ERA24LA239

A partial loss of engine power due to the failure of the No. 2 cylinder rocker shaft bosses from fatigue cracking.

The student pilot and flight instructor reported that the engine lost power shortly after takeoff. ADS-B data showed that the airplane reached a maximum altitude of 213 ft above ground level (agl) before entering a descending left turn back toward the runway. The airplane impacted the ground before reaching airport property. Examination of the wreckage found substantial damage to the engine mounts, fuselage, both wings, the right horizontal stabilizer, and elevator. Examination of the engine revealed that the rocker shaft bosses for the No. 2 cylinder had failed. Metallurgical examination found that a portion of the cylinder head, which contained the rocker arm shaft, fractured due to fatigue cracking that had initiated along the bore for the shaft. Once this section of the cylinder head fractured, the remainder of the cylinder head holding the rocker shaft fractured from overstress, freeing the shaft and rocker arms. These fractures would prevent the proper function of the drivetrain, preventing the valves from opening and leading to the failure of the No. 2 cylinder. The housing boss bore walls exhibited wear significant enough to allow for movement of the bushings and shaft. The bore exhibited wear marks consistent with rotational and lateral movement of the shaft, and the adjacent bushings exhibited lateral wear marks. Many of these marks matched those observed on the shaft, indicating movement along these surfaces. This uneven wear likely led to the rocker arms pushing erratically on the valves, as evidenced by the off-center wear marks on the rocker arm and valve stem faces. This uneven movement and wear likely led to wobbling and sliding. This created more wear, which eventually resulted in fatigue cracking and the subsequent failure of the rocker shaft bosses. A review of maintenance records was unable to determine the number of hours on the cylinder. The review found no record of the No. 2 cylinder being removed and reinstalled on the engine. While the logs described that about 7 years before the accident three unspecified cylinders had been removed and replaced, no information regarding which position they were installed to, or their serial numbers were recorded. The engine was subject to an FAA airworthiness directive (AD), which stated that the cylinder rocker shaft bosses must be inspected “At the next cylinder removal from the engine, or engine overhaul, whichever occurs first, after the effective date of this AD, inspect the cylinder rocker shaft bosses for cracks….” Based on the records, it could not be determined whether the No. 2 cylinder had been inspected in accordance with the AD.

On June 1, 2024, at 1416 Atlantic standard time, a Cessna 150M, N6198K, was substantially damaged when it was involved in an accident near San Juan, Puerto Rico. The student pilot and flight instructor sustained serious injuries. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 instructional flight. Review of ADS-B data showed that the airplane began the takeoff roll on runway 9 at Fernando Luis Ribas Dominicci Airport (TJIG), San Juan, Puerto Rico, at 1415:23. At 1415:57, about 1,000 ft before the departure end of the runway, the airplane began a climbing slight right turn. The airplane reached a maximum altitude of 213 ft agl, then began a left descending turn back toward runway 27, with the last ADS-B target recorded at 1416:23. The airplane impacted the ground and came to rest on the edge of a parking lot about 0.13 nautical miles before the runway 27 threshold. Postaccident photographs showed the airplane sustained substantial damage to the engine mounts, fuselage, both wings, the right horizontal stabilizer, and elevator. Fuel samples collected from the wing fuel tanks by a Federal Aviation Administration (FAA) inspector were absent of any visible contamination, and the fuel was consistent with 100LL aviation fuel.  An FAA inspector interviewed the student pilot and instructor after the accident. The student pilot reported that, shortly after takeoff, between 300 ft and 600 ft, the engine lost total power, and the instructor assumed control of the airplane. The student pilot had no recollection of events after the instructor took control of the airplane. The instructor reported that the engine lost partial power shortly after takeoff “around 400 ft,” at which time he took control of the airplane. He initially turned right about 30° followed by a left 270° turn in an attempt to return to the airport. He had no further recollection of the accident after the left turn. A postaccident examination and teardown of the engine was conducted by a mechanic under the supervision of an FAA inspector. This inspection found the No. 2 rocker shaft bosses were fractured and separated. The No. 2 cylinder, rocker shaft boss pieces, rocker shaft, and rocker arms were sent to the National Transportation Safety Board (NTSB) Materials Laboratory for further examination. No other anomalies were noted with the engine during the examination. Examination of the fractured parts found a portion of the cylinder housing that held the rocker arm shaft fractured from fatigue cracking that initiated along the bore with the shaft. Once this section of the housing fractured, the remainder of the cylinder housing also fractured, freeing the shaft and rocker arms. The bore exhibited wear marks consistent with rotational and lateral movement of the shaft. The rocker and valve stem faces exhibited comet-shaped, off-center witness marks consistent with uneven wear. The bushings, shaft, and housing bore exhibited wear signatures consistent with circumferential and lateral movement of these components. The fatigue cracking began at multiple initiation sites along the edge of the fracture surface with the shaft bore. While the housing contained solidification voids, and fatigue cracking had propagated through some of these internal voids, no features were observed that were consistent with initiation from or along the void surfaces. On February 18, 1994, the FAA issued airworthiness directive (AD) 94–05–05, applicable to Teledyne Continental Motors (TCM) Models C75, C85, C90, C125, C145, O–200, O–300, and GO–300 series reciprocating engines, to require inspection of the cylinder rocker shaft bosses for cracks, and inspection of the cylinder rocker shaft for looseness and replacement, if necessary, with a serviceable part. The AD stated that the action was prompted by reports of cracked or improperly repaired cylinder rocker shaft bosses, and that the condition, if not corrected, could result in engine power loss and engine failure. Review of aircraft maintenance records showed the AD had been complied with on November 21, 1999, at an aircraft total time of 6,788.5 hours. There were no accompanying maintenance entries in the engine logbook noting which cylinders were inspected at that time. The records also showed that 3 cylinders were replaced on December 4, 2017, at an aircraft total time of 7,937.6 hours, but did not specify which cylinders were replaced. At the time of the accident, the engine had a total time of 8,600.3 hours.