What happened
On December 4, 2015, a privately owned Maule Air M-7-235C, registration JA30HT, was involved in a serious incident at Otone Temporary Airfield in Ibaraki Prefecture, Japan. After completing an airworthiness inspection flight, the aircraft landed at approximately 11:30. While taxiing toward the apron at a speed of roughly five knots, the pilot noticed an unusual sensation and brought the aircraft to a stop.
Upon inspection, the pilot discovered that the tailwheel had detached from the airframe. The investigation revealed that the bolt responsible for connecting the tailwheel bracket assembly to the tail spring had fractured at the bolt head, causing the assembly to fall away.
The investigation
Investigators examined the physical condition of the hardware and the maintenance history of the aircraft. The inspection of the failed bolt showed a fracture at the neck, just below the bolt head. Additionally, a 0.6 mm dent was found around the bolt hole on the bracket assembly, and the washer on the bolt head side was noticeably curved and thinned due to repeated crimping.
Technical analysis by the National Institute for Materials Science confirmed that the bolt failure was a fatigue fracture. This was driven by the loosening of the bolt, which allowed for increased vibration and impact loads during landing and taxiing. The investigation also noted that the maintenance company had previously identified the dented bracket and deformed washer during inspections in 2014 and 2015 but had only replaced the bolt, opting to continue using the existing washers and nut.
Furthermore, the investigation found that the maintenance company lacked access to the specific maintenance manuals, parts catalogues, or drawings for the tailwheel assembly, as the original manufacturer no longer existed. The parts in use—specifically a bolt with a larger diameter and longer grip length than standard—did not align with the recommended maintenance criteria for aircraft repair.
Findings
- The tailwheel detached because the bolt connecting the bracket assembly to the tail spring suffered a fatigue fracture.
- The fracture originated from the loosening of the bolt, which led to excessive stress concentrations on the bolt neck.
- The loosening was likely exacerbated by insufficient tightening torque and the presence of a pre-existing dent in the bracket assembly, which caused the washer to deform and lose thickness over time.
- Improper maintenance practices contributed to the incident, as the maintenance provider failed to replace the worn washers and nut despite observing visible signs of deformation and wear during previous inspections.