Accident
The accident occurred on May 15, 2021, at approximately 1647 eastern daylight time, involving an experimental amateur-built Zodiac 601 XL, registration N493TG. The airplane was substantially damaged after the propeller separated from the engine during the initial climb following takeoff from Carroll County Airport (HZD) in McKenzie, Tennessee. The student pilot, who was the sole occupant, reported no injuries. The flight was conducted under Title 14 Code of Federal Regulations Part 91 as a personal flight.
According to the student pilot, before departure he performed an uneventful engine run. With calm wind, he chose to depart on runway 1 to avoid a back-taxi to runway 19. The takeoff and climb were normal until reaching about 300 feet above the runway surface, when the airplane experienced a momentary violent shudder and the propeller separated from the engine. At that moment, the airplane was directly over the runway. After the propeller detached, the aircraft yawed right, and the pilot, unable to realign with the runway, executed a forced landing onto a berm. The airplane then crossed the berm, dropped into a 3-to-4-foot depression, and struck a fence, resulting in substantial damage.
The propeller was recovered about halfway between the departure point and the landing site. One blade was broken in half, with the separated portion found approximately 100 feet away.
Maintenance History
The engine, manufactured in January 2004 (about 17 ½ years before the accident), had a previous propeller strike on May 19, 2012, at 672.6 hours of operation, which destroyed the propeller blades. The replacement propeller was installed by the airplane owner on September 3, 2016. This propeller had about 154 hours of service on another aircraft before installation. The owner, who was not the original builder, did not hold a mechanic certificate. The logbook entry for the installation lacked reference to the technical data used.
On October 16, 2020, maintenance logs recorded a “Jabiru Maintenance Manual prop strike test” and replacement of recommended bolts in the propeller flange and flywheel. The mechanic noted using “Brighton Best NF Socker Head Cap Screws 4001-0285, 4001-0345, 3/8 x ¾ UNF for prop flange” but changed to longer bolts (3/8 x 1-5/16 1-1/4 UNF) for the flywheel due to insufficient thread engagement. The entry also stated that Nord-Lock washers were installed per Jabiru Service Bulletin JSB 012-3. However, representatives of the engine manufacturer observed that using 3/8” x 1” bolts and Nord-Lock washers on the propeller flange could cause the threads to bottom out, preventing correct torque. Furthermore, the mechanic had referenced a flywheel installation bulletin (JSB 012-3) rather than the propeller installation bulletin (JSB 014-3).
Investigation
Postaccident examination of the cap screws securing the propeller flange to the crankshaft revealed fractures. The fracture features were consistent with fatigue cracking that initiated along thread roots where the nuts were threaded. The initiation occurred at multiple sites without material or mechanical defects, indicating the cap screws could flex or move, allowing fatigue crack propagation. No thread-locking adhesive, as required by the engine manufacturer, was visible on the threads. Energy-dispersive X-ray spectroscopy also detected no foreign elements consistent with adhesive.
The propeller blades' damage was consistent with impact without power; only one blade showed aft bending. The hub shell exhibited inward compressive impact damage from ground contact.