Incident Details
On February 20, 2015, about 0750 central standard time, an Agusta A119 Mk II helicopter, registration N802SM, experienced an abrupt, uncommanded left yaw during cruise flight over the Gulf of Mexico. The helicopter was operated by Era Helicopters LLC under 14 Code of Federal Regulations Part 135 as an on-demand air taxi flight. Visual meteorological conditions prevailed, and the flight operated on a company visual flight rules flight plan. It had departed the Houma-Terrebonne Airport (HUM), Houma, Louisiana, at 0720 and was en route to an off-shore platform.
Following the uncommanded yaw, the pilot returned to HUM and performed a run-on landing. A post-flight inspection revealed that one of the tail rotor pitch change links (PCL) had fractured. No further damage to other helicopter components occurred, but the fractured PCL adversely affected flight characteristics and required replacement, meeting the definition of substantial damage to the helicopter. The pilot and passenger were not injured.
Pitch Change Link Description and Maintenance History
The tail rotor pitch change link assembly comprises a single-piece cast aluminum body with two circular ends, each containing a spherical bearing. The end connected to the tail rotor blade contains the larger bearing (referred to as the large bearing), while the end connected to the tail rotor pitch change mechanism contains the smaller bearing (small bearing). Maintenance documentation indicated that the fractured PCL had been overhauled by Precision Heliparts (PHP) and issued an authorized release certificate on December 8, 2014. It was installed on N802SM on February 13, 2015. The pitch link failed at 14.8 flight hours since overhaul and 1,129.8 total flight hours.
Investigation and Examination
Representatives from the National Transportation Safety Board (NTSB), AgustaWestland, PHP, Era Helicopters, and the Federal Aviation Administration (FAA) convened at AgustaWestland facilities in Philadelphia, Pennsylvania to evaluate the bearing friction for the tail rotor PCLs. The method of inspecting PCL bearing rotational torque was found in the maintenance manual (MM) and overhaul manual (OM). The earlier version of the OM, dated November 25, 2012, differed from the more recent version dated November 10, 2014. The 2012 version required rotational torque inspection by rotating the inspection tool (a 100 mm lever arm) about the axis of the bearing bolt hole, while the 2014 version required rotating the tool away from the axis. This revision changed the bearing staking procedure. The bearing replacement procedures remained the same between the two versions. AgustaWestland identified the 2012 version as containing the correct method.
PHP confirmed that they had used the 2014 version of the bearing rotational torque inspection method. Postaccident examination of the fractured PCL used both the 2012 and 2014 procedures to measure rotational torque. Both the large and small bearings exceeded the maximum allowable bearing rotational torque when measured with either procedure. For the fractured PCL, the 2012 method consistently yielded a higher rotational torque value than the 2014 method.
The NTSB Material Laboratory examined the fractured PCL. The fracture surface exhibited evidence of fatigue that originated at a raised dash that was part of the part number marking on the PCL. Fatigue cracking was characterized across 60% to 70% of the fracture cross section. Details are contained in the Materials Laboratory Factual Report.
Airworthiness Directives and Corrective Actions
On March 4, 2015, the FAA issued emergency airworthiness directive (EAD) No. 2015-05-52 for owners and operators of certain A109-series and AW119-series helicopters. The directive required inspection of PCL part number 109-0130-05-117 for freedom of movement, corrosion, excessive friction of the spherical bearings, and cracks. This EAD was prompted by European Aviation Safety Agency (EASA) EAD No. 2015-0035-E issued on February 27, 2015. According to AgustaWestland and the FAA, no reports of tail rotor PCL spherical bearings exhibiting excessive friction had been received after inspections directed by these EADs.
Based on the investigation findings, AgustaWestland corrected the figure in the replacement procedures to display the correct method of inspecting spherical bearing rotational torque. Additionally, the PCL bearing replacement procedures were removed from the MM and are now found only within the OM.