LAX94LA054

During a swath run to apply a mixture of pesticides, the pilot heard a loud 'pop'. The helicopter's collective control began to oscillate and the aircraft experienced a severe vibration. The helicopter also began to oscillate and the pilot slowed to about 15 knots and attempted to land. A spray boom attached to the helicopter contacted the ground and the helicopter rolled on its left side. Examination revealed the presence of fretting corrosion and a fatigue failure of the tension-torsion bar pin, into which the main rotor blade drag brace is attached. A second tension-torsion pin from a sister helicopter was discovered to also have corrosion and a fatigue crack. Pins failed at 369.5 hours and 489.4 hours respectively. The retirement life of the pin is 643 hours.

On November 20, 1993, about 0845 hours Pacific standard time, a Hiller UH-12E helicopter, N161HA, crashed during an emergency landing about 8 miles southeast of Salinas, California. The helicopter was being operated as a visual flight rules (VFR) aerial application flight in the local area under Title 14 CFR Part 137 when the accident occurred. The helicopter, operated by R and B Helicopters Inc., sustained substantial damage. The certificated commercial pilot, the sole occupant, received minor injuries. Visual meteorological conditions prevailed. The flight originated from an adjacent field at about 0844 hours.

The pilot reported that while on a swath run to apply a mixture of pesticide about 2 feet above a field, he heard a loud "pop". The helicopter's collective control began to oscillate and the aircraft experienced a severe vibration. The helicopter also began to oscillate, and the pilot slowed to about 15 knots and attempted to land. A spray boom contacted the ground and the helicopter rolled onto its left side.

The pilot reported that the outboard end of one-of-two tension-torsion bar pins separated at the eye bolt end. This allowed the main rotor blade drag brace fitting to pull out of the tension-torsion pin, permitting the main rotor blade to rotate freely on the blade hub.

The helicopter had accumulated a total time in service of 7,725.3 flight hours. The most recent annual inspection was accomplished on November 1, 1992, 772.8 flight hours before the accident. In addition, a 100-hour inspection was completed on October 14, 1993, 73.5 hours before the accident.

The tension-torsion bar pin, part number 51452, serial number 26536, has a retirement life of 643 hours. The pin had a new serviceable part tag from the manufacturer that was dated April 29, 1993. The pin was installed on the accident helicopter by the operator on June 1, 1993. It had accumulated 369.5 hours before the accident. As a result of Air Worthiness Directive 86- 17-2, dated August 22, 1986, the pin is subject to a recurring 100-hour inspection that includes a dye penetrant examination.

The tension-torsion pins are also subject to Rogerson Hiller Inc. Service Bulletins 51-2 and 51-9. Bulletin 51-2, dated March 31, 1978, addresses proper alignment between the tension-torsion pin and the main rotor blade drag brace fitting to preclude an imposition of large twisting moments on the head of the pin. An assessment of any drag brace misalignment, limited to 1/16 of an inch, is included in each 100-hour inspection.

Bulletin 51-9, dated April 8, 1983, addresses the addition of shims between the tension-torsion pin and the drag brace fitting to reduce fretting corrosion. Compliance with the service bulletin is not mandatory; however, if performed, a logbook entry is required. Repeated examinations of the shims were not required. The operator reported that the service bulletin was not performed.

The operator reported that he examined a second company helicopter, N5374V, also a Hiller UH-12E, on December 1, 1993. During a dye penetrant inspection of the tension-torsion bar pins, a crack was discovered in the interior radius of the eye bolt of one of the two pins. The pins were included as part of a complete rotor head assembly that had a part tag dated March 3, 1993. The rotor head assembly was part number 51439-19, serial number 10529-A. The head assembly was installed on N5374V by the operator on March 27, 1993. The pins did not have individual component record cards and were either serial number 46168 or 46169. The rotor head assembly had accrued a total time in service of 489.4 hours.

The separated tension-torsion bar pin from N161HA, and the cracked pin from N5374V are manufactured from AN 4140 steel and coated with cadmium. The pins were submitted to the National Transportation Safety Board, Materials Laboratory for examination. The separated pin from the accident aircraft bolt through-hole contained areas of high luster indicative of wear from the pin moving relative to the retaining bolt. The fracture surface exhibited beach marks typical of a fatigue crack. The crack emanated from multiple origin areas on the inside radius of the bolt through-hole. The surface area of the fatigue origin area exhibited corrosion pits. The pits contained deposits of cadmium and sulphur. The cadmium coating at the fatigue origin area was not present on the inside diameter surface.

A magnetic particle inspection of the cracked tension-torsion bar pin from N5374V revealed that the crack emanated from the inside diameter of the bolt through-hole and traversed about 80 percent of the bolt sidewall. Opening of the fracture surface revealed beach marks typical of a fatigue crack originating from multiple origins on the inside radius of the bolt through-hole. The surface area of the fatigue origin area exhibited corrosion pits. The pits also contained deposits of cadmium and sulphur.