Robinson Helicopter Company R44 II N444KD accident near Ketchum, Oklahoma

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.

NTSB analysis

The private pilot stated that, before the flight, he had intended to refuel the helicopter but that the pumps were not operating. He added that he did not loosen the fuel cap before departure and that 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 private pilot reported that he was flying between about 300 and 400 ft above ground level (agl) at 80 to 90 kts when there was a sudden "boom," followed by a yaw. 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 the helicopter around 40 kts and 40 ft agl and the nose yawed hard left. He pulled the collective and the skids hit about level. The helicopter rolled over on its right side in a flat, grass pasture.

The first parts of the tailboom (including the helicopter's left tank fuel cap) were located about 1,700 ft from the main wreckage. The tail rotor gearbox and tail rotor blades were found near the initial tailboom parts. The metallurgical examination of the tailboom attachment point (bulkhead), tail rotor gearbox, tail rotor hub, and tail rotor blades revealed that the fractures were consistent from overstress fractures with no evidence of preexisting damage.

The tail rotor separation from the helicopter was likely due to a severe blade imbalance associated with an impact to one of the blades. However, examination of the components could not determine what object was impacted. No evidence of impact with a foreign object such as a bird, vegetation, or drone was observed, although such an impact could not be entirely ruled out since one of the blade tips was missing and subsequent blade contact with the vertical stabilizer could have masked a prior impact. Although the fuel cap was found near the start of the debris trail, no evidence of impact damage or paint transfer corresponding to contact between a blade and the fuel cap was identified.

Although the pilot stated that he had not loosened the fuel cap before departing on the accident flight, the fuel cap was found 1,700 ft from the main wreckage. While the exact sequence of events could not be determined because there was no impact mark on the fuel cap, it is likely that the fuel cap was not properly secured at some point and subsequently departed the helicopter during the accident flight. Whether or not it was the fuel cap that impacted the tail rotor blade or some other unknown object could not be determined based on the available evidence.

Text from the NTSB. Open the NTSB docket for CEN15LA379 for the full record.