NTSB Factual Report

On August 25, 2015, about 0750 central daylight time, a Robinson Helicopter R44 II, N444KD, sustained substantial damage when it impacted terrain about 0.8 nm northeast of the South Grand Lake Regional Airport (1K8), Ketchum, Oklahoma, after a loss of the tailrotor gearbox. The pilot received minor injuries. The helicopter was registered to the Marcotte Veterinary Clinic PC and operated by the pilot under the provisions of the 14 Code of Federal Regulations Part 91 as a personal flight. Visual meteorological conditions prevailed at the time of the accident, and no flight plan was filed. The flight departed from 1K8 about 1748 on a local flight.

The pilot reported that he conducted a preflight of the helicopter before departing from his home located about 8 miles from IK8. He flew to IK8 where he intended to refuel the helicopter. The pilot stated that he landed near the self-service fuel pumps and grounded the helicopter. He tried to swipe his credit card in the self-service fuel pumps but the fuel pumps were not operating. He ungrounded the helicopter, started the helicopter, and checked the magnetos and sprague clutch before departing to Grove, Oklahoma, located about 16 miles from IK8. The pilot stated that he had not loosened the fuel cap prior to departing from 1K8, and additionally, he did not remove the fuel cap when he performed his aircraft preflight. He stated that he checked the fuel level by observing the fuel quantity gauge during his preflight.

The pilot reported that there was nothing abnormal about the takeoff from 1K8. The wind was calm and he was flying about 300-400 ft agl (his typical cruising altitude) with 80 – 90 kts airspeed. He had flown about 1/2 mile from the airport heading northeast when there was a sudden "boom." There were no abnormal vibrations or noises before the "boom." He stated that there was a sudden yaw; although, he was uncertain if it was to the left or right. 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 estimated that he flared the helicopter around 40 kts and 40 ft agl and the nose yawed hard left. He pulled the collective – although he wasn't exactly sure if he did or not. The skids hit about level and the helicopter rolled over on its right side in a flat, grass pasture.

The pilot stated the he wasn't sure how long it was from the "bang" to landing with any certainty, but estimated 15 – 20 seconds, and not more than 30 seconds. He wasn't sure about how high he was when he flared, and he had never done a practice autorotation from 300 ft agl. He stated that he didn't hit a tree and he didn't see any birds close to the helicopter, although there were some buzzards a few hundred yards away. He wasn't sure if there was anything mechanically wrong with the tail rotor or gearbox that caused the problem.

The pilot held a private certificate with a helicopter rating. He had 144 total hours of flight time with 89 hours in the accident helicopter.

At 0735, the surface weather observation at the Grove Municipal Airport (GMJ) located about 23 nm east of 1K8, was: wind light and variable, 10 miles visibility, skies clear, temperature 12 degrees C, dew point 12 degrees C, altimeter 30.17 inches of mercury.

The examination of the accident site revealed that the first parts found in the wreckage path were located about 0.5 miles from 1K8, and were primarily tail boom sheet metal coverings of the boom structure just forward of the drive shaft flange. The debris path from the initial tail boom parts to the main wreckage was about 1,700 ft on a 063 degree heading. The helicopter's fuel cap was found about 125 ft from the initial tail boom parts. The tail rotor gearbox and tail rotor blades were found about 340 ft from the initial tail boom parts. The vertical stabilizer and horizontal stabilizer were found about 1,600 feet from the initial tail boom parts.

The helicopter was on its right side and the cabin remained intact. The left tank fuel cap was not attached to the filler neck and not located at the main wreckage site. The main rotor and blades remained attached the main transmission. One of the main rotor blades was bent opposite the direction of rotation and partially separated about 5 ft from the end of the blade tip. The other main rotor blade was bent but otherwise intact. A 6 ft section of the end of the tail boom was found near the main wreckage. The tail boom guard parts were located near the main wreckage. A prominent ground scar/slash was observed next to the end of the tail boom that remained attached to the fuselage.

The tail boom attach point (bulkhead), tail rotor gearbox, tail rotor hub and blades, and fuel cap were sent to the National Transportation Safety Board's Materials Laboratory for metallurgical examination. Examination revealed that the tail cone was fractured around its circumference 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, and the tip was missing from one of the blades (blade labeled "B").

The examinations of the aft bulkhead fracture surfaces revealed that fractures were consistent with overstress failure or post-fracture damage, and there was no evidence of a preexisting crack. The tail rotor gearbox flange fracture surfaces were rough and matte gray consistent with overstress fractures and post-fracture damage. The tail rotor hub was intact, but showed evidence of substantial teetering.

The leading edges and fractures from blades A and B were examined closely for evidence of contact with a foreign object. Small areas consistent with insects fluoresced when examined under a black light, but no larger areas of fluorescing material were observed. No evidence of feathers or vegetation was found. Paint transfer observed on the blade surfaces was either red or white, consistent with contact with the blade guard or the empennage structure (the helicopter was painted white). Fractures of the skin on both blades formed on slant angles consistent with ductile overstress fractures.

A visual examination of the fuel cap was conducted. No evidence of heavy impact marks or paint transfer from the tail rotor blades were observed on the fuel cap.The fuel cap was placed on the exemplar R-44 left fuel tank filler's neck and twisted until it was tight. The fuel cap fit snugly and could not be loosened without exerting considerable pressure.

On May 30, 2012, the Robinson Helicopter Company issued Service Bulletin SB-84, which concerned fuel cap decals. The service bulletin stated the following:

"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. This bulletin requires inspection of fuel cap alignment stripes, refurbishing as required, and installation of decals to aid in use of the stripes."

The service bulletin had not been complied with and there were no alignment stripes or decals on the fuel cap.

NTSB Probable Cause

The separation of the tail rotor gearbox from the tailboom aft bulkhead due to overstress loads imposed by a blade imbalance from an in-flight impact with an undetermined object.