Accident Overview
On September 19, 2018, at 1504 central daylight time, a Cessna 206H airplane, registration N247F, struck power lines and terrain after the engine lost power while on landing approach to Sugar Land Municipal Airport/Hull Field (SGR), Sugar Land, Texas. The two pilots aboard were not injured; the observer sustained minor injuries. The airplane was registered to Silver Creek Aviation Services and operated by an entity of the United States Government as a public use instructional flight under 14 Code of Federal Regulations Part 91. Visual meteorological conditions prevailed, and no flight plan was filed. The local flight originated from Conroe-North Houston Regional Airport (CXO), Houston, Texas.
The commercial-rated pilot, under the oversight of a flight instructor, had flown in Houston Class B airspace for an hour and was flying the RNAV GPS instrument approach to runway 17 at SGR. After crossing the final approach fix CASOB and before reaching the missed approach point, the engine lost power. The instructor took control and maneuvered the airplane over a set of power lines. While attempting to maneuver under a second set of power lines, the right wing struck a line. The airplane touched down on a roadway, where its left wing struck a passing vehicle. The landing gear then struck a concrete median, yawing the airplane clockwise, and the tail struck a second vehicle. According to the operator, the airplane had about 30 to 32 gallons of fuel in each fuel tank.
Engine Examination
The airplane was transported to a secure facility in Alliance, Texas, for examination. The engine, a six-cylinder Lycoming IO-540, was installed new on February 2, 2005. It had been overhauled between November 4, 2013, and March 17, 2014, at which time it had accrued 1,983.9 hours. During the overhaul, all 12 hydraulic lifters were replaced, and the intake and exhaust rocker arms were reworked and reused.
Manual rotation of the propeller moved about a 200-degree arc before stopping. The engine was removed and disassembled. Borescope examination revealed failure of the No. 1 cylinder intake valve. Fractured pieces of the valve were drawn into the intake plenum. The No. 1 cylinder and piston head were extensively damaged; other piston heads had impact marks or embedded valve pieces. No metal was found in the oil pump, but blow-by was noted on piston skirts. Damage was observed to spark plugs 1 and 2.
Selected components were sent to Lycoming Engines for metallurgical examination. The report indicated that the No. 1 intake valve failed in fatigue initiating from the key recess fillet radius surface, with the fatigue crack radiating across approximately two-fifths of the cross section. Both intake and exhaust rocker arms had nonconforming gauge shoe heights after removal of about 0.01 inch of material from the shoe pads during overhaul. Nine of the twelve hydraulic lifters, including the No. 1 intake, failed leak-down testing. All other intake valve train components were undamaged. The report stated that the root cause of the fatigue failure could not be determined with certainty, but noted that the intake lifter and nonconforming rocker arm may have contributed to a malfunction in the No. 1 intake valve train. The valve stem tip separated from the stem at the keeper fillet radius; the stem was bent and the head fractured into pieces, with damage from post-fracture contact with the piston and combustion chamber.
Propeller and Seatbelt Examination
The propeller assembly was examined at McCauley Propellers in Wichita, Kansas, under FAA oversight. Both blades sustained impact damage consistent with low rotational energy absorption; no propeller failure or malfunction was found. The propeller governor was functionally tested and met manufacturing requirements.
The observer's seatbelt was found released despite being tightened prior to impact. Examination at AMSAFE revealed that the system housing may have taken crushing damage that created drag on the push button assembly, allowing partial connection only when lightly engaging. The connector engaged 100% in tests; abrasion on buckle covers indicated possible smashing or restriction.
Fuel Analysis
A fuel sample from the left wing was tested at Aviation Laboratories. The report indicated slightly elevated water content but stated that sample test results appeared normal.