Accident Summary
On June 11, 2024, at approximately 1250 mountain standard time, a Cessna 150M airplane, registration N3156V, was substantially damaged during an off-airport landing near Roll, Arizona. The pilot, who was the sole occupant, reported no injuries. The flight was conducted under Title 14 Code of Federal Regulations Part 91 as a personal flight.
The pilot stated that she departed Yuma International Airport (NYL), Yuma, Arizona, with full fuel. While cruising at about 5,500 feet, the engine emitted a loud bang followed by a complete loss of power. Attempts to restart the engine were unsuccessful. She identified a nearby private airport but was parallel to the runway. After turning south and then back toward the runway, she realized she would not reach it and executed an off-airport landing in a field. During the landing, the aircraft struck trees, causing substantial damage to both wings.
Engine Examination
Postaccident examination revealed that the engine could only be partially rotated. The cylinders and pistons were removed from the crankcase. Internal crankcase surfaces and components were undamaged and coated with light oil, except for the No. 2 cylinder intake valve head, which was embedded in the No. 2 piston head. The intake valve head had separated from its stem near the head-to-stem radius, and the stem was found in the carburetor throat. The separation surfaces were jagged. The No. 2 piston was cracked in half. The No. 2 exhaust valve exhibited pitting corrosion about 1 inch from the head. The No. 2 cylinder assembly and valves from all cylinders were sent to the NTSB Materials Laboratory for further examination.
Maintenance History
The engine had accumulated 3,182.5 hours since new and 846.7 hours since overhaul. The engine was overhauled in February 2017, with new Superior (part number SA10200-A20P) cylinders installed. Over 75 percent of the engine time since overhaul was accrued after February 2022.
Valve and Component Findings
Further examination showed the No. 2 intake valve fractured at the neck, with the stem bent. The piston had impact marks consistent with contact from the valve head and stem, and cracks originated from the impact area. Piston rings were fractured near the crack. Intake valve impact damage was noted on the cylinder head internal surfaces and the inboard end of the valve. The No. 2 pushrods were straight.
The cylinder head external surfaces had a uniform yellow-gray color with no high-temperature tinting. The rocker box contained black flaky coked oil deposits, especially around the exhaust valve guide and springs. Both rocker arms had oil channels with drive screws at the pushrod end but lacked oil orifices for valve stem lubrication at the rocker pad end. According to the Continental Aerospace Technologies Standard Practice Maintenance Manual (SPMM) section on rocker arm identification, the exhaust rocker arm (part number 639615) includes an oil channel with drive screw and orifice at the pad end, while the intake rocker arm (part number 639614) does not. The SPMM notes that installing rocker arms in incorrect positions may cause oil starvation or shorten cylinder life.
The No. 2 intake valve keeper half was loosely held within the valve spring seat and could not be separated by hand. Both keeper halves had severely worn inside diameters and missing locking tabs. Outside diameter surfaces were worn, with corresponding wear on the valve spring seat inside diameter. The inboard edges of the keeper halves were deformed outboard, trapping one keeper half.
The No. 2 intake valve stem had a dark tint extending up to 2.5 inches from the head, with substantial pitting between 1.5 and 2.2 inches. The valve seat contact area was predominantly black and glazed, with uneven circumference contact and isolated dark gray deposits with radial streaks.
Intake valves from cylinders 1, 3, and 4 showed smooth, worn tip end faces with circular wear patterns and isolated pits. Remnants of original linear grinding marks were observed on Nos. 1 and 3. Each had circumferential gouges and linear scrape marks near the tip end. Thick black deposits were on the stem side of the head, and flattened deposits on seat contact surfaces.
Exhaust valves from cylinders 1, 3, and 4 had smooth, worn tip end faces with circular wear patterns and isolated pits. Coked oil was present between the stem shoulder and tip end face. The keeper lock contact areas were slightly worn. Dark tinting extended up to 2 inches on the stems, with pitted areas. The seat contact faces were black and glazed, with isolated dark gray deposits and radial streaks.
The No. 2 intake valve inside and outside diameters were within specified range, and the stem-to-guide clearance was less than the service maximum.