Date: 202107
Local time of day: 1801-2400
Location: ZZZ.Tower, US
Flight conditions: VMC
Weather elements / visibility: 10
Light: Night
Aircraft: Helicopter
Operator: Government
Operating under: Part 91
Flight plan: VFR
Flight phase: Cruise
Aircraft component: Helicopter Gearbox Drive Shaft
Component problem: Failed
Reporter function: Pilot Flying
Reporter qualification: Flight Crew Commercial
Anomaly: Aircraft Equipment Problem Critical; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy; Ground Event / Encounter Other / Unknown
Detector: Automation Aircraft Other Automation
When detected: In-flight
Result: Flight Crew Landed in Emergency Condition; General Maintenance Action
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: I was piloting the helicopter.  My [co-pilot] was in the co-pilot's seat and operating the FLIR system.  We had already been in the air for approximately 35 minutes when we made our way to the area of [location X]. We made several orbits at 1800 ft MSL which were uneventful.  When turning northbound on our last orbit; the LOW ROTOR RPM light and MASTER CAUTION light illuminated; the LOW ROTOR RPM horn began to sound; the SAJEM digital engine instrument panel lost functionality; and we lost all hydraulic assistance of the flight controls.   The engine was still audible; but given I was unable to determine rotor RPM and didn't know if the engine was truly running at 100%; I decided to perform a power on auto-rotation. I began a downward; right-hand; controlled spiral to purposefully lose altitude and maneuver the helicopter away from the heavily populated areas of [location X].  I established a glidepath and trajectory with a football practice field that is located close to [location Y].  At approximately 500 feet MSL; my night vision goggles (NVGs) fogged over due to the abrupt change in temperature from the cool night air of 1;800 feet MSL to the humid and muggy air at 500 feet MSL.  I was unable to see through my NVGs and was forced to rely on the peripheral vision of my right eye (I did not want to take my hands off of the controls to raise my NVGs) and my co-pilot who was telling me whether or not we were clear of obstacles.  At or around 20 feet MSL; I began establishing the helicopter for a RUN-ON landing. The ground on the practice field was very saturated due to heavy rains which have been soaking the grounds for days.  The skids did not slide; but instead; stuck into the ground and only moved forward about 5 feet.  This equated to an almost sudden stop of the helicopter.Both myself and my co-pilot were able to walk out of the helicopter.  The helicopter did not suffer any damage from the landing.  It has been determined that the malfunction is possibly a sheared shaft (inside the main rotor transmission) that runs through the main tach generator; the hydraulic pump; and into the main rotor transmission.  The main rotor transmission is being removed and will be sent to an offsite maintenance facility to disassemble the main rotor transmission for inspection and repair.




Synopsis: Helicopter pilot reported performing an autorotation to a landing after losing hydraulic assistance of the flight controls.