History of Flight
On April 20, 2020, about 0802 local time, a Sikorsky S-61N, registration 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 onto 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 Part 135 and a contract with the US Department of Defense.
The flight was transporting cargo from Camp Bastion to Camp Dwyer. A cockpit voice recorder (CVR) captured that at 0746:13, the right seat pilot stated, “Dwyer’s in sight,” which the left seat pilot acknowledged. An Appareo Vision 1000 image recorder, installed on the cockpit ceiling, provided parametric data, still images, and audio. At 0751:44, all engine and transmission instruments appeared normal; the auxiliary hydraulic pressure gauge indicated about 1,500 psi.
At 0758:37, the crew received clearance to land. At 0801:57, an unintelligible word was recorded. The image recorder showed the helicopter at 2,600 ft mean sea level, with auxiliary hydraulic pressure decreasing to about 1,300 psi, near the bottom of the normal operating range. The left seat pilot’s left pedal began moving forward without pilot input, reaching the fully forward position within a second. The pilot’s foot lost contact after the sudden movement.
At 0801:59, both pilots were jostled; the left seat pilot placed his foot back on the left pedal and made a momentary forward cyclic input. The heading indicator showed a left yaw continuing until ground impact, with pitch and roll excursions. Engine and transmission indications remained normal. At 0802:00, the left seat pilot asked, “do you have the controls?” Both pilots’ feet were on respective pedals; the helicopter had rotated about 90 degrees and pitched down approximately 20 degrees. Three seconds later, the left seat pilot said, “let’s get this/us down on the ground,” and the right seat pilot’s arm was near the speed selector levers.
The left yaw rate increased from about 20° per second to about 80° per second. At 0802:05, an expletive was heard; the helicopter had pitched up to 36 degrees nose up, completed one full rotation, and increased right roll to 36 degrees. One second later, the auxiliary hydraulic pressure caution lights illuminated. The roll reached 73 degrees right roll by 0802:06.7, and pitch reached 75 degrees nose up about one second later. The image recording ended at 0802:08, about 9 seconds after the left pedal moved fully forward. The CVR ended at 0802:10.
During a postaccident interview, the pilots recalled a loud bang, initial left yaw with slight right roll, and that the right seat pilot moved both speed selector levers to off for autorotation. They reported no pedal or cyclic authority, and a right spin ensued. Recorded data showed no change in yaw direction and several excursions of pitch, roll, and acceleration.
Aircraft Information
The helicopter was a Sikorsky S-61N. Its directional flight control system transmits cockpit inputs to the auxiliary servo cylinder assembly via control tubes. The auxiliary servo cylinder provides hydraulic assistance to the tail rotor control. Within the yaw channel, hydraulic fluid ports are located in the bypass valve, pedal damper, and input valve. The pedal damper design was intended to prevent sudden large pedal movements.
Maintenance
The operator’s continuous airworthiness maintenance program required a safety inspection every 15 hours, including an auxiliary servo cylinder inspection. The last such inspection was on April 18, 2020. Five phased inspections were performed at 30-hour intervals; the phase V inspection (every 150 hours) included inspection of directional control cables, pulleys, rods, and hydraulic accessories. The last phase V inspection was on April 11, 2020.
The auxiliary servo cylinder assembly had a 2,500-hour overhaul interval. It was last overhauled from May to September 2017, with 34,184 hours time since new. The overhaul included a fluorescent penetrant inspection of the pedal damper check valve housing, which found no cracks. On April 19, 2020, the assembly had 35,455 hours since new and 1,270 hours since overhaul. Overhaul records did not show if the check valve housing had been replaced; the overhaul facility reported no cracks were found and the housing was not replaced. Bolts were reportedly replaced per the manufacturer’s manual.
At the time of the accident, there was no specific inspection for the pedal damper check valve housing and its bolts after installation, and no service life limit. On October 17, 2022, Sikorsky released Alert Service Bulletin ASB 61B65-25, providing instructions for a one-time fluorescent penetrant inspection of the auxiliary servo unit for cracks, along with a daily visual inspection and a 30,000-hour life limit for the check valve housing.
Wreckage and Impact
The helicopter came to rest on its right side on a magnetic heading of about 74°. The main fuselage partially collapsed, and the tailboom was twisted right. All five main rotor blades exhibited fragmentation and chordwise fractures. The tail rotor remained attached, with three blades showing chordwise bending. Tail rotor control cables had overstress fractures.
The primary hydraulic pressure gauge indicated normal pressure; the auxiliary hydraulic pressure gauge indicated 0 psi. Both engine speed selector levers were in the shutoff position. The left seat pilot’s left pedal was fully forward, right pedal fully aft. The collective-mounted hydraulic switch was in “AUX OFF.” The right seat cyclic upper portion was missing; the collective head was fractured. The auxiliary servo cylinder remained installed, with surrounding structure collapsed to the right. A small pool of hydraulic fluid was observed to its right.
Examination of the auxiliary servo cylinder assembly found that on the yaw channel pedal damper check valve housing, the forward right bolt was fractured (head attached to safety wire), and an extruded O-ring piece was at the interface. A crack was visible on the check valve housing near the forward left bolt. Bench testing showed a leak at the check valve housing at just under 200 psi. Disassembly revealed the forward left bolt had a crack on the threaded shank and a slight bend. The crack on the check valve housing extended toward the threaded bore for a plug. The extruded O-ring was separated. No other anomalies were found in the pedal damper or subcomponents.
The yaw power piston rod end exhibited a slight bend. All O-rings and backup rings were in good condition. Scanning electron microscopy of the fractured forward right bolt showed fatigue signatures; the fracture surface was about 75% fatigue and 25% overload. The forward left bolt had multiple cracks in the first three engaged thread roots. Both bolts conformed to requirements; no evidence of over- or under-torque.
The check valve housing crack exhibited fatigue on most of the surface, with hydraulic fluid present. The fatigue origin was near the radius of the forward left lug. The radius measured about 0.003 inches, not conforming to drawing requirements of 0.005 to 0.015 inches. Material composition, hardness, and microstructure were typical.
Additional Information
According to Sikorsky, in 1978 and 1979, one operator reported three instances of loss of hydraulic pressure to the auxiliary servo system on S-61 helicopters. Two occurred during flight, one on the ground. In all three, the auxiliary hydraulic pressure gauge indicated 0 psi. In two instances, two pedal damper check valve bolts had fractured, causing hydraulic fluid loss. In the third, the flight crew reported a “violent 45-degree yaw to [the left] and a 5 degree roll to [the right],” and inspection found a cracked pedal damper check valve housing with two fractured bolts.