CHI04TA193 · PZL-Mielec M-18T

The airplane sustained substantial damage during a forced landing to a wheat field when a loss of power occurred. The airplane was a PZL-Mielec M-18T that had been modified with a Lycoming T53 turbine engine and propeller assembly from an Army OV-1 Mohawk aircraft. The pilot reported that the flight was uneventful until he 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 increase in thrust and the N2 (propeller) warning light illuminated, indicating an overspeed condition. He immediately reduced power and brought the propeller to high pitch to try regain control of the propeller. When he reapplied power, the N2 overspeed light illuminated again. It became obvious to him that he would not make the runway so he 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. The propeller control assembly, a Mohawk control, part number 557996, serial number SE 15579, was bench tested. 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. The overhaul technician 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." He 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 airplane operator reported that he used 20-weight (20W) or 30-weight (30W), non-detergent automotive engine oil in the propeller control assembly. 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 pilot reported that 30W, non-detergent automotive engine oil was used in the propeller control assembly. After one flight, the propeller control assembly needed servicing and the pilot added 4 ounces of 50W detergent oil instead of a non-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." The airplane had flown 19 hours since the last 100 hour maintenance inspection.

Verbatim NTSB analysis and probable cause from the NTSB dataset