What happened
On February 20, 2015, an Agusta AW11CO MKII helicopter, registration N802SM, was performing an on-demand air taxi flight from Houma-Terrebonne Airport (HUM) in Louisiana to an offshore platform. While in cruise flight over the Gulf of Mexico, the aircraft experienced an abrupt, uncommanded left yaw. The pilot was able to return to the departure airport and completed a run-on landing. There were no injuries to the pilot or passenger, but the helicopter sustained substantial damage.
The investigation
Post-flight inspections of N802SM revealed that a tail rotor pitch change link (PCL) was fractured. The PCL, a single-piece cast aluminum component, had been overhauled by Precision Heliparts (PHP) and installed on the aircraft on February 13, 2015, only 14.8 flight hours prior to the accident.
Investigators examined the overhaul manual (OM) revisions. A version of the OM dated November 10, 2014, contained a different method for inspecting bearing rotational torque compared to the 2012 version. The 2014 version changed the bearing staking procedure by requiring the inspection tool to be rotated away from the axis of the bearing bolt hole. The overhaul facility had used this 2014 method.
Testing of the fractured PCL showed that both the large and small bearings exceeded the maximum allowable rotational torque when measured using both the 2012 and 2014 procedures. Furthermore, the NTSB Material Laboratory found that the fracture surface showed evidence of fatigue originating at a raised dash part of the part number marking on the PCL. The fatigue cracking spanned 60% to 70% of the fracture cross-section.
Findings
- The PCL fractured due to undetected fatigue cracking.
- The fatigue cracking was caused by bending loads resulting from stiff bearings.
- The bearing stiffness was caused by the installation of the bearings in exceedance of the maximum allowable torque.
- The overhaul facility used an incorrect inspection method for bearing rotational torque due to a revision in the overhaul manual.