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Aviation Accident

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NTSB Identification: ANC15LA015
Nonscheduled 14 CFR Part 135: Air Taxi & Commuter
Accident occurred Friday, March 13, 2015 in Anchorage, AK
Probable Cause Approval Date: 04/04/2016
Aircraft: AIRBUS AS350 B2, registration: N814EH
Injuries: 3 Uninjured.

NTSB investigators may not have traveled in support of this investigation and used data provided by various sources to prepare this aircraft accident report.

The commercial pilot was conducting a 14 Code of Federal Regulations Part 135 on-demand charter flight in the helicopter. The pilot reported that, while the helicopter was in cruise flight, about 1,000 ft above ground level, he felt a “clunk” in the tail rotor control pedals. Immediately thereafter, the helicopter began to yaw left. The pilot attempted to counteract the yaw by pressing the right tail rotor control pedal up to its forward stop, but the helicopter did not respond. The pilot declared an in-flight emergency with air traffic control, returned to the departure airport, and executed an emergency run-on landing.
An examination of the helicopter revealed that the tail rotor pitch change spider assembly had fractured into two pieces with rotational scarring present along the fractured surfaces; the inside of the spider assembly exhibited dark discoloration consistent with thermal damage. Half of the inner race of the ball bearing within the spider assembly was found loose on the tail rotor gearbox output shaft. Further examination revealed that the spider assembly failure was consistent with bearing seizure.
No evidence of grease was found on the bearing surfaces or the bearing housing. A review of maintenance records revealed that, about 13 months before the accident, the pitch change spider assembly was overhauled by a certified repair station, during which the original ball bearing was replaced. According to the helicopter manufacturer’s spider assembly overhaul procedures, grease was to be applied during the installation of the new bearing.
The spider assembly was installed on the helicopter by the operator’s maintenance personnel about 1 month later, at which time a mechanic signed off completing the helicopter manufacturer’s 600-flight-hour/24-month inspection checklist, which included regreasing the bearing. The bearing failed about 10 months and 141 flight hours after the spider assembly was installed on the helicopter.
Given that no grease was found on the fractured components and that the signatures were consistent with thermal damage due to a bearing seizure, it is likely that the overhaul facility did not follow the helicopter manufacturer’s overhaul procedures and failed to apply grease to the bearing. During the subsequent installation of the overhauled spider assembly, it is likely that the mechanic presumed that the newly overhauled spider assembly bearing contained grease and, therefore, did not complete the bearing regreasing procedure in accordance with the inspection checklist, which led to the lack of grease in the bearing going undetected.


The National Transportation Safety Board determines the probable cause(s) of this accident as follows:
  • The failure of the ball bearing within the pitch change spider assembly due to its operation with no grease within the bearing, which resulted in the subsequent fracture of the spider assembly and a loss of tail rotor control authority. Also causal to the accident were the overhaul facility’s failure to follow the helicopter manufacturer’s spider assembly overhaul procedures, which resulted in the assembly leaving the facility with no grease in the bearing, and the mechanic’s failure to complete all of the tasks on the 600-flight-hour/24-month inspection checklist, which led to the lack of grease in the bearing going undetected.