Following their investigation of a helicopter accident involving a Eurocopter AS350B3 Ecureuil, the Accident Investigation Board Norway (AABN) recommends that owners and operators of similar helicopters be reminded of the phenomenon of ‘Hydraulic Transparency' (Safety Recommendations SL 2012/08T/09T).
The purpose of this SAND is to draw the attention to ‘Hydraulic Transparency', also known as ‘Servo Transparency/Reversibility' or ‘Jack Stall'. A full description of this feature is provided below through the information issued by Eurocopter in their Service Letter 1648-29-03, dated 4 December 2003. The Operational Evaluation Board (OEB) report for the Ecureuil family, published by EASA, defines additional awareness training to prevent the phenomenon.
The content of this SAND is based on the Eurocopter Service Letter (No. 1648-29-03), the accident report of AABN (SL 2012/13) and of the UK AIC (P 043/2009):
On single hydraulic system equipped helicopter, the Hydraulic Transparency phenomenon can occur when the general load of the main rotor increases excessively during immoderate maneuvering. The general load on the main rotor increases under the following conditions:
- High speed
- High torque (high collective pitch)
- High mass
- High g-load
- Increasing density altitude, i.e.:
- Increasing flying altitude
- Increasing temperature
- Increasing humidity
On this helicopter type there is no indicator or warning which alerts the pilot that the helicopter is about to enter Hydraulic Transparency. It occurs in combination of the factors referred to above. If the load of the main rotor increases smoothly, the phenomenon could begin with vibrations of the "cyclic" control. If the load increases quickly, hydraulic transparency may occur instantly (equates HYD OFF on "collective"). The pilot may experience that the stick force increases from almost zero to 10 daN (equivalent to about 10 kg) over a short period of time, and may perceive that the controls are jammed.
Why to avoid Hydraulic Transparency?
If the pilot is prepared and adds this extra force, she/he may keep the helicopter on intended flight path. If the pilot is surprised by the phenomenon, the helicopter's flight path (altitude and track) may change: a sudden rolling movement to the right (may exceeding 90º of bank) and the helicopter's nose pitch up (speed drop). If this phenomenon is encountered in close proximity to terrain or obstacles, especially in a right turn, it could be hazardous.
How to avoid hydraulic transparency?
Basic airmanship should prevent encountering this phenomenon by avoiding excessive maneuvers leading to high rotor loads (>1.5g): Therefore avoid combinations of high speed, high gross weight, high bank and pitch angles, especially at high density altitude.
How to react if hydraulic transparency occurs?
The pilots reaction to the first indication of control forces feedback should be to IMMEDIATELY contract the unintended roll to the right, reduce the severity of the maneuver (to reduce the load) and to allow the collective pitch to decrease (of course, monitor main rotor rpm speed at very low pitch). Further smoothly counteract the right cyclic tendency to prevent an abrupt left cyclic movement as hydraulic assistance is restored.