Middletown, DE, USA
N126GW
SIKORSKY S-58JT
The pilot had flown two uneventful external load lifts to place 2,900-lb air conditioning units on a warehouse roof. The pilot reported that, during the third lift, he felt vibration in the pedals that became violent, and the helicopter then began to rotate about its vertical axis. The air conditioning unit touched down on the roof as the helicopter was spinning. The pilot could not stop the helicopter's rotation, so he released the cable attached to the air conditioning unit and then maneuvered the helicopter away from the warehouse. He then increased forward speed, turned right to line up with a street, and conducted a roll-on landing. Examination of the helicopter revealed that the entire aft portion of one of the four tail rotor blades had separated just aft of the blade's spar where a bond line existed. Examination of the tail rotor blade revealed high-stress progressive crack growth features at the root end of the fracture, buckling deformation adjacent to the fracture, and bending deformation of the leading edge, all of which were consistent with the tail rotor blade fracturing due to dynamic instability in the tail rotor. The progressive crack growth features observed on the fracture surface were associated with relatively high stress and few cycles and likely occurred after the deformation associated with the buckling. In addition, all four tail rotor blades exhibited bending deformation, indicating that they all experienced loads that exceeded the allowable design loads, and the deformation pattern was consistent with an external input on the tail rotor assembly overloading all of the blades rather than a failure in the blade causing it to become unstable. The helicopter manufacturer confirmed that such damage can be caused by dynamic tail rotor instability and that such instability can be accompanied by tail rotor vibration, as was experienced during the accident flight. Although dynamic tail rotor instability rarely occurs, it has been known to occur on the accident helicopter make and model. To improve tail rotor stability, the helicopter manufacturer had introduced two modifications to the tail rotor system, and both of these modifications had been installed on the accident helicopter. Even with the modifications, dynamic tail instability can occur, and high values of left pedal, improper tail rotor cable tension (too high or too low), bottoming of the tail rotor control system spring, higher rotor speed, or relative wind from the right forward quadrant could increase susceptibility. However, a review of the helicopter's maintenance records did not reveal that any of the mechanical factors that could contribute to tail rotor instability existed, and wind was calm at the time of the accident. Additionally, after the damaged components were replaced, the helicopter was returned to service. The helicopter's flight manual also contained guidance stressing that pilots should immediately decrease the tail rotor pitch after encountering pedal vibration. If the pilot had recognized that the pedal vibration was indicative of tail rotor instability and immediately taken the proper corrective actions in accordance with this guidance, the accident might have been prevented.
HISTORY OF FLIGHTOn August 1, 2012, about 0900 eastern daylight time a Sikorsky S-58JT, N126GW, operated by Aircrane Inc., was substantially damaged when it incurred a failure of a tailrotor blade in Middletown, Delaware. The certificated commercial pilot was not injured. No flight plan had been filed for the local commercial flight conducted under Title14 Code of Federal Regulations (CFR) Part 133. According to the 'Safety Officer," who was helping with the external load operation, placing air conditioning units on the roof of the warehouse, the helicopter flew in to the site around 0730. After gathering all of the personnel, she gave a safety briefing and gave instructions to them on how they were going to conduct the operation. She then split the personnel up into two 4 man crews and sent one of the crews off the roof to reduce the number of personnel they had in the way of the operation. Once she had done this, she gave the "High Sign" to start the operation. The first two lifts were good but, on the third lift, when the helicopter came up over the roof, it did not sound right, and was swerving with the air conditioning unit swinging below the helicopter. The helicopter than started spinning and she yelled for the people on the roof to move. Then while the helicopter was spinning and the nose dropping, the air conditioning unit landed on to the roof, and rolled upside down while it was still attached to the helicopter by the cable. She continued to yell for everyone to get away as the helicopter continued to spin with the nose dropping even after the air conditioner had fallen onto the roof. The pilot then released the cable, and the helicopter then began