Amateur-built Mark IV N20MN accident near Gonzales, Texas

Probable cause

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.

NTSB analysis

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.

Text from the NTSB. Open the NTSB docket for CEN24LA141 for the full record.