CEN24LA121

The stuck No. 2 cylinder exhaust valve, which resulted in a partial loss of engine power and a forced landing due to an undersized exhaust valve guide. Contributing to the accident were the improper manufacture of the engine cylinders, and the operation of the engine in an overly rich fuel condition.

During an instructional flight, the airplane had just taken off when the engine lost partial power. The flight instructor took control of the airplane and conducted a forced landing on the remaining runway. During the forced landing, the airplane departed the runway surface and struck a fence and trees, which resulted in substantial damage to both wings. Engine monitoring data showed that during the takeoff, engine power increased to a maximum of 2,505 rpm. About five seconds after rotation, engine power decreased to 1,446 rpm. Postaccident examination of the engine revealed a partially open, stuck exhaust valve on the No. 2 cylinder. The valve stem had a metallic debris smeared at mid span of the stem, and the valve guide had numerous metallic deposits present. The metallic debris and deposits contained lead, carbon, and bromine, consistent with combustion products, and the valve guide was below the manufacturer-specified minimum guide-to-stem clearance from the interior through mid-span of the guide. Relatively thick deposits were also noted on the piston crown and exhaust port. Based on the composition of the deposits, the No. 2 cylinder exhaust valve likely became stuck due to cumulative engine operations at an overly rich fuel condition, leading to incomplete fuel burn and an accumulation of combustion deposits on the valve stem and guide. The presence of the deposits suggests that the engine was operated at relatively low cylinder head temperatures (CHT). This is consistent with the accident flight data showing that the No. 2 cylinder temperature peaked at 336°F. Additionally, a review of 109 previous flights totaling about 165 hours showed a maximum No. 2 cylinder temperature of 364°F and a median temperature of 293°F. According to the engine manufacturer’s guidance, continued operations at much lower temperatures could be detrimental to engine operation and likely contributed to the build up of combustion products on the valve. The valve guides on all four cylinders were found to be below the minimum inner diameter specification. The cylinders were factory-new when installed on the engine at overhaul by the engine manufacturer on January 11, 2023. Based on the measurements, the exhaust valve guides were not correctly sized during manufacture of the cylinders. It is likely that the undersized valve guide contributed to the valve sticking with the combustion deposits. Given the CHTs values recorded in the accident flight and accumulation of combustion deposits observed in the examined cylinder, it is likely that combustion deposits accumulated in the valve guide and led to valve sticking in addition to the undersizing of the valve guide at factory overhaul.

On February 25, 2024, about 1610 central standard time, a Piper PA-28-181 airplane, N38696, was substantially damaged when it was involved in an accident near Watertown, South Dakota. The flight instructor and student pilot were uninjured. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 instructional flight. The flight instructor stated that while the student pilot was performing a touch and go, just after the airplane had taken off, the engine lost partial power. The flight instructor took control of the airplane and conducted a forced landing on the remaining runway. Engine monitoring data showed that during the takeoff, engine power increased from 851 rpm to a maximum of 2,505 rpm. About five seconds after rotation, engine power decreased to 1,446 rpm. During the forced landing, the airplane departed the runway surface and struck a fence and trees, which resulted in substantial damage to both wings. Postaccident examination of the engine revealed a partially open, stuck exhaust valve on the No. 2 cylinder. Removal of the valve from the cylinder assembly required the use of penetrating oil and a mallet. The neck area and adjacent surfaces on the valve head were covered with reddish-brown deposits. Gray-colored, raised deposits were observed on the side of the valve stem. The piston crown was also covered in similar deposits. Energy-dispersive x-ray spectroscopy of the No. 2 cylinder intake valve deposits by the NTSB Materials Laboratory showed evidence of relatively high levels of lead, carbon, and bromine. A sample from the No. 2 cylinder exhaust valve deposits showed evidence of high lead content with relatively smaller levels of bromine and carbon. With the deposits present, the clearance between the No. 2 cylinder exhaust valve guide and valve stem was between 0.0007 and 0.0009 inches at the middle of the valve guide and between 0.0011 and 0.0029 inches at the inboard (piston side) end of the guide; the engine manufacturer, in Lycoming Service Table of Limits and Torque Value Recommendations (SSP-1176-5), specified that the exhaust valve stem-to-guide clearance should be between 0.0040 and 0.0060 inches. After the exhaust valve guides of all 4 cylinders were cleaned of deposits, the inside diameter of the No. 2 cylinder exhaust valve guide was measured to be 0.4975 inches at the outboard (rocker side) end, 0.4977 inches at middle of the guide, and 0.4983 inches at the inboard (piston side) end, which were all less than the minimum of 0.4985 inches specified by the manufacturer in SSP-1176-5. In addition, the inside diameters of the exhaust valve guides of the other 3 cylinders were each less than the manufacturer-specified minimum in at least 2 locations. The cylinders were factory-new when installed on the engine at overhaul by the engine manufacturer on January 11, 2022. Maintenance records indicated that since being installed, no maintenance work had been performed on the cylinders or valves. As of February 23, 2024, the engine had 646.6 hours since overhaul. Review of recorded engine monitoring data for the accident flight showed that the No 2. cylinder’s peak temperature was 336°F. Review of 109 previous flights totaling about 165 hours showed a maximum No. 2 cylinder temperature of 364°F and a median temperature of 293°F. The engine manufacturer recommends CHT values during normal operations for normally-aspirated engines should be 400°F or less. Additionally, Lycoming Valve Sticking – Early Warning Signs and Indications (SSP-525) states that the engine should be properly leaned at cruise power settings to promote complete burning of the fuel, and that long periods of ground operation should be avoided; an oil-lead sludge will form without proper leaning and during long periods below normal operating temperature.