Incident
A certified flight instructor (CFI) configured a helicopter for a practice emergency maneuver by turning off the hydraulic system. Immediately after disengaging the hydraulics, the CFI noted that an abnormally high force was required on the cyclic control to prevent the helicopter's nose from pitching up and to the left. He decided to continue the landing with the hydraulics off and reduced airspeed as the helicopter entered the final approach. Believing that a normal landing would cause damage due to the severe control restriction, the CFI performed a run-on landing at about 10 knots, which was completed without incident.
After the helicopter came to a stop, the pressure on the cyclic was released, and the second pilot restored hydraulics via the collective switch. Immediately thereafter, the cyclic moved to the left in a hard-over displacement against the CFI's leg. The CFI attempted to center the cyclic with both hands but could not move the control. After approximately 40 seconds, the pressure released, and the second pilot centered the cyclic with ease.
Investigation
The helicopter's hydraulic servos were tested, and each servo underwent a complete teardown. Pressure testing of one servo revealed that its unlocking pressure was out of tolerance (too high). Disassembly of that servo showed wear on the conical surface of the locking finger and on the diameter in contact with the bearing. According to the servo manufacturer, the wear caused excessive friction in the bearing, leading to the high unlocking pressure.