Columbus, MT, USA
N765HV
Hughes 369D
During a stable hover about 120 feet above ground level for the purpose of visually inspecting a power line support structure, the helicopter had a complete loss of engine power. The pilot's maneuvering options were limited by the support structure, and he attempted an autorotation to a forced landing. During that sequence of events the helicopter collided with terrain. The investigation determined that the engine's fuel nozzle was supplying a nonuniform fuel spray, leading to a localized over-temperature in the turbine section, which lead to turbine blade rub, the eventual fatigue separation of a second stage turbine blade, and the loss of engine power.
On May 28, 2007, approximately 1340 mountain daylight time, a Hughes 369D helicopter, N765HV, impacted the terrain during an attempted forced landing, after experiencing a complete loss of engine power near Columbus, Montana. The commercial pilot and one crewmember received serious injuries, and a second crewmember was killed. The helicopter, which was owned by Dylan Aviation, LLC., and operated by Haverfield Corporation, was destroyed by the impact and post-crash fire. The 14 CFR Part 91 aerial observation flight, which departed Billings Logan International Airport, Billings, Montana, about 50 minutes prior to the accident, was being operated in visual meteorological conditions. The flight, for which a company visual flight rules (VFR) flight plan had been filed and activated, was intended to terminate back at Billings. The ELT was activated by the accident sequence. According to the pilot, the helicopter, which was being used for comprehensive visual inspection of power lines and power line support structures, was in a stationary hover about 120 above ground level (AGL), when it experienced a sudden and complete loss of engine power. The pilot, who's direction of maneuver was limited by the presence of the power line support structure that was being inspected, initiated an autorotation to an emergency forced landing. Although the helicopter contacted the relatively level grassy ground in a upright attitude, the impact forces where high enough that both front seat crewmembers sustained serious injuries, and the helicopter's airframe was distorted. The two front seat crewmembers were able to exit the helicopter through the broken out front windscreen. The back seat crewmember had sustained "significant blunt traumatic injuries," and according to the opinion of the medical doctor who performed the autopsy, he expired prior to the initiation of the fire that started a few minutes after the impact. The majority of the helicopter's airframe, except for about the last five feet of the tail boom, and most of the engine accessory systems were consumed by the fire. Both the engine and airframe underwent initial inspections after being recovered to Arlin's Aircraft Service in Bozeman, Montana. After those inspections, the engine was shipped to Rolls-Royce Corporation, where under the oversight and guidance of the Federal Aviation Administration's Indianapolis Flight Standards District Office, it underwent a further teardown inspection, testing, and metallurgical examination. These further investigative actions determined that a second stage turbine blade had separated from its turbine wheel due to high cycle fatigue (HCF), which initiated at the suction (forward) side blade-to-rim fillet. There were no metallurgical anomalies found at the fatigue crack initiation site, and microstructure, hardness, and chemistry of the turbine wheel conformed to the requirements of the engineering drawings. Further inspection of the second stage turbine wheel found evidence of blade tip rub, including on the blades immediately preceding and following the failed blade. Fluorescent penetrant inspection (FPI) and binocular examination revealed cracking on the suction side blade-to-rim fillets of many of the remaining blades. Dimensional inspection of the seconds stage turbine nozzle assembly determined that both the first and second stage turbine blade tracks were circumferentially distorted (out of round), and inspection of the power turbine-3 and power turbine-4 (P/T-3 and P/T-4) determined that the wheel shroud knife seals were worn non-uniformly around the circumference of the turbine wheel shrouds. Inspection of the first stage turbine nozzle assembly revealed that four adjacent airfoils near the 11:00 o'clock position (aft looking forward view) showed extensive thermal damage, to include incipient melting near the surface, and complete mid-span burn-through along the entire length of one of the airfoils. The second stage turbine nozzle showed thermal damage, although to a lesser degree, near the same position. The combustion liner showed a non-uniform discoloration indicative of a non-uniform burner profile. Although the aforementioned thermal damage is consistent with a streaking fuel nozzle, the fuel nozzle itself suffered heat distress due to the post-crash fire, and was therefore tested on the flow bench, in accordance with the 250-C20B overhaul manual, in its as-found condition. During the test sequence, streaking was observed, and the fuel flow (in pounds per hour) was lower than the minimum specified in three out of the four defined nozzle pressure differential test points. After the as-found flow test, the nozzle was disassembled, and no evidence of foreign contamination was observed. During the investigation, a review of the operator's engine maintenance records revealed that during only two of the last seven 100-hour inspections (a period of about 810 engine hours) was there documentation showing that the fuel nozzle had been removed, inspected, and cleaned, as called for in the June 1, 2004, revision of the Rolls-Royce 250-C20 Series Operation and Maintenance Manual 72-00-00. According to the records, at the time of the accident, it had been about 236 hours since the nozzle inspection/cleaning requirements had been met. During the other five 100-hour inspections, the checklist used did not contain the current June 1, 2004, revision page (page 609) where item 21A (Remove, inspect and clean the fuel nozzle) had been added. The investigation also revealed that item 21A in the June 1, 2004 revision contained a note that stated, "Operators may find it necessary to inspect and clean the fuel nozzle more often depending on past experience or operating conditions."
The fatigue failure of a turbine blade due to a localized over-temperature of the turbine section as result of a non-uniform fuel distribution by the fuel nozzle. Contributing to the accident was the close proximity of a power line support structure which restricted maneuverability for a successful autorotation.
Source: NTSB Aviation Accident Database
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