N467PH
Bell 407
The helicopter was in cruise flight over open ocean water at 130 knots and 800 feet MSL, when the pilot felt a "violent jerk or yaw" and heard a grinding sound coming from the rear of the aircraft. The pilot attempted to correct the yaw with the anti-torque pedals, but there was no response, and the aircraft continued to yaw to the right and the nose pitched down. The pilot reduced the throttle and initiated an autorotation. He stated that the helicopter made "almost" a 360-degree right turn before contact with the water at a level pitch attitude with "slight" forward speed. One of the 4 passengers recalled seeing an object fall and hit the water, "leaving a circular motion" on top of the water, just prior to the helicopter landing in the water. The helicopter remained afloat on its deployed skid mounted float system and was recovered on the day of the accident, with the exception of the aft section of the tail boom, including the tail rotor gearbox, vertical fin, and tail rotor hub and blade assembly, which was missing. The aft section of the tail boom was recovered approximately 3 months later. Detailed examination of the tail boom separation area revealed multiple fractures and tears between tail boom stations 148 and 162. Examination of the fractured area revealed no evidence of fatigue. There was evidence of 4 tail rotor blade strikes between tail boom stations 150.9 and 153.2. One of the tail rotor blades was fractured about 14 to 18 inches from the inboard end, with the outboard section missing. The metallic leading edge of the remaining inboard section of this blade was deformed and twisted toward the tail boom. The other tail rotor blade was relatively intact with an indentation near the leading edge on the side toward the tail boom located about 18 inches from the inboard end of the blade. White paint, similar to the paint on the tail boom was found smeared onto the tail rotor blade's black paint in the area of the indentation. Detailed examinations of the tail rotor hub and blade assembly, tail rotor mast, tail rotor controls, tail rotor drive system, and the 90-degree gearbox mounting system did not reveal the reason for the in-flight contact of the tail rotor blades with the tail boom.
HISTORY OF FLIGHT On September 18, 1997, at 0745 central daylight time, a Bell 407 helicopter, N467PH, owned by G.E. Capital Corporation, and operated by Petroleum Helicopters Incorporated (PHI), was substantially damaged when the tail rotor blades struck the tail boom while in cruise flight. The pilot executed a forced landing (autorotation) into ocean water, approximately 70 miles south west of Fourchon, Louisiana, in the Gulf of Mexico. The commercial pilot was not injured and his four passengers sustained minor injuries. Visual meteorological conditions prevailed and a company flight plan was filed for the 14 Code of Federal Regulations Part 135 on demand air taxi flight. The flight originated at 0729 from the Ewing Banks 826 offshore platform, and was en route to a company field base located in Venice, Louisiana. In an interview with the NTSB investigator-in-charge (IIC), the pilot stated that the helicopter was in cruise flight at 130 knots and 800 feet mean sea level (MSL), when suddenly there was a "violent jerk or yaw of the aircraft and a loud noise like a bang, followed by a grinding like sound." The pilot attempted to correct the yaw with the anti-torque pedals, but there was no response, and the aircraft continued to yaw to the right and the nose pitched down. He then applied aft cyclic to try to "level" the aircraft, and reduced the collective. Losing altitude and still in a turn to the right, the pilot reduced the throttle and initiated an autorotation. He stated that the helicopter made "almost" a 360-degree turn before contact with the water at a level pitch attitude with "slight" forward speed. Upon entering the water, the chin bubbles broke out and then the pilot deployed the skid mounted floats. The helicopter came to rest floating upright on the water surface. After about 1 hour, a rescue boat arrived and the pilot and his 4 passengers were transported to the nearest offshore platform. The helicopter was kept afloat by a recovery crew and transported by barge to Lafayette, Louisiana. All of the 4 passengers aboard provided written statements to the NTSB IIC. Excerpts of their statements follow: Passenger #1, seated in the left front seat across from the pilot, stated that he was talking to the pilot on the intercom when he heard a "loud noise from the back off the aircraft and saw the foot controls go to the floor" as the helicopter "yawed or shoved" to the left. The