N908CH

HISTORY OF FLIGHTOn April 20, 2020, about 0802 local time, a Sikorsky S-61N, N908CH, entered an uncommanded left yaw while approaching Camp Dwyer, Afghanistan. The flight crew made an emergency landing, during which the helicopter impacted the ground and rolled on its right side. The two pilots and the crew chief aboard the helicopter were seriously injured, and the helicopter sustained substantial damage. The flight was operated by Construction Helicopters Incorporated, doing business as CHI Aviation, under the provisions of Title 14 Code of Federal Regulations Part 135 and a contract with the US Department of Defense. In accordance with Annex 13 to the International Civil Aviation Organization, the National Transportation Safety Board (NTSB) accepted delegation of this accident investigation from the Afghanistan Civil Aviation Authority.

The purpose of the flight was to transport cargo from Camp Bastion (also in Afghanistan) to Camp Dwyer. The helicopter was installed with a cockpit voice recorder (CVR), which showed that, at 0746:13, the right seat pilot called, “Dwyer’s in sight,” which the left seat pilot acknowledged.

The helicopter was also equipped with an Appareo Vision 1000 image recorder, which was installed on the ceiling of the cockpit. The image recorder was forward looking with a full view of the instrument panel, a partial view of the left and right seat cockpit controls, and a partial view of the outside via the lower portion of the windscreen. The accident recording consisted of parametric data, still images, and audio. The image recorder transcription began at 0751:44; at that time, all engine and transmission cockpit instruments appeared normal. The auxiliary hydraulic pressure gauge indicator in the cockpit showed about 1,500 pounds per square inch (psi), which indicated normal operation.

According to the CVR, at 0758:37, the flight crew received clearance to land. At 0801:57, the CVR recorded an unintelligible word from either the left or right seat pilot. The image recorder showed that, at that time, the helicopter descended to an altitude of 2,600 ft mean sea level and that the auxiliary hydraulic pressure decreased to about 1,300 psi, which was near the bottom of the normal operating range. Also, the left seat pilot’s left and right feet had been resting on the pedals, but the left pedal began to move forward without pilot input. Within the next second, the left seat pilot’s left pedal moved uncommanded to the fully forward position, and the pilot’s foot lost contact with the pedal after the sudden movement.

At 0801:59, the image recording showed that both pilots were jostled in their seats and that the left seat pilot placed his left foot on the left pedal again and made a momentary forward cyclic input. (The cyclic grip and the left seat pilot’s hand were then out of the camera’s view.) The right seat pilot’s left foot was not on the left pedal for that position. The heading indicator showed a left yaw that continued until ground impact along with pitch and roll excursions. The engine and transmission cockpit indications remained within normal limits for the rest of the flight.

At 0802:00, the CVR and image recording showed that the left seat pilot stated, “do you have the controls?” At that time, both pilots’ feet were visible on their respective pedals. The helicopter had rotated about 90 degrees from the previous heading and was pitched down approximately 20 degrees. Three seconds later, the left seat pilot stated, “let’s get this/us down on the ground,” and the right seat pilot’s arm was near the speed selector levers (engine throttles). Afterward, the left yaw rate substantially increased from an average of about 20° per second to about 80° per second.

At 0802:05, an expletive was heard on the image recording. The helicopter had pitched up to 36 degrees nose up, had completed one full rotation from the cruise heading, and had increased right roll to 36 degrees. One second later, the auxiliary hydraulic pressure caution lights illuminated on the master warning panel; the auxiliary hydraulic pressure gauge was not visible at this time and for the remainder of the recording. As the helicopter continued to rotate to the left, the roll reached a maximum value of 73 degrees right roll by 0802:06.7, and had pitched up to a maximum of 75 degrees nose up about one second later. The image recording ended at 0802:08, about 9 seconds after the left pedal moved to the fully forward position. The CVR recording ended at 0802:10.

