight was uneventful until descending through 4,500 feet mean sea level (msl) to enter the right downwind for runway 30. At 4,000 feet msl, the pilot reapplied power but experienced no increase in thrust. The N2 (propeller) warning light illuminated, indicating an overspeed condition. The pilot immediately reduced power and moved the propeller to high pitch in an attempt to regain control. When he reapplied power, the N2 overspeed light illuminated again. Realizing he would not reach the runway, the pilot executed a forced landing to 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.
Propeller Control Assembly Examination
The propeller control assembly, a Mohawk control (part number 557996, serial number SE 15579), was bench tested. The technician reported that when the test was initiated at 600–700 rpm, the unit went to either high or low pitch with high torque. An extreme pressure build-up erupted out the rear-seal flange. Teardown of the 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. No other anomalies were found. The overhaul technician stated that with the scavenge pump plugged, no oil moved to the pressurized pump, resulting in an inability to control pitch changes. He noted that without oil pressure, a pitch lock occurs, leading to an overspeed.
Maintenance Information
The airplane operator reported that 20-weight (20W) or 30-weight (30W) non-detergent automotive engine oil was used in the propeller control assembly. The Army maintenance manual (Technical Manual 55-1510-217-23) specifies that the propeller control unit shall be serviced with hydraulic fluid conforming to Spec MIL-H-5606. The pilot reported that 30W non-detergent automotive engine oil was used. After one flight, the pilot added 4 ounces of 50W detergent oil instead of a non-detergent oil. The pilot noted that the Federal Aviation Administration had not approved the use of automotive engine oil in the propeller control assembly in place of the required hydraulic fluid. Federal Aviation Regulation 43.13 requires that maintenance be performed using methods prescribed in the manufacturer's maintenance manual or instructions for Continued Airworthiness, or other methods acceptable to the Administrator. The airplane had flown 19 hours since the last 100-hour maintenance inspection.