Zachary, LA, USA
N7148Z
Hughes 369D
While performing aerial side-trimming of trees and bushes for a pipeline right-of -way using an external saw at an altitude of about 200 feet, the 14,000-hour pilot experienced a left yaw and what he perceived to be an engine failure. The pilot released the external load as he attempted an autorotation from an out-of-ground effect hover at 200 feet agl. Crushing damage to the airframe was consistent with a ground impact at a high rate of descent. A detailed examinations of the engine confirmed the reported loss of engine power. The engine examination revealed the Rolls-Royce manufactured compressor adapter coupling, which had been installed approximately 414 hours earlier, had disconnected the compressor assembly from the turbine assembly as a result of fatigue cracking initiated by fretting on the pilot diameter.
HISTORY OF FLIGHT On December 9, 2004, approximately 1515 central standard time, a single-engine Hughes 369D turbine powered helicopter, N7148Z, sustained substantial damage when it impacted terrain following a loss of engine power while hovering out of ground effect near Zachary, Louisiana. The commercial pilot, sole occupant of the helicopter, was seriously injured. The helicopter was registered to and operated by Aerial Solutions, Inc., of Tabor City, North Carolina. Visual meteorological conditions prevailed and a flight plan was not filed for the 14 Code of Federal Regulations Part 133 rotorcraft external load operation flight. The local flight originated from a staging area located adjacent to the accident site approximately five minutes prior to the time of the accident. The operator reported in the Pilot/Operator Aircraft Accident Report (NTSB Form 6120.1/2) that the pilot had been performing aerial side-trimming of trees for a pipe line right-a-way using an external saw at an altitude of approximately 200 feet above ground level (agl). The pilot stated that while performing the trimming, the helicopter experienced a left yaw and what he perceived to be an engine failure. The pilot added that he then released the external load, as he attempted an autorotation. Subsequently, the helicopter impacted terrain in a flat wooded area, coming to rest in an upright position. The operator added that the pilot had refueled the helicopter with 40 gallons of Jet-A fuel prior to departure. The operator reported that Aerial Solutions utilizes a patented aerial power-saw attached to the helicopter by an 80-foot extension constructed from 3-inch aluminum tubing. This device is used by helicopters operating under an external load certificate, to trim vegetation from rights-of-way for several power companies throughout the United States. The aerial saw, which has a series of circular saws attached to a common shaft controlled by a transmission, provides side trimming/pruning of trees/brush from the ground upward. The unit weighs approximately 825 pounds, and is capable of extending up to 110 feet below the bottom of the helicopter. PERSONNEL INFORMATION The pilot holds a commercial pilot certificate with airplane single-engine land and helicopter ratings. The pilot was last issued a second-class Federal Aviation Administration medical certificate on February 9, 2004, with the limitation of "must possess glasses for near/intermediate vision." The operator reported that the pilot had accumulated over 14,000 hours of flight time, of which 830-hours was in the accident make/model helicopter. The pilot also held an aircraft mechanic certificate with airframe and powerplant ratings. AIRCRAFT INFORMATION The 1977-model Hughes 369D helicopter, serial number 570136D, was equipped with an external load hook and configured for a maximum of one occupant. The helicopter was powered by an Allison 250-C20B turboshaft engine, serial number CAE 270419, rated at 420 horsepower, driving a five-bladed main rotor system and a two-bladed tail rotor. At the time of the accident, the total airframe time on was 19,906.5 hours. The most recent 100-hour airframe inspection, including the following AD 100-hour inspections: 94-24-04 (12/31/94), T/R swashplate nut and tang inspection, 89-02-01 R1 C2 (07/19/90), visual inspection of main rotor (M/R) straps, and 88-17-09 R1 (12/26/89), tail rotor (T/R) attachment inspection retorque of nuts, were performed on December 5, 2004, with total time of 19,887.1 hours. The 100 and 300-hour inspections from November 2001 to the last 100-hour inspection in December 2004 were reviewed. No anomalies were noted, including deferred maintenance items or discrepancies in the airframe and engine logbooks. The engine was a rental unit from Standard Aero, of Winnipeg, Manitoba, Canada, and was installed on N7148Z on November 20, 2004. At the time of the installation, the engine had accumumulated a total of 4,986.6 hours and 6,128 cycles. The last 100-hour engine inspection was performed on December 5, 2004, at 5,023.0 hours, 6,164 cycles, which was 19.4 hours prior to the accident. Maintenance engine records from