Kekaha, HI, USA
N615CK
SIKORSKY S-61N
The accident helicopter was under contract to the United States Navy. The mission for the accident flight involved locating a training torpedo in the open waters, retrieving the torpedo using a recovery basket/cage system, then returning the torpedo to Pacific Missile Range Facility (PMRF) by sling load. According to automatic dependent surveillance-broadcast (ADS-B) data, after the helicopter departed, it proceeded north-northwest to an area about 44 miles away. After maneuvering in the area, the helicopter proceeded south-southeast to return to PMRF. As the helicopter approached the facility, it crossed the shoreline and began a shallow left turn as it maneuvered to the north, into the prevailing wind. As the helicopter neared the predetermined drop-off site, the left turn stopped, and the helicopter proceeded in a northeasterly direction. Multiple witnesses located near the accident site reported that as the helicopter continued the left turn towards the drop-off site, the turn stopped, and it began to travel in a northeast direction. The witnesses noted that as the helicopter flew about 200 ft above the ground, it gradually pitched nose down and impacted nose first, in a near-vertical attitude. An examination of the wreckage revealed the flight control fore/aft servo input link remained connected at its clevis end to the flight control fore/aft bellcrank, located adjacent to the main gearbox. However, the rod end was partially connected to the fore/aft servo input clevises and its bolt had mostly backed out of its normally installed position. The bolt exhibited no evidence of fractures or visible deformation and its threads exhibited no unusual wear. Therefore, the bolt likely backed out of its normally installed position during the accident flight due to the absence of its nut and cotter pin. This would have caused an uncommanded input to the fore/aft servo, resulting in the helicopter’s nose-down attitude, and the inability of the crew to control the pitch attitude of the helicopter. The fore/aft primary servo was installed on December 28, 2021. About 7.5 flight hours had elapsed from the time the fore/aft primary servo was installed until the day of the accident. The mechanic who installed the fore/aft servo input link to the fore/aft primary servo likely failed to correctly install the attaching hardware. The company’s certified inspector and who oversaw and inspected all of the work at completion, failed to ensure the hardware attaching the fore/aft servo input link to the fore/aft primary servo was installed correctly.
On February 22, 2022, about 1020 Hawaii-Aleutian Standard Time, a Sikorsky S-61N helicopter, N615CK, was destroyed when it was involved in an accident at the Pacific Missile Range Facility (PMRF), Barking Sands, Kekaha, Hawaii, on the island of Kauai. The two pilots and two crewmembers were fatally injured. The helicopter was operated as a Title 14 Code of Federal Regulations Part 133 flight. The accident helicopter, owned and operated by Croman Corporation, was under contract to the United States Navy, being used to retrieve inert training torpedoes from the Pacific Ocean as part of the Navy’s ongoing, Pacific submarine training operations. According to the director of operations for the operator, the accident mission involved locating a training torpedo in the open waters, retrieving the torpedo using a recovery basket/cage system, then returning the torpedo to PMRF by sling load. The helicopter was one of two helicopters stationed at a hangar in PMRF Barking Sands. At the time of the accident, three mechanics were located at PMRF Barking Sands to maintain the two helicopters. The helicopter was equipped with ADS–B, which provided aircraft tracking to determine its position via satellite navigation or other sensors and periodically broadcasts it, enabling it to be tracked. The information can be received by air traffic control ground stations as a replacement for secondary surveillance radar, as no interrogation signal is needed from the ground. According to archived Federal Aviation Administration ADS-B data, after the helicopter departed PMRF, it proceeded north-northwest to an area about 44 miles away. After maneuvering in the area, the helicopter proceeded south-southeast towards PMRF to return to the facility. As the helicopter approached PMRF, it crossed the shoreline and began a shallow left turn as it maneuvered to the north, into the prevailing wind. As the helicopter neared the predetermined drop-off site, known as the ordnance recovery clear area (ORCA), the left turn stopped, and the helicopter proceeded in a northeasterly direction before the data ended. Multiple witnesses located near the accident site consistently reported that as the helicopter continued the left turn towards the ORCA, the turn stopped, and it began to travel in a northeast direction. The witnesses noted that as the helicopter flew about 200 ft above the ground, it gradually pitched nose down and impacted nose first, in a near-vertical attitude. Figure 1. N615CK at accident site The helicopter came to rest on its left side on a heading of about 230° magnetic. Three ground scars consistent with main rotor blade impact marks were present near the initial airframe ground impact location. The nose bay door for avionics was found near the start of the debris trail, followed by pieces of debris from the cockpit structure and cockpit instruments, and then the remainder of the helicopter. The initial ground impact mark and debris trail leading up to the main wreckage was oriented about 65° magnetic. A postcrash fire consumed most of the cockpit and the cabin, though remnant frame sections were present near the main (forward) landing gear as well as the transmission deck. The cockpit voice recorder was found near the forward end of the main wreckage. The main transmission gearbox remained whole and installed on the airframe. The main gearbox exterior was coated in soot from the postcrash fire but was not consumed by the postcrash fire. The main rotor blades exhibited considerable fragmentation, with numerous fragments of main rotor blades found throughout the vicinity of the accident site. An examination of the wreckage revealed that the flight control fore/aft servo input link remained connected at its clevis end to the flight control fore/aft bellcrank, located adjacent to the main gearbox. The rod end was partially connected to the fore/aft servo input clevises, but the attaching hardware had mostly backed out of its normally installed position and the bolt was cocked (Figure 2). Figure 2. Fore/Aft Primary servo N615CK at accident site This bolt remained partially within the rod end bearing inner race, which was also cocked, exposing a portion of the bearing’s rolling elements. The bolt head-side bushing and three washers were present between the bolt head and rod end. The nut, nut-side bushing, nut-side washers, and cotter pin were not present. A search of the main transmission deck found a loose bushing within the right-side longitudinal beam. The bolt between the fore/aft servo input link and the fore/aft servo input clevises was removed and it exhibited no evidence of fractures or visible deformation of the bolt shank. According to maintenance records, from December 17-29, 2021, multiple maintenance actions were performed. The director of maintenance and another mechanic traveled from the operator’s base in Oregon to PMRF Barking Sands and worked with two additional mechanics, based in PMRF Barking Sands, to complete these maintenance actions. The fore/aft primary servo of the flight control system was installed on December 28, 2021. About 7.5 flight hours had elapsed from the time the fore/aft primary servo was installed until the day of the accident. According to both the director of maintenance and a mechanic who traveled to PMRF, when the main gearbox assembly is removed from the helicopter, the primary servos typically remained installed on the main gearbox housing. Furthermore, the primary servos were typically disconnected from the flight control system at each servo input link’s clevis connection to the main gearbox bellcranks. During a primary servo replacement, the servo input link would be removed from the old primary servo and transferred to the new primary servo. The condition of the removed hardware, such as bolts and washers would be checked and replaced as needed. One-time-use hardware such as cotter pins and nuts with nylon locking features would be discarded after each removal. After all the work on a work order was complete, a company certified inspector inspected all work performed.
The improper installation of the fore/aft primary servo by maintenance personnel, which resulted in the attaching hardware backing out and which subsequently rendered the helicopter uncontrollable. Contributing to the accident was the company’s quality control personnel to identify the improper installation before certifying the helicopter for flight.
Source: NTSB Aviation Accident Database
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