WPR10FA440 · Hiller UH 12E

Two state biologists planned to conduct an aerial wildlife survey in a commercially-owned helicopter equipped with a three-abreast bench seat and a fully enclosed cabin. After briefing with the biologists, the pilot stowed most of the biologists' equipment and personal effects on the helicopter's external racks, and all three boarded the helicopter, with the biologists in each of the outboard seats. The plan was to conduct a fuel stop at one of the operator's fuel trucks located about 80 miles east of the departure airport, and then conduct the survey in the region near the fuel truck. The helicopter departed, and 6 minutes later, the state communications center received the first automated flight-following transmission. About 33 minutes later, the pilot broadcast that the helicopter was “landing at Kamiah,” which was about 35 miles short of the planned destination. An exact correlation between the time of the transmission and the accident time could not be determined, but the transmission was very likely within 4 minutes of the accident, and possibly much closer. No further transmissions were received from the helicopter. Several witnesses observed the helicopter transiting west to east, then heard unusual noises emanating from the helicopter and observed objects separating or falling from it. Several witnesses reported that it was rotating as it descended; one witness stated that the nose was “dipping” up and down, and other witnesses reported that the trajectory steepened as the helicopter descended. The main wreckage was found in the driveway of a residence, and a 1,500-foot debris field was oriented back along the helicopter's flight path; some of the items at the beginning of the debris field included tail rotor blade and tail rotor gearbox segments, and fragments of a metal clipboard that belonged to one of the biologists.

Witness marks on the tail rotor and clipboard clearly indicated that the clipboard struck and separated the tail rotor, which resulted in the loss of control of the helicopter. Helicopter geometry and aerodynamics suggested that the clipboard originated from the left side of the helicopter.

The investigation was unable to determine why the helicopter diverted to Kamiah. One of the biologists was reported to be susceptible to airsickness. Anti-nausea wristbands were found in the external luggage, but they could not be definitively associated with any particular person on the helicopter. The landing diversion could have been to allow a biologist to access the wristbands, to prevent the biologist from getting sick in the helicopter, or to allow the biologist to discontinue the flight altogether. Other speculative reasons for the diversion include a problem with the helicopter, the need for one of the biologists to retrieve something other than the medication from the externally-stowed luggage, or the need to retrieve the clipboard that was inadvertently left unsecured on one of the external racks. Because the fuel stop was planned to occur prior to beginning the survey, that stop would have provided the opportunity to retrieve any survey-related articles from the stowed luggage; thus it is unlikely that survey equipment was needed at that time. Aside from the clipboard-induced tail rotor system damage, examination of the helicopter and engine did not reveal evidence of any preimpact condition or failure that would have precluded normal operation or continued flight. There was no evidence that anyone actually got sick during the flight. Therefore, the landing diversion was likely either to provide an opportunity to somehow address the airsickness issue, or to retrieve the misplaced clipboard.

It could not be determined whether the clipboard originated from inside or outside the cabin. If the clipboard were inside the cabin at the beginning of the flight, the only exit path would be via an open door. If a door were opened either intentionally or unintentionally, the clipboard could have exited either because it was near the door or because it was already resting on the bubble window at the time. Although one witness stated that the right cabin door was open in flight, the damage patterns indicated that both cabin doors were closed at impact. However, this does not mean that the door was not opened during the flight, and then closed during the descent.

Although a witness stated that all external cargo items were secure before takeoff, it is possible that the clipboard was inadvertently left on an external baggage rack, went unnoticed until the helicopter was in flight, and was the reason for the diversion. Flight path data indicated that the helicopter was in a continuous climb until about 5 minutes prior to the accident. Since the cargo racks had mesh floors, the clipboard might have been held in place by the external airloads during the climbing portion of the flight and was only dislodged by altered airflow during the descent for the diversionary landing.

Although two witnesses saw geese in the vicinity of the helicopter, there was no evidence of a bird strike.

Several noteworthy safety-related discrepancies were revealed during the investigation, even though they did not directly contribute to the accident or its severity. The genealogy of the helicopter could not be clearly established and remained suspect. Although the helicopter bore what appeared to be the manufacturer's original data plate, the accident helicopter configuration differed significantly from the as-delivered configuration, and there were no appropriate means of converting the as-delivered configuration to the as-found configuration. In addition, the helicopter was not in conformance with at least two contractual requirements regarding flight safety equipment. Finally, the state flight-following service, which was responsible for monitoring the progress of the flight, had to be prompted by the operator to determine the status of the flight after the helicopter disappeared from the operator's flight-following display.

Verbatim NTSB analysis and probable cause from the NTSB dataset