CEN24LA141

Maintenance personnel’s failure to properly torque the crankshaft idler gear hardware according to the engine manufacturer’s service instructions, which resulted in separation of the crankshaft idler gear’s fasteners and disengagement of the camshaft, crankshaft, and crankshaft idler gear and a subsequent total loss of engine power.

While enroute, at 9,500 ft above mean sea level (msl), the engine lost all power. After multiple attempts to restart the engine, the pilot executed a forced landing to a nearby airport. The airplane landed short of the runway threshold and its nose landing gear separated as the airplane transitioned onto the asphalt runway. The left wing and left canard sustained substantial damage. Postaccident examination of the engine revealed that the left crankshaft idler gear shaft separated from its attach points, which prevented the crankshaft idler gear from engaging its corresponding crankshaft and camshaft gears. The disengagement prevented camshaft rotation and valvetrain timing to the crankshaft. The left crankshaft idler gear’s attachment hardware were both separated into two sections from abrasion. The capscrew was lockwired to the idler shaft but no lockwire was observed through the stud and slotted shear nut. It is likely that maintenance personnel did not follow the engine manufacturer’s service instructions and undertorqued the fasteners during installation, resulting in insufficient clamping force. Additionally, the absence of lockwire likely permitted the shear nut to back off during engine operation. Because the airplane’s maintenance records were not available for review, the investigation was not able to identify when the crankshaft idler gear was last serviced.

On March 25, 2024, about 1853 central daylight time, a Cozy Mark IV airplane, N20MN, was substantially damaged when it was involved in an accident near Gonzales, Texas. The pilot was not injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight. The pilot reported that while enroute, about 9,500 ft msl, the airplane sustained a total loss of engine power. After multiple attempts to restart the engine, the pilot executed a forced landing to a nearby airport. The airplane landed short of the runway threshold and its nose landing gear separated as the airplane transitioned onto the asphalt runway. The left wing and left canard sustained substantial damage. Postaccident examination of the engine revealed there was no valvetrain continuity when the engine was rotated through at the propeller. Further examination revealed the left crankshaft idler gear was disengaged from the crankshaft and camshaft gears. The 5/16-inch stud, slotted shear nut, and capscrew fasteners that attached the crankshaft idler gear shaft to the crankcase were abraded and separated into two sections. The capscrew was lockwired to the idler shaft but no lockwire was observed through the stud and slotted shear nut and there were no separated sections of lockwire observed in the oil sump. Abrasion was noted to the heads of the cap screw and slotted shear nut. Additionally, circular abrasion observed to the back of the crankshaft idler gear was consistent with the fasteners contacting the idler gear during engine operation. The idler shaft recess in the crankcase exhibited deformation and abrasion. The idler gear shaft contained a bushing that was consistent with the Lycoming Engine’s Service Instruction No. 1197A, Repair of Idler Gear Shaft Recesses, dated March 29, 1974. The bushing was fractured in two sections and deformed. According to the Lycoming Engine’s Service Instruction No. 1310A, Lockwire for Idler Shafts, dated February 25, 1977, the required capscrew torque is 200-inch pounds and the slotted shear nut torque is 150-inch pounds. Additionally, it requires both fasteners to be lockwired. The pilot’s flight logs and airplane maintenance records were not made available during the course of the investigation. Additionally, the pilot did not submit the Pilot/Operator Aircraft Accident/Incident Report Form despite multiple requests.