Accident Overview
On August 25, 2015, about 0750 central daylight time, a Robinson Helicopter R44 II, registration N444KD, sustained substantial damage when it impacted terrain approximately 0.8 nautical miles northeast of the South Grand Lake Regional Airport (1K8) in Ketchum, Oklahoma. The accident occurred after a loss of the tail rotor gearbox. The pilot, who was the sole occupant, received minor injuries. The helicopter was registered to Marcotte Veterinary Clinic PC and operated by the pilot under 14 Code of Federal Regulations Part 91 as a personal flight. Visual meteorological conditions prevailed, and no flight plan was filed. The flight had departed from 1K8 about 1748 on a local flight.
Pilot Actions and Flight Sequence
According to the pilot, he conducted a preflight inspection at his home before flying to 1K8 to refuel. Upon landing near the self-service fuel pumps, he found the pumps inoperative. He then started the helicopter, checked the magnetos and sprague clutch, and departed for Grove, Oklahoma, about 16 miles away. The pilot reported that he did not loosen or remove the fuel cap during preflight and relied on the fuel quantity gauge to check fuel level. The takeoff from 1K8 was normal, with calm winds, and he climbed to a cruising altitude of 300–400 feet above ground level (agl) at 80–90 knots. About half a mile from the airport, heading northeast, he heard a sudden "boom" without prior abnormal vibrations or noises. A sudden yaw occurred, though the pilot was uncertain of the direction. He immediately lowered the collective, rolled the throttle to idle, and entered an autorotation. The nose dropped, and he pulled back on the cyclic to keep the helicopter level. He flared at approximately 40 knots and 40 feet agl, after which the nose yawed hard left. The skids contacted the ground level, and the helicopter rolled onto its right side in a flat, grass pasture. The pilot estimated the time from the "bang" to landing at 15–20 seconds. He had never practiced an autorotation from 300 feet agl and reported no bird strikes or tree contacts.
Pilot Experience
The pilot held a private certificate with a helicopter rating and had logged 144 total flight hours, with 89 hours in the accident helicopter.
Weather Conditions
At 0735, the surface weather observation at Grove Municipal Airport (GMJ), about 23 nautical miles east of 1K8, reported wind light and variable, visibility 10 miles, clear skies, temperature 12°C, dew point 12°C, and an altimeter setting of 30.17 inches of mercury.
Wreckage and Component Examination
Examination of the accident site revealed a debris path approximately 1,700 feet long on a 063-degree heading. The first parts found, about 0.5 miles from 1K8, were tail boom sheet metal coverings forward of the drive shaft flange. The fuel cap was located 125 feet from the initial parts. The tail rotor gearbox and tail rotor blades were found 340 feet from the initial parts. The vertical and horizontal stabilizers were 1,600 feet from the initial parts. The main wreckage came to rest on its right side with the cabin intact. The left fuel cap was not attached to the filler neck and was not located at the main wreckage site. Both main rotor blades remained attached to the main transmission; one blade was bent opposite the direction of rotation and partially separated about 5 feet from the tip, while the other was bent but intact. A 6-foot section of the tail boom was near the main wreckage, along with tail boom guard parts. A prominent ground scar was observed near the end of the tail boom still attached to the fuselage.
Metallurgical examination at the National Transportation Safety Board's Materials Laboratory focused on the tail cone, tail rotor gearbox, hub and blades, and fuel cap. The tail cone was fractured circumferentially just forward of the aft bulkhead. The tail rotor drive shaft was fractured at the aft flexible coupling, and the tail rotor gearbox was fractured from the aft bulkhead. The tail rotor blades were fractured into multiple pieces, with the tip missing from one blade (blade "B"). Fracture surfaces on the aft bulkhead and gearbox flange were consistent with overstress failure or post-fracture damage, with no evidence of preexisting cracks. The tail rotor hub showed substantial teetering but was intact.
Examination of blade leading edges for foreign object damage revealed small insect residues under black light, but no large fluorescing areas. No feathers, vegetation, or significant paint transfer (other than red or white, consistent with contact with the blade guard or empennage) were found. Blade skin fractures were on slant angles typical of ductile overstress.
The fuel cap showed no heavy impact marks or paint transfer from tail rotor blades. When placed on an exemplar R44 left fuel tank filler neck, it fit snugly and required considerable pressure to loosen.
Service Bulletin Compliance
Robinson Helicopter Company Service Bulletin SB-84, issued May 30, 2012, addressed fuel cap decals, stating that "Fuel cap alignment stripes are required by regulation and necessary for the pilot to verify fuel caps are properly secured. An improperly secured cap may come off in flight." The bulletin required inspection and installation of decals to aid in using the stripes. The accident helicopter had not complied with the bulletin; no alignment stripes or decals were present on the fuel cap.
Probable Cause
The investigated source does not state a probable cause.