moving away from the building. A portion of a tail rotor blade then landed on the roof. According to the "Guide Man" who was on the roof, after flying to the warehouse, the helicopter landed and was unloaded. The rigging was than attached to the helicopter. About 45 minutes later, He called for the helicopter and advised that they were ready on the roof. The helicopter lifted the first air conditioning unit and it was placed "dead on" to its mounting location. The second unit was then lifted and it also was "dead on." The helicopter then began lifting the third air conditioning unit, did a normal left turn but, he suddenly heard a high rotor rpm sound. The helicopter then turned into the wind and began spinning over the roof with the air conditioning unit about 12 feet off the roof. The air conditioning unit then touched down on the roof, but the helicopter was still spinning. He then began calling over the radio for the pilot to "break it loose". At this point the air conditioning unit was on the roof upside down, the helicopter then moved away from the building and landed. According to the pilot, He flew the helicopter over from Summit Airport (EVY), Middletown, Delaware, and landed at the job site. They completed the "Safety Brief" for the area and personnel; and the extra people they did not need for the lift operation were moved off the roof. The lifts consisted of 2,900 pound roof top air conditioning units. The first two lifts were uneventful. However, during the third lift, while over the curb about 12 feet above ground level, the pilot felt vibrations in the pedals for a moment. The vibrations became violent, which activated the emergency locator transmitter and the landing light. The helicopter started to rotate about its vertical axis, and though he tried, he could not stop the rotation. He reduced power, then moved the aircraft from above the roof and jettisoned the cable. He then flew the helicopter away from the building, and cleared the roof. He then picked up forward speed, turned to the right to line up with a street, and did a roll on landing. PERSONNEL INFORMATIONAccording to Federal Aviation Administration (FAA) records, the pilot held a commercial pilot certificate with ratings for helicopter and instrument-helicopter. He also held type ratings for the S-58 and S-61. His most recent FAA second-class medical certificate was issued on January 3, 2012. He reported that he had accrued 16,500 total hours of flight experience, of which, 7,000 hours were in the accident helicopter make and model. AIRCRAFT INFORMATIONThe commercial version of the S-58 helicopter was certificated by the FAA on August 2, 1956. The S-58 featured a 56 foot diameter, 4 bladed main rotor, and a 4 bladed tail rotor. Both main and tail rotor blades used the symmetrical NACA 0012 airfoil. The fuselage was all metal, and was equipped with conventional landing gear (main wheels in front, tail wheel in back). According to FAA and maintenance records, the helicopter was manufactured in 1959. It was modified on July 7, 1971 with the removal of its radial engine and installation of a Pratt & Whitney Canada PT6T-3 Twin-Pac Turbine engine. A few years later, it was upgraded to a PT6T-6. Its last continuous airworthiness inspection was completed on July 31, 2012. At the time of the accident the helicopter had accrued 11,064.7 hours of operation. METEOROLOGICAL INFORMATIONThe recorded weather at New Castle Airport (ILG), Wilmington, Delaware, located 10 nautical miles northeast of the accident site, at 0851, included: calm winds, 8 miles visibility, clear skies, temperature 23 degrees C, dew point 22 degrees C, and an altimeter setting of 29.90 inches of mercury. AIRPORT INFORMATIONThe commercial version of the S-58 helicopter was certificated by the FAA on August 2, 1956. The S-58 featured a 56 foot diameter, 4 bladed main rotor, and a 4 bladed tail rotor. Both main and tail rotor blades used the symmetrical NACA 0012 airfoil. The fuselage was all metal, and was equipped with conventional landing gear (main wheels in front, tail wheel in back). According to FAA and maintenance records, the helicopter was manufactured in 1959. It was modified on July 7, 1971 with the removal of its radial engine and installation of a Pratt & Whitney Canada PT6T-3 Twin-Pac Turbine engine. A few years later, it was upgraded to a PT6T-6. Its last continuous airworthiness inspection was completed on July 31, 2012. At the time of the accident the helicopter had accrued 11,064.7 hours of operation. WRECKAGE AND IMPACT INFORMATIONPost accident examination of the helicopter by a Federal Aviation Administration (FAA) inspector revealed that the entire aft portion of one of the tail rotor blades was missing. Further examination revealed that it had separated at a point just aft of the broken tailrotor blade's spar where a bond line existed. The separated aft portion of the broken tailrotor blade was later recovered from