helicopter "stayed in a downward" angle for "a few seconds" and then "came back around to the right and went nose down" and "side to side nearly vertically." He observed the pilot "fighting the controls" as the helicopter "went to the right again and almost turned upside down. The pilot then "pulled the ship out of [the] corkscrew," and the helicopter impacted the water. Passenger #2, seated in the cabin left seat directly behind passenger #1 and facing to the rear of the helicopter, stated that he heard a "loud boom" through his ear protection. The helicopter "pitched to the left" and began a "spiral nose dive." The helicopter "made several revolutions" prior to impacting the water. He observed the pilot "struggling with the stick" during the descent. Passenger #3, seated in the cabin left rear seat facing forward, stated that he heard a "loud bang" and the helicopter started "spinning out of control." The helicopter "kept spiraling down" as the pilot was "fighting with the control stick to stabilize the helicopter." He stated that the helicopter "hit the water" at about a 20-degree angle and the "windshield and pilots foot control pedals busted." Passenger #4, seated in the cabin right seat facing aft, stated that he was reading when he heard a "whop" sound come from behind his head and then "suddenly, the aircraft yawed hard to the left and rolled slightly to the right." At this point, he was looking to see "our relationship with the water when [his] eyes caught a glimpse of a falling object. He further stated that, "as fast as [his] eyes picked the object up, it (the object) hit the water leaving a circular motion on top of the water." The aircraft then "went nose downward while descending to the water." Just before the aircraft hit the water, he remembered hearing a loud high pitch sound coming from the main rotor area as the "nose of the aircraft came up, almost leveling off, but still nose forward somewhat." He recalled that the aircraft "went into the water rather hard" and water immediately came "flooding through the ship." The helicopter "went deep into the water, then suddenly there was a loud noise and the helicopter came rushing back to the surface." PERSONNEL INFORMATION The pilot was employed by PHI on February 6, 1988, and held a valid commercial pilot certificate with helicopter, single engine land, and multi-engine land airplane ratings. He held airplane and helicopter instrument ratings. His total flight time, as of September 18, 1997, was 10,773 hours, of which 9,330 hours were in light helicopters. His total flight time in the Bell model 407 as pilot-in-command was 75 hours. He had flown 163 hours in single engine helicopters in the 90 days preceding the accident. AIRCRAFT INFORMATION Manufactured at Bell Helicopter Textron Canada, the Bell model 407 helicopter, serial number (S/N) 53142, was certified on May 16, 1997, with 7.2 total flight hours. After certification, the helicopter was ferried to the HAS Corporation, Mount Pleasant, Pennsylvania, for customization and avionics installations. The helicopter was delivered to PHI, Lafayette, Louisiana, on June 19, 1997, as a new aircraft with a total time of 20.5 hours. The following information is a summary of the manufacturing history of the helicopter: The helicopter was equipped with tail boom, S/N 53143. The tail boom serial number is usually matched with the helicopter serial number; however, according to the manufacturer, it is not unusual to see a difference in the helicopter serial number and some major assemblies such as the tail boom. The tail boom was equipped with tail-rotor gearbox, S/N A-777, which was test run (called a "green" run) on two occasions for routine manufacturing testing purposes. On the first run, the gearbox performed to manufacturer's specifications. However, following a green run, gearboxes are normally disassembled as part of a visual inspection process to ascertain gear wear patterns and other details not visible from the outside. During the disassembly of the gearbox, it was discovered that the output journal needed to be replaced. The following green run was successful and the gearbox was accepted. During manufacturing, the tail boom was equipped with tail rotor hub and blade assembly, S/N 53117, which included the following components: Hub (yoke/flexure) S/N A-160 Tail Rotor Blade S/N A-1692 (later replaced by blade S/N 1757 due to damage) Tail Rotor Blade S/N A-1693 After the tail rotor hub and blade assembly was installed on the helicopter, blade S/N A-1692 was found to have a dent on the outboard side. The damage was assessed by manufacturing quality assurance, and the blade was removed