McKenzie, TN, USA
N493TG
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).
On May 15, 2021, about 1647 eastern daylight time, an experimental amateur-built Zodiac 601 XL airplane, N493TG, was substantially damaged when it was involved in an accident in McKenzie, Tennessee. The student pilot was not injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight. According to the student pilot, he departed from Carroll County Airport (HZD), McKenzie, Tennessee for a solo local flight. Before takeoff he conducted an uneventful engine run, and since the wind was calm, decided to take off on runway 1 to avoid a back-taxi to runway 19. The takeoff and start of the climb were normal. About 300 ft above the runway surface, the airplane momentarily shook violently and he noticed that the propeller had separated from the engine. The airplane was directly over the runway when the propeller separated from the engine. After the propeller came off, the airplane turned right and the pilot did not have time to get the airplane back over the runway, so he landed on a berm. The airplane then traveled off the other side 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. One blade was broken in half. The separated portion of the blade was discovered about 100 ft away. The engine was manufactured about 17 ½ years before the accident, in January, 2004. Logbook entries indicated that on May 19, 2012, at 672.6 hours of operation, the airplane incurred a propeller strike which was severe enough to destroy the blades of the propeller. A replacement propeller was installed by the airplane owner on September 3, 2016. The propeller was originally on another airplane and had about 154 hours in service when it was installed on the accident airplane. The airplane owner did not hold a mechanic certificate, nor did he have a certificate for the airplane, as he was not the original builder. The logbook entry did not indicate what technical data the airplane owner used for the installation. The maintenance logs also indicated that on October 16, 2020, A “Jabiru Maintenance Manual prop strike test was performed and recommended bolts” were replaced in the propeller flange and flywheel. The mechanic that performed the maintenance noted in the entry that “Brighton Best NF Socker Head Cap Screws 4001-0285, 4001-0345, 3/8 x ¾ UNF for prop flange, factory recommended ¾ length did not engage enough so changed to 3/8 x1. 5/16 1-1/4 UNF for flywheel.” The entry went on to say: “Changed to Nord-Lock washers for the prop flange and flywheel per Jabiru Service Bulletin JSB 012-3. Nord-Lock 1526 3/8” and Nord-Lock 1523 5/16.” Representatives of the engine manufacturer reviewed these maintenance log entries an noted that installation of 3/8” x 1” bolts and Nordloc washers on the propeller flange could result in the threads bottoming out and the bolts not being able to be torqued correctly. The maintenance log entry also indicated that during the installation, the mechanic used guidance for a flywheel installation (Jabiru Service Bulletin JSB 012-3) and not the guidance for a propeller installation (Jabiru Service Bulletin JSB 014-3). Postaccident examination of the cap screws that were installed to hold the propeller flange on to the engine crankshaft revealed that they had fractured. No evidence of thread locking adhesive, as required by the engine manufacturer during installation, was visible on the threads. Detailed examination of the cap screws revealed that they displayed fracture features consistent with 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 suggested that the cap screws were able to flex or move, allowing fatigue crack propagation to occur. The fracture damage to the propeller blades was consistent with impact without driven power—only one of the blades had fractured in a manner consistent with aft bending. Further, the propeller hub shell damage was consistent with impact with the ground, exhibiting features consistent with inward compressive impact. Further examination by the NTSB Materials Laboratory to confirm whether a thread locking adhesive had been applied to the cap screw threads revealed that not only were there no visual indications consistent with application to the thread screws, nor were any fluorescing substances observed within the threads. Examination using energy-dispersive X-ray spectroscopy also did not find any elements dissimilar to alloy steel along the threads.
The improper installation and inspection of the propeller, which resulted in an in-flight separation of the propeller assembly.
Source: NTSB Aviation Accident Database
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