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The Servo Transparency phenomenon, also known as Servo Reversibility or Jack Stall, can be encountered during abrupt manoeuvring of any single hydraulic system equipped helicopter, particularly at high speeds. The phenomenon marks a flight envelope boundary.
This aircraft phenomenon occurs smoothly and is not dangerous if properly anticipated by a pilot during an abrupt or excessive high load manoeuvre such as a high positive g turn or pull-up. The factors that affect Servo Transparency are airspeed, collective pitch input, gross weight, "G" loads and density altitude.
How does it Happen?
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 manoeuvres. Some manufacturers state that the design of the flying controls hydraulic system is to limit its power so as to protect the helicopter from excessive flight loads. 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.
Note that the effects of Servo-Transparency are generally pitch-up and a roll towards the retreating side of the rotor disk. In this respect the phenomenon presents similarly to Retreating Blade Stall.
Servo Transparency can be managed if the pilot anticipates it during an abrupt or high-G load manoeuvre. 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 the accidents attributed to servo transparency 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.
Accidents and Serious Incidents
- AS50, Dalamot Norway, 2011: On 4 July 2011, an Airlift Eurocopter AS 350 making a passenger charter flight to a mountain cabin in day VMC appeared to suddenly depart controlled flight whilst making a tight right turn during positioning to land at the destination landing site and impacted terrain soon afterwards. The helicopter was destroyed by the impact and ensuing fire and all five occupants were fatally injured. The subsequent investigation came to the conclusion that the apparently abrupt manoeuvring may have led to an encounter with ‘servo transparency’ at a height from which the pilot was unable to recover before impact occurred.