Hydraulic Control System Servo Transparency Phenomenon
On December 19, 2003, the FAA issued Special Airworthiness Information Bulletin (SAIB) No. SW-04-35 alerting owners and operators of Eurocopter France AS350B, BA, B1, B2, B3, D, AS355E, and EC120B model helicopters that pilots can encounter a phenomenon known asservo transparency, servo reversibility, or
jack stall during high airspeed, high collective pitch, high gross weight, high G-loads, and high density altitude conditions. The bulletin stated, in part, the following:
Pilots and operators may misunderstand this phenomenon. This aircraft phenomenon occurs smoothly, and can be managed properly if the pilot anticipates it during an abrupt or high load maneuver such as a high positive g‑turn or pull-up.
Servo transparency begins when the aerodynamic forces exceed the hydraulic forces and is then transmitted back to the pilot’s cyclic and collective controls. On clockwise turning main rotor systems, the right servo receives the highest load when maneuvering, so servo transparency results in uncommanded right and aft cyclic motion accompanied by down collective movement. The pilot control forces to counter this aerodynamically induced phenomena are relatively high and could give an unaware pilot the impression that the controls are jammed. If the pilot does not reduce the maneuver, the aircraft will roll right and pitch-up.
You should understand that servo transparency is a natural phenomenon for any flyable helicopter. BASIC AIRMANSHIP should prevent encountering this phenomenon by avoiding combinations of high speed, high gross weight, high‑density altitude, and aggressive maneuvers, which exceed the aircraft’s approved flight limitations.
During the investigation of another accident
29 in which the servo transparency phenomenon test data were reviewed, Eurocopter representatives stated that the transparency phenomenon is transitory (lasting only 2 to 3 seconds) and can be immediately corrected by pilot actions to reduce G loads. If uncorrected by the pilot, the resultant aircraft reaction to the servo transparency will reduce the factors that contribute to the severity of the maneuver. The Eurocopter representatives also stated that the controls are fully operable throughout the entire transparency event; however, the force required to effect movement of the flight controls against the rotor system dynamic feedback loads increases significantly. Eurocopter stated that the force feedback for each control channel would be dependent in part on the amount of G loads experienced; however, the company estimated that about 22 pounds of force would be required to move the collective in the “up” or increased-pitch direction, with the same amount of force required to move the cyclic to the left. In addition, flight test data indicated that servo transparency could not be encountered if the collective was raised less than 50 percent.
Eurocopter’s chief pilot and an FAA test pilot conducted additional servo transparency test flights in December 2003. Flight test results also indicated that servo transparency was impossible to encounter if the collective was less than 50 percent and also indicated that the flight condition was difficult to enter at airspeeds less than 100 knots. All servo transparency entries and sustainment were accomplished with intentionally high G forces, at high gross weights, and at high entry speeds and were difficult to sustain. All servo transparency conditions during these flight tests ceased when the collective was reduced.
In addition, a Safety Board investigator who is also a rated helicopter pilot conducted several test flights in an AS350 helicopter with Eurocopter’s chief instructor pilot at the company’s Grand Prairie, Texas, facility in December 2003. A Safety Board study of servo transparency data collected from Eurocopter flight tests, SBs, the FAA and Safety Board staff flights indicated that the onset of the servo transparency condition can be predicted based on the helicopter’s weight, altitude, and maneuver loading. The study indicated that, unless the entry maneuver or atmospheric condition inducing the servo transparency condition is violent, pilots would receive a warning, via cyclic control vibration, that servo transparency onset was occurring and that properly trained pilots can exit or avoid this flight condition by proper manipulation of the flight controls.
In addition, flight test data indicated that, when the collective flight control position was at or below the 50 percent raised position, servo transparency did not occur regardless of the helicopter’s maneuvering load, weight, or altitude. Servo transparency onset was possible when the helicopter was subjected to high maneuvering loads at high forward airspeeds during operations at high gross weights and high pressure altitudes but ceased immediately after the pilot reduced the flight control forces, especially the collective, which reduces total maneuvering loads.