September 1996, including overhaul and repair that were supplied by Standard Aero, were also reviewed. No anomalies were noted. Compressor Adapter Coupling (CAC) P/N 23039791, S/N 72979, was installed new (0 hours and 0 cycles) in compressor assembly P/N 6890550, S/N 41685, on October 1, 2001. At the time of installation, the compressor assembly had accumulated a total time (TT) of 4,168 hours, time since overhaul (TSO) 982.0 hours, cycles since overhaul (CSO) 1,275, cycles since new (CSN) 5,239. The compressor assembly previously had another CAC (S/N 65514), which was installed on September 24, 1996, with TT of 3,186 hours. On November 1, 2001, the compressor assembly, S/N 41685, was installed in CAE 270419, with engine TT of 4,628.4 hours. According to the operator, at the time of the accident, the engine had a total time (TT) of 5,042.4 hours, and had been on the helicopter for a total of 58.8 hours. The new CAC (S/N 72979) had been on the engine for 414.0 hours. The remaining-time listing status of components by serial number and part number for airframe (S/N 570136D) and engine (CAE 270419) from Aerial Solutions were reviewed, and no anomalies were noted. The Airworthiness Directives (ADs) list for airframe and engine were reviewed, including Rolls-Royce commercial engine bulletin CEB A-1392, and no anomalies were noted. METEOROLOGICAL INFORMATION The closest weather reporting station to the accident site was located at the Baton Rouge Metropolitan Airport, Ryan Field (BTR), near Baton Rouge, Louisiana, approximately 11.8 nautical miles southwest of the accident site. At 1453, the automated surface observing system at BTR reported wind from 220 degrees at 7 knots, visibility 10 statute miles, sky clear, temperature 75 degrees Fahrenheit, dew point 50 degrees Fahrenheit, and an altimeter setting of 29.77 inches of Mercury. WRECKAGE AND IMPACT INFORMATION Examination of the helicopter by an Federal Aviation Administration (FAA) inspector, who responded to the accident site, revealed the tail boom was separated from the fuselage, and the fuel tank was breeched by the landing skids. The cargo hook was open and the mechanical release plastic tie was missing. The cargo hook circuit breaker on the electrical panel was out. The wreckage was recovered to Air Salvage of Dallas, near Lancaster, Texas, for further examination. TESTS AND RESEARCH On December 16, 2004, at the facilities of Air Salvage of Dallas, representatives from The Boeing Company, Rolls Royce, Standard Aero, and Aerial Solutions, examined the helicopter under the supervision of the NTSB investigator-in-charge (IIC). Examination of the fuselage revealed the forward left section of the helicopter fuselage was crushed inward. The seat pan was intact with no buckling, and the seat belt straps/buckles, and shoulder harness were not damaged. The left landing skid was separated and the right landing skid was removed to facilitate transport. All five main rotor blades were also removed for transport. The tail boom was separated 52 inches aft of it's attach point. The tail rotor remained attached to the tail rotor gearbox. No binding or ratcheting was noted within the tail rotor gearbox assembly when rotated by hand using the tail rotor draft shaft. No metal chips were observed within the upper and lower chip detectors. When rotated by hand, the main rotor transmission rotated freely. The main rotor hub assembly and swashplates also rotated freely. The lead/lag link bolts were free of damage. The lead/lag link for the yellow main rotor blade was damaged. The lead/lag dampers for the green, yellow, and red main rotor blades were damaged. The main rotor feathering bearing studs, feathering studs were free of damage. The droop stop plungers were free of damage. Damage was noted on the droop stop ring. The main rotor blade pitch control links for the green, white, blue, yellow, and red main rotor blades were free of damage. The main rotor blade pitch setting was observed at its maximum setting. Examination of the main rotor blades revealed all the blades were wrinkled throughout their entire length. All blades displayed trailing edge separation. Red paint was observed on the yellow main rotor blade. The damage to all main rotor blades was consistent with a low RPM setting at the time of impact. Flight control continuity was established throughout the airframe. Damage was observed on the cyclic interconnect torque tube and longitudinal trim motor. The collective jackshaft was damaged due to the crushing of the fuselage structure. The pilot's twist grip throttle controlled the linkage at the firewall when operated. The pilot's anti-torque pedals were separated from the airframe. The cyclic and collective remained attached to their respective mounts. Examination of the engine revealed no visible damage between the engine and main transmission. Continuity was not established between the compressor rotor and the N1 drive pads on the accessory gearbox. When turned by hand, the fourth stage turbine wheel