NTSB Factual Report

HISTORY OF FLIGHT

On 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 INFORMATION

According 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.

ORGANIZATIONAL AND MANAGEMENT INFORMATION

Aircrane Inc. was established in 1993 as a construction helicopter operator, specializing in Heavy lift Aerial Crane Services or "External Loads."

At the time of the accident they held 14 CFR Part 133 (External Load), 14 CFR Part 135 (Air Taxi), and 14 CFR Part 137 (Agricultural Application) certificates.

AIRCRAFT INFORMATION

The 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 INFORMATION

The 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.

WRECKAGE AND IMPACT INFORMATION

Post 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.

TESTS AND RESEARCH

Tail 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 structural adhesive film. According to a technical representative for 3M, their unsupported adhesive films such as Scotch-Weld AF 30 do not have a fiber mesh.

The skin fractures were examined visually. The skin fractures were all on slant planes, consistent with ductile overstress fracture and closer examination of the blade surface in close proximity to one of the fractures near the root end of the airfoil revealed that the outboard skin was bent consistent with compression buckling. The surface adjacent to the fracture on the inboard side of the blade was relatively straight which was consistent with the tension side of a bending fracture.

Sections of the fracture surfaces at the blade tip and the root end of the airfoil were cut from the rest of the blade and examined using scanning electron microscopy (SEM). The flat fracture features of the flange attached to the aft side of the spar showed elongated dimples consistent with ductile overstress fracture from shear loading. The orientation of the dimples was consistent with a loading where the trailing piece of the airfoil was moving radially outward (away from the hub) relative to the spar.

Most of the SEM images of the fracture surface showed ductile dimple fracture features. However, portions of the fracture surface showed small regions of flat features with curving boundaries, features consistent with progressive crack growth.

At the inboard side of the fracture, evidence of progressive crack growth was also observed on the inboard side of the skin up to approximately 2.5 inches from the root end of the fracture. The progressive crack features were relatively small.

Evidence of progressive crack growth was also observed on both the inboard and outboard sides of the skin and strap and on the inboard side of the root end channel. None of the progressive crack regions extended through the thickness. Rubbed fracture features consistent

NTSB Probable Cause

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.