the roof of the warehouse by the operator. All four tailrotor blades including the seperated aft portion from the broken blade were then retained by the NTSB for further examination. According to the operator after the accident, the yaw spring was inspected and returned to service on the accident helicopter. The tail rotor assembly and intermediate gearboxes were scrapped along with the tail rotor drives. The chip detectors were inspected and found to be clean and the main rotor gearbox was inspected, and its chip plugs and screen were also found to be clean. The main rotor gearbox oil was changed, and a gear box penalty run of one hour was performed, the chip plugs and oil screen were then inspected again and were still found to be clean, and it was returned to service. All of the hanger bearing supports, gearbox mounting flanges, and the pylon fittings then were subjected to a die penetrant inspection for cracking and no defects were noted. All of the tail rotor drive shafts were scrapped and replaced with overhauled ones. The pylon was also inspected for structural integrity and loose rivets and all of the inspection panels were opened on the helicopter and inspected with no defects being noted. The helicopter was returned to service in October of 2012 and at the time of this report had been operating without incident. ADDITIONAL INFORMATIONIn 1970, Sikorsky had set up a production line to remanufacture S-58 helicopters to the S-58T configuration which included replacing the R-1820-84 Radial engine with a Pratt & Whitney Canada PT6T-3 Twin-Pac Turbine engine installation. FAA approval for the modifications was received in April 1971. Sikorsky also produced kits which allowed S-58 helicopter operators to convert their helicopters to the S-58T Configuration. An improved version of the S-58T, the S-58T Mark II, added a more powerful engine which improved one engine inoperative capabilities and installed a bifilar vibration absorber which reduced rotor induced vibration. The bifilar provided improved pilot and passenger comfort and reduced aircraft maintenance. In 1981, Sikorsky sold the manufacturing rights, and support for the S-58/S-58T to California Helicopter International (California Helicopter Airways). This included all tooling, jigs, fixtures, drawings, and spares inventory. Sikorsky however, retained the type certificate and the responsibility for the manufacturing, repair, and overhaul of the main and tail rotor blades. Over the years though, as Sikorsky moved on to manufacture other helicopters; they ceased manufacturing, repair and overhaul of the main and tailrotor blades. As a result to maintain operational safety support, on May 6, 2015, Sikorsky transferred the type certificate for the S-58, as well as the S-55, and S-62 models to California Helicopter Airways. TESTS AND RESEARCHTail Rotor Control System The tail rotor was controlled through a hydraulically boosted cable system with push-pull rods which connected a rear fuselage bell crank to the rotor. The hydraulic servo operated on the tension difference between the two cables and, as the system was a boosting system with the power piston in series with one of the cables and not a fully powered system, the yaw pedals had a direct mechanical link to the rotor blade pitch change mechanism. The cables were rigged to a specific tension and a spring inserted in one of the cables had as its main function the tuning of the rate of the whole tail rotor system to avoid unwanted resonances. The spring also maintained the tension over a wide range of ambient temperatures. Tail Rotor Blades The tail rotor blades were of aluminum alloy construction. The structural supporting member of the blade assembly consisted of a solid spar around which the skin was wrapped and bonded. The skin was bonded together at the trailing edge and formed an integral part of the blade structure. An aluminum foil honeycomb core was sandwiched and bonded between the top and bottom skins and the trailing edge side of the spar to form structural support for the skin. The tip of the blade was sealed by means of a riveted tip cap, and the root was sealed with cemented balsa filler. The root end of the blade assembly was also reinforced by a strap which was wrapped and bonded to both sides and around the leading edge of the blade. Review of the helicopter's maintenance manual revealed that the tail rotor blades had an unlimited life, provided that the following flight restrictions were complied with: 25 knots maximum sideward flight, minimum 10 seconds hovering turns (360 degrees), and minimum 88 percent Nr (main rotor rpm) on all taxi turns. Review of the maintenance records did not reveal however, whether the helicopter ever exceeded any of the specified flight restrictions nor could it be ascertained if a robust mechanism had ever been set up by the manufacturer or operators of the S58 that would capture these types of exceedances. Examination