Bell 407 technicians reported that pilots experienced uncontrolled yaw motion in the helicopter during lift off. The technicians discovered that the control tube for the tail rotor pitch change was not connected to the bell crank under the tail rotor gearbox.
Synopsis
Bell 407 technicians reported that pilots experienced uncontrolled yaw motion in the helicopter during lift off. The technicians discovered that the control tube for the tail rotor pitch change was not connected to the bell crank under the tail rotor gearbox.
Narrative
During a training flight; the pilot in command; accompanied by a new hire; experienced a loss of tail rotor effectiveness. The pilot reported attempting to reposition the aircraft from a field (approximately 75 feet) to the apron. Upon lifting off; the aircraft exhibited an uncontrolled and uncommanded yaw motion. The pilot immediately reduced collective input and successfully shut down the aircraft.There were no injuries reported; and no damage was noted to the aircraft.An investigation revealed that the Rod Assembly; connecting the Tail Rotor Trunnion and Lever assemblies; was not properly attached to the Aft Bell Crank. Although the bolt securing the rod assembly was tight and cotter keyed; a visual inspection suggested that the rod assembly was wedged in the Bell Crank; giving the false appearance of being installed. This caused a slight rubbing between the parts until they eventually released.The Bell Crank had previously been removed during the 300-hour/12-month scheduled phase inspection due to unacceptable damage. Prior to the incident; a secondary maintenance check had been conducted; including the following procedures:Tail Rotor Flight Control RiggingTail Rotor BalancingMain Rotor Track and BalancingMain and Tail Rotor Mast Nut Torque ChecksThese checks; along with the majority of the above mentioned training flight; were performed over 2.2 flight hours before the rod assembly came loose.The appearance of the rod assembly being properly installed; coupled with its apparent functionality during post-action checks; allowed the issue to persist undetected. The pilot's quick recognition of the problem and prompt action prevented further damage or injury. Time pressures on the maintenance team to quickly return the aircraft to service may have contributed to the rushed installation and subsequent checks. Although no procedure mandates forceful verification of control installations; complacency during visual inspections--such as inadequate use of a mirror or proper lighting--may have led to missed details. The mechanic who installed the Bell Crank had previous experience with the procedure; while the mechanic verifying the installation had over 20 years of experience with Bell model helicopters.
Second reporter narrative
During pilot training on Day 12 at approximately XA:00 the pilots noted a reduction in tail rotor authority during low level autorotation training. They increased torque and started to pick aircraft up to reposition from where they had touched down to the helicopter transport on the ramp when they realized they had lost all tail rotor control. They shut the aircraft down at that time and called maintenance. Upon arrival at the aircraft maintenance found that the control tube for the tail rotor pitch change was not connected to the bell crank under the tail rotor gearbox. Bolt; washers; nut and cotterpin were installed in bellcrank.Previous to this on Day 0 the tail rotor directional control bell crank was replaced due to damage identified during inspections. On Day 6 I looked over the installation prior to installing the covers and cowlings. I did not notice any issues with the installation and all hardware was installed; unable to turn by hand and had cotter pins installed.
Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.