Pilots who learn to fly in smaller helicopters probably hear very little about servo transparency, yet this phenomenon has caused or played a role in several accidents. When giving flight reviews I have found some helicopter pilots who totally misunderstand why and how it happens. However, the concept is not too difficult to understand.
Because of the higher control forces in larger helicopters, hydraulically boosted servo actuators are used to assist the flight controls. The maximum force that these servo actuators can produce is constant and is a function of hydraulic pressure and servo characteristics. Engineers design the hydraulic system to adequately handle all aerodynamic forces required during approved maneuvers. Even so, with certain aggressive maneuvering it is possible for the aerodynamic forces in the rotor system to exceed the maximum force produced by the servo actuators. At this point, the force required to move the flight controls becomes relatively high and could give an unaware pilot the impression that the controls are jammed. To prevent servo transparency, pilots should avoid abrupt and aggressive maneuvering with combinations of high airspeed, high collective pitch, high gross weight, and high-density altitude.
The good news is that this phenomenon occurs smoothly, and can be managed properly if the pilot anticipates it during an abrupt or high-G load maneuver. On clockwise-turning main rotor systems the right servo receives the highest load, so servo transparency produces an un-commanded right and aft cyclic movement accompanied by down collective. The pilot should follow (not fight) the control movement and allow the collective pitch to decrease while monitoring rotor rpm, especially at very low collective pitch settings. The objective is to reduce the overall load on the main rotor system. It normally takes about two seconds for the load to ease and hydraulic assistance to be restored. However, be aware that if the pilot is fighting the controls when this happens, the force being applied to the controls could result in an abrupt undesired opposite control movement.
Many of these accidents have happened while aggressively flying the helicopter at low altitudes, leaving very little time to recover. Most important for avoiding this kind of accident is to follow the aircraft limitations published in the helicopter’s flight manual.