of the Tail Rotor Blades As part of the examination, the tail rotor blades were lettered from A to D in sequence in the direction of tail rotor rotation such that each blade trailed the next higher blade, and blade D trailed blade A. Blade A was fractured with most of the airfoil separated from the spar. Blades B, C, and D were intact. All of the tail rotor blades were Sikorsky part number 1615-30100-045. According to component log cards, all blades were installed on June 28, 2011 at 162.3 hours prior to the accident. The component log cards stated that prior to installation on the accident helicopter, blade B was last removed from another helicopter in 1993 for painting, and blades C and D were last removed from another helicopter in 1990 for vibration troubleshooting. The prior installation history for blade A was not noted on the component log. At the time of failure, the component log stated blade A had a total time of 2,494.40 hours. The total times for blades B through D were unknown. Data plates affixed to the inboard sides of the blades indicated the blades had been inspected and repaired at Sikorsky. Blades C and D were each marked inspected and repaired in May, 1979. Blades A and B were marked inspected and repaired in September, 1980, and in May, 1983, respectively. The data plate for blades A and C listed total times of 2,562.10 hours and 0.0 hours, respectively. The hours for blades B and D were marked unknown. A stainless steel wear strip covered the leading edge along nearly the entire length of the airfoil back to approximately 1.44 inch from the leading edge. The wear strip is bonded to the skin with Scotch-Weld AF 30 structural adhesive film manufactured by 3M. The intact areas of the blades were initially examined for paint condition, dents, and other anomalies. As shown in figures 1 and 2, the paint was eroded away from the leading edges of the blades. The paint erosion on blade A was less than that of the other blades. A dent was observed on the outboard side of blade B at a location approximately 14.5 inches from the butt end of the blade and is indicated in figure 1. This dent, measuring approximately 1.75 inches in diameter, was the largest and deepest dent observed on the 4 blades. Smaller and shallower dents were observed on other areas of blade B and on blades A and D. A slight bulge was observed on the inboard side of blade B near the trailing edge of the leading edge wear strip. The bulge was approximately 1 inch long and was located approximately 36.25 inches from the hub end of the blade. Blade A was fractured into 2 pieces. One piece included the intact main spar, and the other piece which was recovered from the roof contained most of the airfoil. The spar was bent with the tip displaced toward the leading direction and outboard relative to the hub end. The fracture in blade A intersected the hub end of the airfoil at a location approximately midway between the spar and the trailing edge. Along most of the length of the blade, the skin was fractured at the spar trailing edge on both the inboard and outboard sides of the blade. At the blade tip, a flange bonded to the trailing side of the spar was fractured and showed flat fracture features. The tip cap on blade A was removed to facilitate examination of the fracture surfaces near the tip of the blade. When the tip cap was removed, it was noted that no lock wire was installed on the bolt attaching the tip weights. The fracture features of the flange at the blade tip were generally flat, and edges at the hub end of the flange piece attached to the spar were bent outward toward the tip. The leading edge wear strip was intact and remained attached to the piece of the skin that wrapped around the leading edge spar. The trailing edges of the leading edge wear strip had a wave pattern deformation. The wear strip was disbonded from the pieces of the skin on the inboard and outboard sides of the separated trailing airfoil piece of the blade. The fracture was mostly an adhesive fracture at the interface between the skin on the trailing piece and the adhesive that remained bonded to the wear strip. Similar features were observed along the entire length of the blade where the skin was disbonded from the wear strip on both the inboard and outboard sides of the blade. The adhesive was teal green in color and was impregnated with an open-weave fiber mesh. Portions of the adhesive appeared to be stained brown. Data sheets for Scotch-Weld AF 30, the leading edge wear strip adhesive specified in the engineering drawings for the blade assembly, state that Scotch-Weld AF 30 is an unsupported str
The pilot's failure to recognize that the helicopter was experiencing tail rotor dynamic instability and to take immediate corrective actions during an external load lift, which resulted in the failure of a tail rotor blade.
Source: NTSB Aviation Accident Database
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