What happened
A certified flight instructor was performing a practice emergency maneuver in a helicopter with the hydraulic system intentionally turned off. Immediately after disengaging the hydraulics, the pilot observed that abnormal force was required on the cyclic control to prevent the aircraft from pitching up and to the left.
As the helicopter approached final approach, the pilot began reducing airspeed. Due to the severe control restrictions, the pilot believed a landing without incurring damage would be impossible. The pilot executed a run-on landing at approximately 10 knots, which was completed without mishap. Once the aircraft came to rest, the second pilot restored the hydraulic system via the collective switch. Upon restoration of pressure, the cyclic experienced a hard-over movement to the left, striking the instructor's leg. The instructor attempted to center the control with both hands but could not move it for approximately 40 seconds, until the pressure eventually released and the control could be centered easily.
The investigation
Investigators performed a complete teardown of each hydraulic servo and conducted pressure testing. Testing of one servo revealed that the unlocking pressure was out of tolerance and too high. Upon disassembly of this specific servo, investigators found wear on the conical surface of the locking finger and on the diameter in contact with the bearing. The manufacturer stated that this wear caused excessive friction within the bearing, which resulted in the high unlocking pressure.