According to the operator, during a postaccident interview, the accident pilots recalled that after a loud bang the helicopter initially yawed to the left with a slight roll to the right. The accident pilots also recalled that the right seat pilot had moved both speed selector levers to the off position during the uncommand yaw to start the autorotation procedure. Both pilots stated they had no pedal or cyclic authority, and the aircraft started a right spin. The cockpit image recorder and data showed that the helicopter did not change yaw direction during the event, and the recorded data showed the helicopter experienced several excursions of pitch, roll, and of acceleration magnitude and direction. AIRCRAFT INFORMATIONDirectional Flight Control System

Inputs from the cockpit cyclic control, collective control, and pedals are transmitted to the auxiliary servo cylinder assembly via control tubes. The auxiliary servo cylinder is mechanically connected to the directional (tail rotor) control system and is the system’s only source of hydraulic assistance.

Figure 1. S-61N Flight control system showing the major components of the cockpit flight controls and the directional control system (Image courtesy of Sikorsky and edited by the NTSB)

Within the auxiliary servo cylinder yaw channel, pressurized hydraulic fluid ports are located in the bypass valve, pedal damper, and the input valve. A hydraulic fluid return port is located between the input valve and power piston. The piston pushes hydraulic fluid from one side of the piston to the other side through a restrictor, and an internal spring allows for limited movement without hydraulic dampening. This pedal damper design was intended to prevent sudden, large-displacement pedal movements by pilots.

Movement of the input linkage via the pedals results in movement of the input valve, porting pressurized hydraulic fluid to one side of the power piston and exposing the return port to the other side of the power piston. As a result, the power piston hydraulically actuates the control cables and control tubes of the directional control system to change the pitch of the tail rotor blades for directional control of the helicopter.

Helicopter Maintenance

The operator’s S-61 continuous airworthiness maintenance program (which was approved by the Federal Aviation Administration) required a safety inspection at 15-hour intervals. The safety inspection comprised general visual inspections of components and fluid levels throughout the helicopter, including an inspection of the auxiliary servo cylinder. According to the helicopter’s daily flight log, a recurrent 15-hour safety inspection was last performed on April 18, 2020, 2 days before the accident.

The continuous airworthiness maintenance program also included five phased inspections performed at 30-hour intervals. Each phased inspection (identified as phases I through V) addressed one or more specific areas of the helicopter. During the phase V inspection (performed every 150 hours), the directional control cables, pulleys, rods and rod ends, and control quadrants and their supports were inspected for security, damage, and wear. The hydraulic accessories, lines, and fittings were inspected for leaks and damage, and their general condition was assessed. The phase V inspection was last performed on April 11, 2020, 9 days before the accident.

According to the operator’s continuous airworthiness maintenance program manual, the auxiliary servo cylinder assembly had a 2,500-hour interval for overhaul. The accident auxiliary servo cylinder assembly was last overhauled from May to September 2017; at that time, the auxiliary servo cylinder assembly had a time since new of 34,184 hours. The overhaul included a fluorescent penetrant inspection of the pedal damper check valve housing, which found no evidence of cracks or fractures. On the day before the accident (April 19, 2020), the auxiliary servo cylinder assembly had a time since new of 35,455 hours and a time since overhaul of 1,270 hours.

The available overhaul records for the auxiliary servo cylinder assembly did not show if the pedal damper check valve housing had been replaced and, if it had, the date of the last replacement and the hours of service that the housing had accumulated at that time. According to a representative of the overhaul facility, when the facility performed overhauls of the auxiliary servo cylinder assembly, no cracks were found in the pedal damper check valve housing, and the housing was not replaced. In addition, the representative stated that the bolts for the pedal damper check valve housing were “always” replaced with new bolts per the manufacturer’s overhaul manual.

At the time of the accident, there was no specific inspection for the pedal damper check valve housing and its bolts after installation of the auxiliary servo cylinder onto the helicopter. Also at the time of the accident, the pedal damper check valve housing had no service life limit. On October 17, 2022, Sikorsky released Alert Service Bulletin ASB 61B65-25, which provided instructions for a one-time fluorescent penetrant inspection of the auxiliary servo unit for cracks. In addition, Sikorsky implemented a daily visual inspection of the yaw pedal damper check valve housing and a 30,000-hour life limit for the check valve housing. AIRPORT INFORMATIONDirectional Flight Control System

Inputs from the cockpit cyclic control, collecti...