ERA21LA223 · Zenith Zodiac 601 XL

The student pilot reported the initial takeoff was normal. About 300 ft above the runway, the airplane shook violently and the propeller separated from the engine. The airplane banked to the right and the student pilot lowered the nose of the airplane and landed on a grass berm. The airplane traveled off of the berm, dropped into a 3-to-4-ft depression, struck a fence, and was substantially damaged. The propeller was found about halfway between the takeoff point and where the airplane touched down on the berm. About 9 years earlier, the airplane had incurred a propeller strike that was severe enough to destroy the blades of the propeller. A replacement propeller was installed by the owner of the airplane. The airplane owner, who was not the original builder, did not hold either a mechanic certificate or repairman certificate for the airplane. About 7 months before the accident, a propeller strike test was performed, and the cap screws were replaced in the propeller flange and flywheel. The mechanic who performed the work noted that the manufacturer’s recommended bolts did not fit, so he installed cap screws that were longer. The bolts that were installed could result in the threads bottoming out and the bolts not being able to be torqued correctly. Postaccident examination of the cap screws indicated they fractured from fatigue cracking that initiated along the thread roots where the nuts were threaded. This fatigue initiated at multiple sites along the thread roots, where there were no indications of material or mechanical defects. This also suggested that the cap screws were able to flex or move, allowing fatigue crack propagation. Once the fatigue crack had progressed about halfway through a cap screw cross section, the remainder of the material would fracture from overstress (likely in bending). Without the cap screws to affix the propeller assembly to the engine, the hub would be able to separate. Examination also revealed that no thread locking adhesive was present on the cap screw threads as required by the manufacturer. The lack of a thread locking adhesive alone did not necessarily account for the lack of torque or clamping force on the cap screws. The fatigue pattern suggested a lack of preload, which may have developed during service as the nut backed off slightly or an indication of over-torquing at installation. If properly installed, the application of a thread locking adhesive to the cap screws would have helped keep the parts in place, reducing the chance of this type of fatigue cracking occurring (assuming the bolts had been torqued to an acceptable preload).

Verbatim NTSB analysis and probable cause from the NTSB dataset