CEN24LA128

The failure of maintenance personnel to properly install the tail rotor pitch change slider attachment hardware (T-bolt), which led to the disconnection of the pitch change slider, a loss of tail rotor control, and subsequent hard landing. Contributing to the accident was maintenance personnel’s failure to complete a maintenance discrepancy entry on the work order for the removal of the T-bolt.

The pilot reported that while in a hover taxi to accelerate for takeoff, he felt a force against his feet from the pedals and the helicopter began to yaw to the right. The pilot was unable to arrest the right yaw with pedal inputs. The helicopter landed hard, which resulted in substantial damage to the fuselage, tailboom, vertical fin, horizontal stabilizer, tail rotor assembly, and one main rotor blade. Postaccident examination revealed that the T-bolt and its attachment bolts, which connects the pitch change bellcrank to the pitch change slider, became disconnected during operation due to inadequate installation. Specifically, the T-bolt attachment bolts were installed but not torqued and safety wired to maintenance manual requirements, allowing them to back out during operation. Once the T-bolt attachment bolts backed out, the T-bolt also backed out and separated from the pitch change slider, resulting in a loss of tail rotor pitch control. The T-bolt likely impacted one of the tail rotor blades, resulting in the separation of the outboard portion of its leading edge and afterbody. The subsequent imbalance of the tail rotor, due to the damaged tail rotor blade, led to overload separation of the upper portion of the vertical fin. During maintenance of the accident helicopter in the days leading up to the accident, the T-bolt and its attachment bolts were removed by a mechanic at the direction of a lead mechanic to facilitate troubleshooting of adjacent components for the tail rotor control system. That mechanic assumed that the T-bolt’s removal was temporary and quick, so he did not follow established procedures by recording the removal in the discrepancy sheet for the work order. The T-bolt attachment bolts were subsequently temporarily installed “finger-tight” by another mechanic to assist a mechanic who was installing the tail rotor blade mounting forks and pitch change links. However, the first mechanic was tasked to another company helicopter shortly after, and while he stated he relayed to the other mechanics that the T-bolt attachment bolts were finger-tight, no one followed up on the installation of the T-bolt attachment bolts to ensure they were torqued correctly and safety wired. Additionally, no one from the maintenance team found that the safety wiring for the T-bolt attachment bolts was missing during final checks of the helicopter before it was released for an operational check flight. Since there was no maintenance discrepancy entry for the removal of the T-bolt, there was no formalized task for the maintenance team to track and verify the complete T-bolt installation, including torquing and safety wiring the T-bolt attachment bolts. Therefore, the lack of an entry in the work order discrepancy sheet for the T-bolt’s removal contributed to this accident.

