WPR24LA194 · Cessna 150M

The pilot reported that, while about 5,500 ft, the engine made a loud bang and then lost all power. Attempts to restart the engine were unsuccessful, and the pilot initiated an off-airport landing after determining the airplane could not reach a nearby private airport. During the landing sequence, the airplane struck trees, which resulted in substantial damage to the airplane’s wings. Postaccident examination of the engine revealed that the No. 2 cylinder intake valve head had separated and was embedded into the No. 2 piston head. The valve head separated from its stem near the radius at the head, and its stem was found in the carburetor throat. The intake and exhaust valves showed evidence of excess heating and valve sticking due to inadequate lubrication, worn valve guides, and poor valve seating. The excess heat was sufficient to produce coked oil deposits at the tip ends of all four exhaust valves and dark stains around the tip area on the No. 2 intake valve. Worn valve guides can be expected for high time engines. However, at approximately 850 hours time in service (TIS), this engine had accumulated less than half the service time between overhauls. An intake rocker arm was incorrectly installed in the exhaust position, which reduced the flow of cooling oil to the exhaust valve. The reduced oil flow to the exhaust valve tip would have led to excessive heating of the exhaust valve with increased valve guide wear and valve stem corrosion, as observed. The condition of the exhaust valves for the remaining cylinders suggested that the valve guides of the remaining cylinders were similarly worn, and the heat signatures on those exhaust valves suggested the intake rocker arms may have also been incorrectly installed in the exhaust positions on the other cylinders as well. Interrupted oil flow to the exhaust valve due to incorrectly installed rocker arm bushings has led to premature exhaust valve failure in previous accidents. Although inadequate oil flow to the exhaust valve would be expected to result in exhaust valve failure, in this case the intake valve failed. The overall condition of the valve seat contact areas and intake valve condition suggest other factors may have contributed to the failure. Although the diametral valve-to-guide clearance of the No. 2 intake valve was within specification at the outboard end and middle of the guide, the area of the guide most susceptible to wear could not be measured due to deformation at the inboard end that occurred after the intake valve dropped. On the intake valve stems from the other three cylinders, circumferential gouges and linear streaks indicated axial misalignments likely existed between the valve stems and guides, consistent with worn or misaligned intake valve guides. Evidence of valve guide wear, axial misalignments, heat signatures, and valve seat leakage on the intake valves indicated that valve seat contact was inadequate. Proper valve seat contact is critical to valve cooling. At manufacture or overhaul, valve seats must be precisely ground to ensure proper contact with the valve faces. The center of the ground valve seat face must align with the valve guide axis, and a correct angle must be achieved to ensure proper contact and sealing with the valve face. If valve guides were not properly aligned to the valve seats during manufacture, the resulting misalignment could cause leakage across the valve seat and excessive wear on the valve guides. Grinding to an incorrect angle can lead to valve leakage or overheating. The extent of combustion deposits and evidence of leakage across the intake and exhaust valve faces suggest that the valve seats likely were not ground to the appropriate angle to properly seal against the valve faces. Poor valve seat contact likely led to excessive heat along the valve stems and increased valve guide wear, compounding the issues with lubrication and valve stem cooling in the rocker box. Thick combustion deposits accumulated on the stem side of the intake valve heads due to leaking seats, and the valve guides became worn, leading to misalignment and valve sticking. The combination of overheated oil in the rocker area and valve sticking likely resulted in excessive wear between the valve tip faces and the corresponding pads on the rocker arms. Fretting damage and excessive wear also occurred on the valve keepers and the mating shoulders on the stem. As a result, the locking tabs on the inside diameter of the No. 2 intake valve keepers were completely worn away. The No. 2 intake valve subsequently dropped into the combustion chamber, where it was likely struck by the piston, resulting in the overstress fracture of the stem adjacent to the head. The reported engine failure likely occurred when the No. 2 intake valve dropped.

Verbatim NTSB analysis and probable cause from the NTSB dataset