History of Flight
On May 25, 2013, at about 1745 Pacific daylight time, a Hiller UH-12E (registration N99090) experienced a hard landing after the main rotor head assembly separated from the helicopter at Pangborn Memorial Airport (EAT) in Wenatchee, Washington. The commercial pilot, who was the sole occupant of the front seat, was not injured; a passenger sustained minor injuries. The helicopter, operated by the pilot under 14 CFR Part 91, sustained substantial damage throughout. The flight originated from EAT at approximately 1730 for a personal flight under visual meteorological conditions. No flight plan was filed.
According to the pilot, after takeoff and about three miles from the airport, he heard a noise from above and behind. He immediately turned back toward the airport. When about one mile away, a second noise was heard. The pilot continued to the airport and hovered about four feet above the ground; at that point, the main rotor head and blades separated and flew upward. The helicopter then impacted the ground hard, causing the tailboom to separate at approximately mid-span. Shortly thereafter, the main rotor head and blades struck the ground about 300 feet away.
Personnel Information
The 61-year-old pilot held a commercial pilot certificate for single- and multi-engine land airplanes and helicopters, as well as a certified flight instructor certificate for helicopters (issued June 10, 2012) and an airplane instrument rating. A second-class medical certificate was issued on July 6, 2012, with a limitation requiring corrective lenses and glasses for near and intermediate vision. The pilot reported approximately 1,555 total flight hours, of which 928 were in the accident helicopter make and model.
Aircraft Information
The three-seat, high-skid helicopter (serial number 1395) was manufactured in 1962. It was powered by a Lycoming VO-540-C2A 305-horsepower engine. The most recent maintenance was a 100-hour inspection performed on November 1, 2012, at an airframe total time of 10,568 hours.
Meteorological Information
At 1755, the weather at EAT was reported as wind from 080 degrees at 5 knots, visibility 10 statute miles, broken clouds at 10,000 feet above ground level, temperature 18°C, dewpoint 3°C, and an altimeter setting of 29.93 inches of mercury.
Wreckage and Impact Information
The National Transportation Safety Board (NTSB) did not conduct an on-scene investigation. Airport management reported that the main fuselage was mostly intact with minimal damage; the landing skids showed no apparent deformation. The tailboom was fractured at mid-span, but the tail rotor drive shaft remained connected along its entire length. The main rotor hub and blades had separated from the main rotor shaft and came to rest about 300 feet from the fuselage.
Tests and Research
Postaccident examination revealed that the main transmission remained attached to the helicopter. The transmission housing had multiple vertical fractures on its center (second stage planetary gear) housing, primarily through thin-walled sections. The main rotor shaft and tail rotor drive shaft were still connected to the transmission. After disconnecting the tail rotor drive shaft, investigators could not rotate the main rotor shaft by hand in either direction. The fracture surface of the main rotor shaft was removed for further examination.
The tail rotor drive assembly was removed from the transmission lower housing; metallic chips and flakes were found in the residual oil and on gear teeth, with one piece showing heat discoloration. The main transmission was removed for detailed examination.
At Hiller Aircraft Corporation's facilities, the transmission was disassembled. Substantial damage was observed to the first stage planetary gear teeth and the input gear teeth. A high quantity of metallic chips and flakes was found at the bottom of the lower housing and throughout the first stage planetary assembly. All six bolts securing the bevel gear to the first stage planetary carrier were fractured; only one of the separated bolt heads remained secured by safety wiring, and the other five were not located. Oil supply lines and nozzles showed no blockages, and components above the first stage planetary assembly exhibited no signs of oil starvation. The center housing, bevel gear, fractured bolts, and the main rotor shaft fracture surface were retained for metallurgical examination at the NTSB materials laboratory.
At the NTSB materials lab, the six bolts were removed and cleaned. All bolt fracture surfaces exhibited comparable features, including fatigue crack arrest and beach marks surrounding a center region consistent with overstress. Fatigue cracks initiated along the thread roots, positioned about 180 degrees from each other, consistent with reverse bending fatigue.
The transmission center housing contained three longitudinal cracks, each across a bolt hole in the flange, progressing through most of the cross section to the opposite flange. Two cracks were backcut; their faces showed overstress fracture from hoop stresses, with no indications of corrosion or progressive failure.
The section of main rotor shaft about two inches from the fracture surface was analyzed. The fracture surface was oriented 45 degrees from the shaft's longitudinal direction, generally dark with a rough texture, and the shaft was deformed to one side—features consistent with overstress failure in tension.