So What Happened?
The investigation focused on the controls. All agreed that the experienced PIC would never have slammed the collective down, jerked it back up and pulled the cyclic full aft. So had that control manipulation sequence led to the accident?
The investigators conducted a biomechanical study and determined that it was feasible that the child sitting on the owner's lap in the left front cockpit seat could fully depress the left-side collective control by stepping on it with her left foot. The collective has a breakout force of between 2.2 and 3.1 lb. and would only need a maximum force of 5 lb. to move the control fully. Thus, the force to displace the collective fully was a maximum of 8.1 lb., which is much less than the child's total weight and less than she would exert with her left foot if pushing to stand up from a seated position.
The study also found that the collective lever's full range of motion was 9.5 in. from full up to full down and that the spacing between the left edge of the seat, the collective and the door are sufficient for the child's foot to rest on the collective and depress it. The study noted that the cyclic control could be moved to the full-aft position even with a small child of this size sitting on her father's lap in various positions.
Because the spacing between the upper partition, which separated the cockpit from the aft cabin compartment, and the ceiling was about 5 in., it is unlikely that the child could shift from the left front cockpit seat to one of the rear seats during the flight. Considering that the child was sitting on the owner's lap in the left front cockpit seat, “it is highly likely that the child inadvertently stepped on the collective with her left foot and displaced it to the full down position,” said the Safety Board. This condition would have then resulted in either the pilot or the helicopter owner raising the collective, followed by a full-aft input pull of the cyclic control and the subsequent main rotor departing the normal plane of rotation and striking the left endplate and the aft end of the tail-rotor driveshaft.
During its investigation of this accident, the NTSB found that the PIC was involved in two incidents while operating the accident helicopter; neither incident was reported to the FAA. On May 8, 2003, the helicopter owner was operating the aircraft, and his seat slid aft while on final approach to landing. The helicopter dropped about 50 ft. before impacting terrain, resulting in damage to the horizontal stabilizer. In this incident, the PIC failed to use proper cockpit discipline when he allowed the helicopter owner to operate the helicopter's controls, particularly during a critical phase of flight.
Although it could not be determined who was flying the helicopter at the time of the accident (and it is not relevant to the cause of this accident), the previous incidents, the statement by the pilot that the helicopter owner dominated cockpit duties, and the PIC allowing the owner's daughter to sit on his lap during flight together indicate that the PIC did not maintain strong cockpit discipline.
The Safety Board determined the probable cause(s) of the fatal accident to be: “The sudden and inadvertent lowering of the collective to near the lower stop, followed by a simultaneous movement of the collective back up and the cyclic control to a nearly full-aft position, which resulted in the main rotor disc diverging from its normal plane of rotation and striking the tail-rotor driveshaft and culminated in a loss of control and subsequent impact with terrain. Contributing to the accident was absence of proper cockpit discipline from the pilot.”
So, the take-home in this story, it would seem, is that pilots need the command presence to be able to say “No” effectively: “No” to unreasonable operational requests; “No” to untrained owners who want to manipulate the controls in critical, non-training environments; “No” to rushed operations that force the crew to ignore SOPs and widely understood best practices.