Anchorage, AK, USA
N814EH
AIRBUS AS350 B2
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.
On March 13, 2015, about 0715 Alaska daylight time, a turbine-powered Airbus Helicopters AS350 B2 helicopter, N814EH, sustained substantial damage as a result of a tail rotor pitch control failure while en route from Merrill Field, Anchorage, Alaska, to an off airport landing site near Beluga, Alaska. The flight was being operated as a visual flight rules (VFR) on-demand commercial flight under Title 14 Code of Federal Regulations (CFR) part 135. The certificated commercial pilot and two passengers sustained no injuries. Visual meteorological conditions prevailed for the flight. Company flight following procedures were in effect, and a company flight plan was filed and activated. During an interview with the National Transportation Safety Board (NTSB) investigator-in-charge, along with a Federal Aviation Administration (FAA) aviation safety inspector on March 13, 2015, the pilot stated that while en route, about 1,000 feet above ground level (agl), he felt a "clunk" in the tail rotor control pedals and the helicopter began to yaw to the left. When he attempted to counteract theyaw by depressing the right tail rotor control pedal, there was no reaction from the helicopter and the right pedal travelled to the forward stop. The pilot declared an in-flight emergency with air traffic control, returned to Merrill Field, and executed an emergency run-on landing. A post flight examination of the helicopter revealed the tail rotor pitch change spider assembly, partnumber 350A33-2030-00, had fractured into two pieces, with rotational scarring and smeared metal present along the fractured surfaces. The inside of the spider assembly contained 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. None of the roller elements of the spider ball bearing were recovered. Bands of circumferential scarring were observed on the tail rotor gearbox output shaft beginning about 3 inches outboard of the gearbox housing. The bands were about 3 inches wide. The tail rotor pitch change spider assembly was sent to the NTSB Materials Laboratory in Washington, DC, for further examination. During the examination, it was determined that the spider assembly failed in a manner consistent with bearing seizure. No evidence of grease was found on the bearing surfaces and the surfaces of the bearing housing. The affected pitch change spider assembly, serial number (S/N) M2057, was overhauled on April 16, 2014, by Able Engineering and Component Services, Inc., Mesa, Arizona. The original ball bearing was replaced with a new ball bearing, S/N NR7842; the lab examination observed "NR7842" stamped on the side of the ball bearing inner race. The Airbus Helicopters AS350 pitch change spider overhaul manual, No. 65.20.31, states in part: (f) Add grease until it comes out of the lip of seal. (g) Add grease in three steps, turning the sleeve through 180° between each step. (j) Final inspection. The overhaul manual for the pitch change spider assembly is located in the public docket for this accident. The pitch change spider assembly was installed on the accident helicopter on May 24, 2014. Following the installation, but prior to any operation of the helicopter, a 600-flight-hour/24-month inspection was signed off in the aircraft records signifying that it had been completed. The AS350 B2 600-flight hour/24-month inspection checklist from Airbus Helicopters, No. 05-22-00, contains an inspection item titled "TRH pitch change unit – greasable bearing." One of the documents referenced for this inspection is the Airbus Helicopters Aircraft Maintenance Manual, "Tail Gearbox – Inspection / Check", No. 65-21-00, Section 6-8. This procedure states in part: 1. Check manually and by touching that there are no rough running points: Rotate in both directions alternately while applying an axial load on stationary spider. Grease the bearing (65-21-00, 3-1). No. 65-21-00, Section 3-1 refers to the "Tail Gearbox – Servicing" section of the Airbus Helicopters Aircraft Maintenance Manual. The document states in part: 1. Remove the protective cap from the grease fitting. 2. Using a hand pump, inject grease CM101 through the grease fitting until new grease appears on the nut side (pump approximately 30 times). 3. During grease injection, turn the pitch change spider ¼ of a turn every 4 pump strokes. 4. Remove the excess grease by means of a clean cloth. 5. Install the protective cap. The notes for this inspection item instruct the person completing this inspection to check the bearing for friction points and greasing. After installation, the accident helicopter operated about 141 hours before the pitch change spider assembly fracture occurred. A second pitch change spider assembly (P/N: 350A33-2030-00, S/N:M2116) overhauled by Able Engineering and Component Services and held by the operator as a spare, was quarantined during this investigation. An examination of the second assembly performed by the operator revealed a rattling noise, consistent with bearing movement, during handling. When grease was applied, it took about 45 pumps of the grease gun before grease came out of the lip of the seal. The rattle and amount of grease required was consistent with the bearing being absent of grease. The closest weather reporting facility is Anchorage International Airport, Anchorage, about 25 miles from the accident site. At 0553, an aviation routine weather report (METAR) from the Anchorage Airport was reporting in part: wind from 020 degrees at 8 knots; sky condition, scattered clouds at 8000 feet; visibility, 10 statute miles; temperature 3 degrees F; dew point -13 degrees F; altimeter 29.48inHG.
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.
Source: NTSB Aviation Accident Database
Aviation Accidents App
In-Depth Access to Aviation Accident Reports
