MARYLAND HTS, MO, USA
N970CC
BELL 206L
THE PILOT OF A TURBINE POWERED HELICOPTER SAID HE BEGAN A HOVER ABOUT THREE FEET ABOVE THE GROUND, TURNED THE HELICOPTER INTO THE WIND, AND INITIATED A TAKEOFF. ACCORDING TO WITNESSES, THE HELICOPTER HAD STARTED TO MOVE FORWARD AND HAD ATTAINED AN ALTITUDE OF ABOUT 25 FEET, WHEN THEY HEARD A CHANGE IN THE SOUND OF THE ENGINE. THE PILOT STATED 'AS POWER WAS BROUGHT IN, A LOUD METALLIC TYPE POP WAS HEARD.' HE SAID THE HELICOPTER YAWED LEFT AFTER THE 'POP' WAS HEARD, AND THAT IT DID NOT RESPOND TO CONTROL INPUTS. THE HELICOPTER FLEW OVER AN EMBANKMENT, AND ITS ROTOR BLADES STRUCK THE SIDE OF THE EMBANKMENT. AFTER THE BLADE STRIKE, THE HELICOPTER ROLLED OVER ONTO ITS LEFT SIDE, THEN ON ITS RIGHT SIDE AND CAME TO REST. EXAMINATION OF THE HELICOPTER AND ITS COMPONENTS REVEALED THE ATTACHMENT BOLTS THAT CONNECTED THE DRIVESHAFT TO THE GEARED PORTION OF THE DRIVESHAFT COUPLING HAD FAILED. METALLURGICAL EXAMINATION REVEALED THAT TWO OF THE BOLTS HAD FAILED FROM FATIGUE, THE OTHER TWO BOLTS HAD EVIDENCE OF OVERLOAD FAILURE. ALSO, AFTER DISASSEMBLY, A TENSION (CENTERING) SPRING WAS DISCOVERED TO BE MISSING FROM THE TRANSMISSION END OF THE DRIVESHAFT (WHICH WOULD HAVE ALLOWED THE BOLT HEADS TO CONTACT THE GREASE RETAINING PLATE). THE LAST ANNUAL INSPECTION OF THE HELICOPTER AND ENGINE WAS ON 11/3/93.
On February 27, 1995, at 0815 central standard time (cst), a Bell 206L, N970CC, operated by Digital Teleport, Incorporated, of St. Louis, Missouri, and piloted by a private pilot, was substantially damaged when it collided with the ground while hovering. Visual meteorological conditions prevailed at the time of the accident. The pilot and four passengers reported no injuries. The flight departed Maryland Heights, Missouri, at 0815 cst. The pilot said he brought the helicopter into a three-foot hover above a parking lot. He said he turned the helicopter into the wind preparing for a normal takeoff. The surface winds in the area were from the Northeast at the time of the accident. He stated, "As power was brought in, a loud metallic type pop was heard. The aircraft immediately yawed to the left, then [turned] east. As controls were executed to bring the aircraft under control and return to the ground, no apparent response was observed. In addition, engine was still operating at apparent full power." According to witnesses, the helicopter attained an approximate altitude of 25 feet above the ground. They said it began moving forward after its pitching its nose slightly down. As the helicopter began to accelerate, the witnesses reported hearing a change in engine sound. One said it sounded as though there was a loss of power followed by an increase. A second witness said the engine sounded as though it had lost power. The helicopter flew over an embankment and touched down on the downwind, sloped, side of the embankment. Its rotor blade struck the ground an undetermined number of times before it rolled over on its side. The helicopter was turned upright, according to witnesses, during the rotor blade ground strikes and then rolled over onto its right side. During the on-scene investigation the helicopter's main drive shaft assembly was examined. The engine driveshaft's coupling end attached to the helicopter's main rotor transmission had separated from the transmission. The centering spring normally positioned between the shaft's end and grease plate was missing. It was not found in the wreckage according to the FAA's Principal Maintenance Inspector (PMI). Examination of the engine was conducted at the manufacturer. A FAA PMI observed the engine inspection and runup. The PMI reported the engine's external areas had what appeared to be long-term uncleanliness. He said the engine's visible compressor section turbine blades were dirty and nicked. The bleed air control valve was dirty with carbon on it. The engine, according to the PMI, was test run using all the accessories it had been equipped with when mounted on N970CC. He stated the engine ran to within 10 percent of the manufacturer's specifications during the tests. The NTSB Materials Laboratory examined the helicopter's drive shaft, two outer and inner couplings of the drive shaft, three separated bolts from the assembly, a spring, and two grease retainer plates. The laboratory's factual report stated that the fracture faces of two bolt pieces displayed fatigue cracking. The fracture face of the third bolt displayed overstress separation. The two bolts showing fatigue failure had almost all of the cadmium plating rubbed off the top of their heads. Cadmium plating on the third bolt had been slightly damaged. The laboratory report continues, "Reassembly of the transmission end of the driveshaft indicated that the positions of scored areas on the grease retainer plate correspond to the positions of the heads of the bolts attaching the inner coupling to the driveshaft flange. Further examination of the plate disclosed contact markings on the periphery of the plate corresponding to the position and size of the gear teeth on the inner coupling." "The surface of the flange at the transmission end of the driveshaft contained circular wear marks around the attachment bolt holes, as if washers were spinning relative to the shaft. Examination showed no evidence of unusual teeth or spline wear pattern in any of the couplings from the driveshaft assembly." The main rotor blade, swashplate, and swashplate support assembly are not connected to the failed driveshaft. The tail rotor system is not connected to the failed driveshaft. Both units are connected to the helicopter's transmission. N970CC's last annual inspection was performed on November 3, 1993. The airframe logbook showed it had 5,576.6 hours as of that date. Records show that the main driveshaft has accumulated 5,651.8 hours total time. It accumulated 176 hours since its last overhaul. According to maintenance invoices dated August 26, 1992, the main driveshaft overhaul was completed on this date. New inner and outer couplings were installed during the overhaul. N970CC's engine logbook showed it had a total time since new of 5,651.7 hours. Its time since overhaul was 2,294.3 hours.
FATIGUE FAILURE OF TWO ATTACHMENT BOLTS THAT CONNECTED THE DRIVESHAFT TO THE GEARED PORTION OF A DRIVESHAFT COUPLING. FACTORS RELATING TO THE ACCIDENT WERE: INADEQUATE MAINTENANCE, UNEVEN TERRAIN IN THE TOUCHDOWN AREA FOR THE FORCED LANDING, AND THE UNFAVORABLE HEIGHT/VELOCITY SITUATION OF THE HELICOPTER WHEN THE DRIVESHAFT DISCONNECT OCCURRED.
Source: NTSB Aviation Accident Database
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