On March 6, 2024, about 2140 eastern standard time, a Eurocopter Deutschland Gmbh MBB-BK117 C-2 helicopter, N191LL, was substantially damaged when it was involved in an accident at Purdue University Airport (LAF), West Lafayette, Indiana. The pilot and two medical crew members were not injured. The flight was operated as a Title 14 Code of Federal Regulations Part 135 helicopter air ambulance flight. The pilot reported that, while in a hover taxi to accelerate for takeoff, he felt a force against his feet from the pedals. The helicopter yawed to the right, so he applied full left pedal, but when the pedal was depressed, there was no resistance and no effect on the helicopter’s yaw. The helicopter landed hard, and the crew egressed without further incident. An on-scene examination by the responding FAA inspector revealed that the helicopter came to rest upright and sustained substantial damage to the fuselage, tailboom, vertical fin, horizontal stabilizer, tail rotor assembly, and one main rotor blade. The tail rotor pitch change bellcrank was not connected to the pitch change slider. The T-bolt (figure 1) and its two attachment bolts were not installed. One of the bolts was found on the ramp near the accident site, but the second bolt and T-bolt were not initially found at the scene. The operator stated to the FAA that there had been recent maintenance completed to the tail rotor assembly. Figure 1. A diagram of the installation of the T-bolt and associated attachment hardware onto the pitch change slider (Courtesy of Airbus, modified by NTSB). Postaccident examination revealed that the tail rotor sliding sleeve assembly remained installed on the tail rotor shaft. The two tail rotor pitch links remained installed between the sliding sleeve assembly and each blade mounting fork. The sliding sleeve assembly was able to be manually moved in the axial (pitch change) and rotational directions with no evidence of binding or restriction. The pitch change bellcrank remained installed on the vertical fin and was connected to the tail rotor hydraulic actuator’s output piston. The pitch change bellcrank was not connected to the sliding sleeve assembly. The spherical bearing for the pitch change bellcrank, to which the stationary sliding sleeve attaches, exhibited smooth movement and did not exhibit anomalous damage. The two attachment bolt holes, on the outer ring of the stationary section of the sliding sleeve assembly, did not exhibit elongation and the internal threads did not exhibit evidence of anomalous damage. The installation hole for the T-bolt, on the outer ring of the stationary section of the sliding sleeve assembly, appeared intact and did not exhibit anomalous damage such as elongation or ovalized appearance. Remnant grease was observed within the bottom T-bolt hole. One T-bolt attachment bolt was found on the ground at the accident site and did not exhibit anomalous deformation or damage, such as excessively worn threads. The second T-bolt attachment bolt was found within the area of the No.1 engine bay, and did not exhibit excessively worn threads or fracture, but one side of its threads exhibited a flattened appearance. Neither of the T-bolt attachment bolts showed remnant safety wiring attached to them or within their safety wire holes. The No. 1 engine exhaust cowling exhibited a puncture, and the engine exhaust duct exhibited an impact mark, both consistent in size with contact by the attachment bolt. The T-bolt was found lodged into a honeycomb panel at the aft-lower area of the fuselage, adjacent to the fuel cell. A puncture into the fuselage skin, just below the aft clamshell doors, was found near the T-bolt. Except for the pitch change slider connection to the pitch change bellcrank, all fasteners remained installed with their appropriate safety wiring or cotter pins. The pedals in the cockpit were moved both full left and full right pedal forward, resulting in movement of the pitch change bellcrank at the vertical fin. The accident helicopter was maintained by the operator under an approved aircraft inspection program (AAIP). On December 15, 2023, at an aircraft total time (ATT) of 7,314.5 hours, a work order was opened, and maintenance was conducted to troubleshoot a discrepancy in which the helicopter was not holding a heading in a hover with feedback in the pedals. The yaw control flexball cable was found to exhibit binding and was replaced on December 19, 2023. Under the same work order, the yaw trim actuator was found serviceable, but removed on December 15, 2023, for use on another company helicopter. The same yaw trim actuator was reinstalled on the accident helicopter on December 20, 2023, and the helicopter was released for service on the same day. From February 29 to March 6, 2024, maintenance to the accident helicopter was conducted at the operator’s hangar in Indianapolis, Indiana. The helicopter had an ATT of 7,434.5 hours at the time this maintenance was conducted. This maintenance included a 3-year weighing and troubleshooting of a discrepancy that involved feedback from the pedals. For troubleshooting the pedal feedback discrepancy, the following was accomplished by the operator’s mechanics in several iterations, which included: - Verification of the yaw smart electro-mechanical actuator (SEMA) angle - Yaw ball bearing (flexball) control cable installation verification - Tail rotor control rigging verification - Replacement of the yaw flexball cable - Replacement of the yaw SEMA - Checking the tail rotor breakout pedal forces - A friction check of the tail rotor blade mounting forks (blade grips) - Removal of yaw trim actuator and installation of serviceable yaw trim actuator Until the replacement of the yaw trim actuator, each troubleshooting step ended with a release for an operational check flight and a subsequent determination of an unsatisfactory operational check flight due to persistence of the pedal feedback discrepancy. A task for the removal of the T-bolt and its attachment hardware was not present in the work order. However, according to interviews with the mechanics involved in the work order, the T-bolt and its two attachment bolts were removed to facilitate the friction check of the tail rotor blade mounting forks. The removal of the T-bolt was not documented on the work order. The mechanic tasked with reinstalling the tail rotor blade mounting forks and pitch change links needed assistance with holding the position of the tail rotor head. Another mechanic reinstalled the T-bolt, with its attachment bolts installed “finger-tight,” to assist in holding the position of the tail rotor head. The mechanic who installed the T-bolt stated that shortly after the T-bolt installation, he was moved to another task on another company helicopter that had just been pulled into the maintenance hangar. Statements from maintenance personnel revealed that the attachment bolts for the T-bolt were not torqued nor were they safety wired. At the end of the maintenance troubleshooting on the accident helicopter, it was determined that the yaw trim actuator replacement had fixed the pedal feedback discrepancy with a satisfactory result for the operational check flight. The helicopter was released and conducted a flight from the maintenance base to LAF for refueling on March 6, 2024. The subsequent flight after refueling was the accident flight.