On the 332L & L2 the flight control servos are in hydraulic lock when there is no control input - ie the fluid is trapped both sides of the piston and the piston cannot move. Operating the flight controls (with or without hydraulic pressure) operates the distributor valve thus breaking the hydraulic lock and letting the hydraulic pressure motor the piston (or in the case of loss of a hydraulic system allows the other piston in the jack to motor the piston, circulating the unpressurised fluid in the duff system).
The main undercarriage legs are held both up and down by hydraulic lock (nosegear has mechanical lock however).
Jackstall can occur on the 332L2 following failure of 1 hydraulic system. Remembering that force = pressure x area (area is fixed and is the area of the piston head) if the pressure from the hydraulic system results in a force less than the feedback forces from the rotor head, the controls will appear to lock or even perhaps motor the wrong way. This can happen during high speed right-banked flight (with 1 hyd system out) and so there is an accumulator on the RH jack to allow level flight to be obtained should the hyd system fail in this flight condition, after which there is a bank limitation imposed by the EOPs.
The EC225 also has the possibility of jack stall when on 1 hyd system though new servos make this less likely than on the L2. As well as the EOPs specifying a bank limit following a hyd failure, there is also an aural warning calculated depending on mass and load factor - if the audio sounds you are approaching the jack stall limit and its time to back off (again, only following failure of 1 hyd system).
HC
Last edited by HeliComparator; 26th Oct 2007 at 12:52.
Reason: Technical correction