Sikorsky S-61N N615CK accident near Kekaha, Hawaii

Probable cause

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.

NTSB analysis

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.

Text from the NTSB. Open the NTSB docket for ANC22FA018 for the full record.