ANNETTE BAY, AK, USA
N21424
Bell B-3
THE HELICOPTER HAD BEEN INVOLVED IN A PREVIOUS TAIL ROTOR STRIKE AND THE DRIVESHAFT WAS INSPECTED. THE ADAPTER ON TAIL ROTOR DRIVE SHAFT NUMBER 4 DEBONDED AND ROTATED ON THE SHAFT. METALLURGICAL EXAMINATION SHOWED DEBONDING BUT THE MODE COULD NOT BE DETERMINED DUE TO ROTATIONAL DAMAGE. OTHER SHAFT ADAPTERS SHOWED DEBONDING AND WERE CAUSED BY PLASTIC BLASTING. THE INSPECTION PROCEDURE CALLS FOR INSPECTIONS ON THE SHAFT USING MAGNETIC PARTICLE OR FLUORESCENT PENETRANT METHODS AND DOES NOT CALL FOR PLASTIC MEDIUM BLASTING.
On June 17, 1993, at 1800 Alaska daylight time, a skid equipped Bell 206 B-3 helicopter, N21424, registered to Rocky Mountain Helicopters (RMH), and operated by RMH Aerologging, experienced a tail rotor drive shaft failure and subsequent loss of tail rotor control while on short final to a field site at Annette Bay located near Ketchikan, Alaska. The business flight, operating under 14 CFR Part 91, departed a different field site and was transporting loggers employed by RMH Aerologging. A company flight plan was in effect and visual meteorological conditions prevailed. The helicopter was substantially damaged and the Pilot-in-Command and the three passengers were not injured. According to the Pilot-in-Command, the helicopter started spinning when he was 20 to 30 feet above the ground during an approach. He rolled the throttle to the flight idle position and was forced to land, striking the tail boom on a log. The tail rotor drive shafts were visually examined and 4 drive shaft segments were submitted for metallurgical examination. The drive shaft segments were numbered from front to rear and the sections submitted were numbers 3, 4, 5, and 8. Shaft number 3 had a fixed coupling adapter at each end and shafts 4, 5, and 8 had a splined coupling adapter at the forward end and a fixed coupling at the aft end. Examination showed that the splined adapter on the forward end of shaft #4 had debonded and had been spinning on the shaft. No corrosion was observed on the shaft or splined adapter at the adhesive bond area. The metallurgical examination showed that multiple fractures and cracks at the aft end of the splined adapter and the forward end of the shaft were a result of low cycle fatigue and overstress. The four drive shaft sections were submitted for analysis because the visual field examination showed jagged edges in the bonding between the shaft and the couplings. Ultrasonic inspections and disassembly of the adapters from the shafts revealed all four shaft assemblies had areas of adhesive separation. The debonded area of shaft number 3 resulted from plastic medium blasting (PMB). The PMB has torn and plastically deformed the aft edges of the forward adapter at its juncture with the shaft. Stresses induced into the adapter material by the plastic deformation caused by PMB resulted in adhesive debond along the aft edge of the forward adapter. The adhesive debond occurred at the adapter surface. Shaft number 5 had a debonded area at the forward edge of the adapter and was an adhesive separation which occurred at the shaft surface. This debond separation is not considered to be a result of PMB. Shaft number 8 area of debonding resulted from a void in the adhesive. The debond was a mixture of adhesive and cohesive separation at the adapter surface. The debond at the void first initiated as a result of an instantaneous fracture and grew larger over time as evidenced by arrest marks visible in the adhesive fracture. The metallurgists report states "The initial instantaneous fracture likely occurred as a result of impact loading." Shafts number 3, 5, and 8 did not fail but were noticed to have different visual characteristics. Shaft number 4, the shaft that failed, had three debonded areas and were a result of adhesive and cohesive separation at the shaft surface. The metallurgical examination of the debonding of the aft adapter did not show the debonding to be a result of PMB. Some corrosion was present on the shaft surface in the debonded area at the aft end of the shaft. It could not be determined if the corrosion was present when the debonding occurred. According to NTSB records, on November 10, 1992, this helicopter, N21424 was involved in an accident, NTSB file number ANC93LA0915. The NTSB Preliminary information reads "The operator reported that the helicopter was equipped with a long line cable which snagged a stump during approach to a logging site. The pilot reportedly felt the cable restrain the forward movement and took emergency action to immediately lower the collective. The helicopter collided with the ground, striking the tail rotor and damaging the left landing skid mounting structure within the fuselage." According to the Operator, the drive shaft installed was the same drive shaft previously involved in the tail rotor strike. It was examined and refinished according to the Bell Helicopter Service manual. According to the Bell Helicopter Conditional Inspection sheet for sudden stoppage of the tail rotor, removal and inspection of all drive drive shaft components is required. The inspection sheet lists the following procedure, "Inspect tail rotor drive shaft system for damage as follows: Inspect flexible disc couplings, drive shaft and adapters for cracks, using magnetic particle or flourescent penetrant methods. Also check above parts dimensionally. Inspect bonds between tail rotor drive shafts and adapters for integrity." The Insepction Sheet does not indicate that the drive shaft should be cleaned with any PMB.
THE FAILURE OF THE TAIL ROTOR DRIVE SHAFT COUPLING AND THE INADEQUATE INSPECTION OF THE TAIL ROTOR DRIVE SYSTEM BY COMPANY MAINTENANCE PERSONNEL AFTER THE FIRST TAIL ROTOR STRIKE.
Source: NTSB Aviation Accident Database
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