LAX95LA096

The pilot was applying chemicals to a field at a slow airspeed about 10 feet off the ground. A severe vibration started through the airframe, and the pilot decided to land immediately. The pilot was turning to a landing area when the vibration suddenly ceased, the helicopter fell to the ground on its nose, and rolled over onto its top. One main rotor blade and half the hub were found more than 100 yards from the helicopter. The second blade was found attached to the remainder of the hub, about 150 feet from the wreckage. The two pieces of the fractured and separated main rotor hub were recovered and metallurgically examined. The hub separated due to a fracture through its center. The examinations disclosed that the fracture through the hub center stemmed from fatigue cracks which initiated from the tension-torsion pin hole on the leading edge side of one hub arm. The origin point of the fatigue crack was determined to be inside the tt pin hole and exhibited corrosion deposits typically seen in aluminum alloys. The crack would not have been visible during an external inspection. The fatigue crack and fracture are in areas not covered by the recurring inspection requirements of AD 73-20-03.

On February 3, 1995, about 1115 Pacific standard time, a Hiller UH-12E, N9293Z, crashed during an aerial application flight near Chowchilla, California. The helicopter was operated by Streeter Flying Service of Merced, California, under the provisions of 14 CFR Part 137 of the Federal Aviation Regulations. Visual meteorological conditions prevailed. The helicopter incurred destroyed damage. The certificated commercial pilot, the sole occupant, was not injured. The flight originated at the Chowchilla Airport on the day of the accident about 1045 as a local area aerial application flight.

In a telephone interview, the pilot/operator said he was applying chemicals to a field at a slow airspeed about 10 feet off the ground. A severe vibration started through the airframe, and the pilot decided to land immediately. The pilot was turning to a landing area when the vibration suddenly ceased, the helicopter fell to the ground on its nose, and rolled over onto its top.

Federal Aviation Administration inspectors from the Fresno, California, Flight Standards District Office responded to the scene and examined the wreckage. The inspectors reported that one main rotor blade, still attached to a portion of the hub, was found more than 100 yards from the helicopter. The second blade was found attached to the remainder of the hub, about 150 feet from the wreckage. The rotor mast and transmission were found about 80 feet away. The mast and transmission were noted to have been torn out of the airframe.

The operator said the main rotor hub, part No. 51437-11, serial No. 2406A, was a military surplus component acquired with a total operating history of 1,571 hours. According to the aircraft records furnished by the operator, the hub had accumulated an additional 404 operating hours in service at the time of the accident.

An engineering representative of Hiller Helicopters stated that the main rotor hub, by serial number sequence, was manufactured in the early 1960s. The component has a definitive 2,500-hour life limit which cannot be extended by overhaul.

Examination of the helicopter maintenance records revealed that the helicopter had accrued a total time of 3,122 hours at the time of the accident. The last 100- and 50-hour inspections were accomplished January 10 and 26 respectively, 54 and 16 hours prior to the accident. Specific inspections for hub cracks in the area of the control rotor trunnion in accordance with AD 73- 20-03 were noted as accomplished on January 10 and 26, and, on the morning of the accident.

A witness was interviewed who was present during the preflight inspection of the helicopter prior to the accident flight. The witness is a helicopter pilot who has no prior experience with the Hiller UH-12 series. He was to ride to a job site in the accident helicopter, then perform his aerial application flights in a Bell 206 already at the field. The witness stated that while he was observing the pilot do the inspection he observed a crack in the main rotor hub. He was asked to describe the crack and its location specifically. He reported that it was about 1-inch long and was inside the conical diameter where the mast goes near some stud holes. When verbally presented with the provisions of AD 73-20-03 and the specific areas to be inspected, the witness stated that the crack was in the area described in the AD. Under questioning, the witness stated that he clearly observed the hub arms at the Tension-Torsion (TT) pin hole area and did not see any crack visible on the external side.

The witness said that after seeing the crack he questioned the pilot about it. The witness said the pilot told him, "They (the hubs) always crack there, don't worry." The witness elected not to fly in the helicopter.

The two pieces of the fractured and separated main rotor hub were recovered and sent to the National Transportation Safety Board Metallurgical Laboratory for examination and analysis. The complete metallurgical examination report is attached to this report.

The examination revealed that the hub separated due to a fracture through its center. Visual, microscopic, and SEM examinations disclosed that the fracture through the hub center stemmed from fatigue cracks which initiated from the tension-torsion pin hole on the leading edge side of one hub arm. The origin point of the fatigue crack was determined to be inside the TT pin hole. The origin point exhibited corrosion deposits typically seen in aluminum alloys. Longitudinal scratches were found on the inside bore of the hole.

In addition to the fatigue crack through the hub center, two smaller preexisting fatigue cracks were observed in the lower control rotor trunnion mounting stud holes. These cracks were found to be about 0.25 and 0.8 inches, respectively, and were in the area identified by the witness as where he observed the cracks during the preflight inspection of the helicopter.