for repair. A replacement blade, S/N A-1757, was then installed on the tail rotor hub and blade assembly. The tail rotor hub and blade assembly was then balanced and operated during test and certification flights before undergoing a final inspection conducted by Bell Canada and Transport Canada. During the final inspection, blade S/N 1693 was found to have a minor scratch on the outside face, which was repaired. Also, the tail skid was found loose and repaired with a sealing compound. The operator provided the following notes on maintenance of the helicopter after delivery: August 14, 1997: 3-month/300-hour lubrication and service as per FAA approved inspection plan. Aircraft total time 84.2 hours. August 21, 1997: 150-hour inspection. Aircraft total time 97.5 hours. August 24, 1997: Removed tail rotor and hub assembly, S/N 53117, to inspect for "metallic noise" coming from the tail rotor when rotated on the flap axis during a preflight inspection. After installation of a different assembly, the same noise was present. Tail rotor and hub assembly, S/N 53117, was reinstalled and the aircraft was returned to service. Aircraft total time 103.5 hours. September 16, 1997: Replaced a cracked coupling disc between the second and third rotating tail rotor drive segments forward of the tail rotor gearbox. Aircraft total time 170.5 hours. The total time of the helicopter at the time of the accident was 177 hours. WRECKAGE AND IMPACT INFORMATION Initial examination of the wreckage was conducted by the NTSB IIC at the operator's main base in Lafayette, Louisiana, on 19-20 September, 1997. The tail boom was separated approximately 33 inches aft of the trailing edge of the horizontal stabilizer, and the aft section including the tail rotor gearbox, vertical fin, and tail rotor hub and blade assembly, was missing. Both of the helicopter's chin bubbles were shattered, and the fuselage belly panels exhibited deformations consistent with hydraulic forces. Detailed examination of the tail boom separation area revealed multiple fractures and tears between tail boom stations 148 and 162. Examination of the fractured area with a stereo microscope (up to 30x) did not reveal evidence of fatigue. There was evidence of what appeared to be 4 tail rotor blade strike marks between tail boom stations 150.9 and 153.2. The tail rotor drive shaft was found fractured at tail boom station 151.9. The tail rotor drive shaft cover was fractured between tail boom stations 150 and 152 and the tail rotor pitch control tube was found fractured and displayed evidence consistent with bending overstress. Strike mark #1 was a 4.8-inch cut that started 1.8 inches above the left edge of the upper tail boom skin panel at station 151.9 and ended at tail boom station 150.9. Strike mark #2 was about 2 inches to the left of the top center of the tail boom and was located at tail boom station 151.2. Strike mark #3 was at the top center of the tail boom at station 151.9, which coincided with the tail rotor drive shaft fracture. Strike #3 left a well defined crease of approximate dimension to the leading edge of an exemplar tail rotor blade. The crease was located on the upper skin panel aft of strike mark #3. A straight edge was placed in the crease and projected outward from the strike mark to statically simulate the approximate location of the tail rotor tip path plane. This projection indicated a distance of about 32.5 inches from the center of where the tail rotor hub and blade assembly would be mounted relative to the tail boom center line. The measured distance along the tail boom center line, from the tail rotor mast to the tip path plane of an exemplar tail rotor hub and blade assembly pivoting the static blade toward the boom, was also about 32.5 inches. Strike mark #4 was 1.8 inches above the right edge of the upper skin and was located at tail boom station 153.2. In mid December 1997, the aforementioned missing section of the tail boom, the tail rotor hub and blade assembly, and vertical fin were recovered from the Gulf of Mexico. Subsequently, all of the recovered tail boom components were examined at the NTSB Materials Laboratory, Washington, DC., on December 19, 1997, and then transported to Bell Textron, Hurst, Texas, for additional teardown examinations of components under NTSB supervision, on 26 May, 1997. The examinations revealed the following: Tail Rotor Hub and Blade Assembly Tail rotor blade, S/N 1757, was attached to the yoke and was fractured about 14 to 18 inches from the inboard end, with the outboard section missing. The metallic leading edge of the remaining inboard section of the blade was deformed and twisted toward the