rotated freely in both directions, and also rotated the transmission drive shaft. The linkage from the collective to the power turbine governor was severed at the bulkhead at the forward section of the engine compartment. With the linkage disconnected, the governor arm moved freely with full travel. The linkage from the throttle to the fuel control unit remained attached to the fuel control arm. Movement of the fuel control arm was observed in the upper end of the quadrant of the fuel control when the throttle was moved. The pneumatic lines between the compressor scroll, power turbine governor, and fuel control remained intact. The delivery and return oil lines between the accessory gearbox and compressor front support, gas producer turbine support, and power turbine were free of anomalies. As the governor linkage bellcrank support bracket was removed from the aft left side during engine removal, a portion of the compressor adapter coupling was found within the gearbox. On February 10, 2005, at the facilities of AeroMaritime, of Mesa, Arizona, the engine (S/N 270419) was examined by representatives from The Boeing Company, Rolls Royce, Standard Aero, and Aerial Solutions, under the supervision of the NTSB IIC. The compressor was removed from the engine as a module, and broken pieces of the spur adapter gearshaft were found with parts of the compressor adapter coupling in the impeller stub-shaft. Most of the Teflon seal was recovered from inside of the gearbox,, and the o-ring was found in the compressor to turbine coupling shaft. The compressor adapter coupling revealed wear at the stub-shaft ends, and the break ran through the internal splines. The following parts were sealed and sent to the NTSB materials laboratory in Washington, D.C. for further examination: compressor adapter coupling (CAC) P/N 23030791-1E, S/N 72979, spur adapter gearshaft (SAG) P/N 23031921-D, S/N CG118107, compressor impeller P/N 23058143, S/N 25346, turbine to compressor coupling P/N 6898977-K, S/N AE500404, number 2 bearing, P/N SOCN6889093, S/N HA-N352, compressor mount pad spacing shims and gearbox magnetic plug. On March 3, 2005, at the NTSB's Material Laboratory in Washington, D.C., an examination of the components listed above were examined by representatives from Rolls Royce, Standard Aero, and Aerial Solutions, with the NTSB IIC. The CAC was fractured into two pieces, and an optical and scanning electron microscope (SEM) examination revealed that the fracture initiated from the outer diameter surface of the pilot. The SAG was found with a spiral crack extending from the forward end of the spline consistent with fatigue propagation. A comparison SAG and CAC removed from another fleet helicopter were supplied by Aerial Solutions. Visual examination of the SAG did not reveal any damage. However, the CAC was shown to have significant wear/fretting damage, especially in the aft area on the pilot diameter. The Materials Laboratory Factual Report number 05-026 from this examination on December 9, 2004, will be submitted as supporting documentation. ADDITIONAL INFORMATION An "Alert Commercial Engine Bulletin" (CEB A-1392) by Rolls-Royce for the compressor adapter coupling (CAC) on M250-C20 engines was issued on September 9, 2003. Revision 1 was issued on September 9, 2004. Among other items, new part numbers for the CAC featured a silver plated pilot diameter to provide better lubrication between the surfaces and alleviate fretting that has been experienced, which could lead to fracture of the part and /or uncommanded power loss. Compliance called for exchanging the CAC the next time the component was at an approved repair/overhaul facility and the compressor rotor was disassembled for any reason. Airworthiness Directive (AD) 2004-26-09 was issued by the FAA and became effective on February 8, 2005. Among other items, the AD calls for the removal of the "baseline" adapter (P/N 23039791) from service the next time the compressor rotor is disassembled, and exchanged with a "phase 1" adapter (P/N 23076559). For purposes of this AD, next access is defined as when the compressor module is separated from the engine and disassembled for any reason, but not later than March 1, 2012. The FAA stated that the AD resulted from nine reports of engine shutdown caused by CAC failure, and was issued to reduce the risk of failure of the CAC and subsequent loss of all engine power. The fuselage wreckage was released to the owner's representative on December 9, 2004. The engine and compressor adapter coupling/internal components were released on June 30, 2005.
The loss of engine power as a result of fatigue cracking of the compressor adaptor coupling initiated by fretting on the pilot diameter and subsequent disconnection of the compressor from the turbine assembly. A contributing factor was the low operating altitude of the helicopter, which made a successful autorotation by the pilot imposible.
Source: NTSB Aviation Accident Database
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