Accident Overview
On August 22, 2018, at about 2300 eastern daylight time, a Cessna T206H, registration N35442, operated by the US Department of Homeland Security, was substantially damaged during a forced landing following a loss of engine power. The accident occurred near Pittsfield Municipal Airport (2B7) in Pittsfield, Maine. The commercial pilot was seriously injured and the passenger sustained minor injuries. Visual meteorological conditions prevailed, and an instrument flight rules flight plan was filed for the public use flight that originated from Bangor International Airport (BGR), Bangor, Maine.
Pilot Narrative
According to the pilot, while returning from a mission flight at an altitude of 8,000 ft, he requested a lower altitude from air traffic control. The controller cleared the pilot to descend to 7,000 ft. During the descent, the pilot heard a "bang" and felt a vibration. He checked the engine monitoring instruments and noted that the No. 4 cylinder was "lost." Shortly thereafter, the engine lost power completely. The pilot declared an emergency and was provided with radar vectors to the closest airport, which was 2B7. While on an extended final approach to runway 36, the pilot determined he would not be able to reach the runway. He attempted to land in an open area of forest about 4,500 ft south of runway 36 but impacted a heavily wooded area of the marsh.
Postaccident Examination
Postaccident examination of the wreckage revealed damage to the elevator and fuselage. Cursory examination of the engine indicated that the No. 4 cylinder exhaust hydraulic roller tappet was fractured and had separated. The engine's No. 4 exhaust roller tappet assembly, camshaft, crankcase, and oil filter were sent to the engine manufacturer's metallurgy laboratory for evaluation.
The engine crankcase generally displayed internal contact damage in the form of various gouging and contact marks. Components of the roller tappet assembly were found and recovered from the oil sump. The crankcase bore holding the No. 4 exhaust roller tappet was in good condition with no abnormal scoring or wear. Below the No. 4 exhaust tappet bore, witness marks on the crankcase were consistent with contact with the roller tappet. Dimensional inspection of all six exhaust roller tappet bores using go/no-go gages found them conforming.
The No. 4 exhaust camshaft lobe displayed severe wear, spalling, and galling. Minor damage along the length of the camshaft appeared as secondary contact marks. While the No. 4 exhaust camshaft lobe was too damaged for metallographic examination, examination of the No. 5 exhaust camshaft lobe revealed a conforming case and core microstructure.
Engine Component Analysis
The No. 4 exhaust roller tappet was fractured, with the roller, needle bearings, and shaft no longer retained by the tappet body. Both ears of the roller body that retained the shaft were fractured. The engine's 11 other roller tappet assemblies were in overall good condition, with each shaft remaining staked. The No. 4 exhaust roller body ear fracture surfaces were examined using stereomicroscopy, digital microscopy, and scanning electron microscopy. Spalling of the case was observed around the shaft bores on the body, along with witness marks from the roller and needle bearings on the inside of the body ears. The witness marks from the roller appeared deeper on one ear compared to the other. Fatigue was observed on one body ear fragment fracture surface. The roller had separated into multiple fragments showing evidence of brittle fracture originating at the outside diameter in a severely spalled region. Another fragment showed fracture origin at the inside diameter. The roller displayed significant wear and spalling on the outer diameter, consistent with severe wear on the corresponding camshaft lobe. Some areas of the inner diameter were galled and/or spalled. The needle bearings were in overall good condition, and the shaft was damaged mostly on the two ends and some areas of the midsection. The No. 4 exhaust roller tappet components were evaluated for conformance, including selected key dimensions, material chemistry, hardness, case hardness, case depths, and microstructure as applicable. Each evaluated component was conforming to engineering requirements.
The oil filter was examined and found to contain fine metallic chips. A representative selection of approximately 18 chips were cleaned and examined using energy dispersive x-ray spectroscopy. Three chips were composed of steel consistent with the type used in the roller tappet assembly roller or shaft; two were copper plating chips consistent with material from the camshaft, valve guides, rocker arms, or connecting rod bolts; five aluminum alloy chips likely from pistons or accessory housing; and eight aluminum alloy chips likely from the crankcase.