tail boom. Tail rotor blade, S/N 1693, was relatively intact with an indentation near the leading edge on the side toward the tail boom located about 18 inches from the inboard end of the blade. White paint, similar to the paint on the tail boom was found smeared onto the tail rotor blade's black paint in the area of the indentation. The tail rotor blade counter weights and supports were all present and relatively intact on both blades. Both of the yield indicator slots on the tail rotor hub static stop were bent closed. The tail rotor hub and blade assembly was removed from the tail rotor mast and placed on an exemplar gearbox and mast assembly mounted on another tail boom. The original static stop which was bent was removed and an undamaged static stop was installed. The outboard feathering bearing for blade S/N 1757 was found to be out of its specified normal position. A static load of 200 pounds was placed on the outboard feathering bearing at end of the yoke. The feathering bearing did not move under the static load. The bearing was cut from the yoke and its outer diameter measured over any primer which was on it. The bearing was found to be staked within the manufacturer’s specifications. Inspection of the tail rotor blade retention bolts after they were removed revealed they were slightly bent. When placed in a V-block and rotated under a dial indicator, the inboard bolt from blade S/N 1757 had a 0.006-inch run out. The blade's outboard bolt had less than a 0.001-inch run out. Run outs on blade S/N 1693 inboard and outboard bolts were 0.004-inches and 0.002-inches, respectively. Tail Rotor Mast Inspection of the tail rotor mast revealed no visible bending. However, when it was placed on centers and rotated, it was found to have a 0.006-inch run out at the splined end where the tail rotor hub had been mounted. The manufacturer's engineering drawing of this component shows a maximum of 0.002-inches run out at that location. It was also observed that the tail rotor mast splines had two locations, 180-degrees apart, where the inboard edges of the splines were deformed. One area of deformation was on the master spline and three adjacent spines (counter clockwise as viewed from the outboard end of the mast). The second area of deformation was evident on 4 splines about 180-degrees around from the master spline. Tail Rotor Controls The tail rotor pitch control tube in the tail boom was fractured adjacent to the tail boom separation. The fracture of the control tube displayed evidence of overload failure. The tail rotor control rod was fractured in the threads near the end attached to the tail rotor control lever assembly. The fracture appeared to be a result of bending overload. The control rod through the mast to the crosshead was intact and functional. Inspection revealed that, relative to the crosshead end, the pitch link corresponding to tail rotor blade S/N 1757 had a 45-degree bend oriented in a counter clockwise direction as viewed looking inboard toward the tail boom and fin. Also, this pitch link had a small crack at the end of the link. The pitch link corresponding to tail rotor blade S/N 1693 was bent about 10-degrees at two locations along its length resulting in a slight "S" bend. Tail Rotor Drive The tail rotor drive shaft was fractured adjacent to the tail boom separation. Examination of the fracture revealed that it corresponded with a tail rotor blade strike exit area in the top of the tail boom skin. Inspection of the mating drive shaft fracture surfaces revealed an area that contained an indentation with the approximate dimension of a tail rotor blade tip. The remainder of the drive shaft fracture displayed evidence of overload failure. The Thomas coupling at the 90-degree gearbox input was intact, and the disks of the coupling were deformed and splayed apart. The tail rotor gearbox magnesium case had dissolved leaving the fractured aft portion of the drive shaft still attached to the aforementioned Thomas coupling and input quill assembly. Both the input pinion and gear were covered with a milky white powdery substance. During inspection, this substance was cleaned from the tail rotor gear and pinion with pressurized air and a non-corrosive cleaner. Inspection of the cleaned gear and pinion teeth revealed no recognizable indentations or marks on the
The tail rotor blade strike(s) that severed the aft portion of the tailboom, subsequently rendering the pilot with no tail rotor control while in cruise flight. A factor was the lack of suitable terrain for the forced autorotative landing.
Source: NTSB Aviation Accident Database
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