TM 1-1520-238-10 2-48 Change 9 actuator position to calculate the equivalent mechanical control command when in BUCS. The SPAD assemblies incorporate two switch assemblies to sense a decoupling of the SPADs and LVDTs are used to sense a mistrack and  send  control  movement  information  to  the  DASEC. The DASEC electronically controls the BUCS servo valve on the primary side of the affected flight control actuator. The  primary  hydraulic  system  and  the  DASEC  must  be operational for BUCS to operate. WARNING · When BUCS is engaged do not release the flight controls until the flight has been  completed  and  the  main  rotor has  come  to  a  complete  stop.  Force trim may or may not be available in the BUCS ON axis. · During the execution of the emergen- cy procedure for either BUCS FAIL or BUCS ON, the APU must be on prior to engine shutdown to provide electrical power to the DASEC. · BUCS  can  become  engaged  when  a mistrack between the controls and the actuator  is  sensed.  This  commonly occurs  when  external  power  is  pro- vided to the aircraft without hydraulic power. Applying hydraulic power and completing  an  DASEC  FD/LS  check may clear the BUCS engagement. b.   Operation When  BUCS  is  engaged,  the  BUCS ON caution/warning light will illuminate and DASE will dis- engage in the affected axis (axes) resulting in ASE cau- tion/warning light illuminating (ASE not applicable for col- lective). Several types of engagements are possible and are covered, by crewstation, in the following paragraphs. WARNING A BUCS jam engagement in the aft cyclic longitudinal  control  direction  or  in  the left or right pedal tail rotor control direc- tion may result in limited control author- ity  (from  the  point  of  the  jam)  because the actuator is unable to break the actua- tor shear pin.  The worst case jam condi- tion in the longitudinal aft cyclic (jam oc- curs at the actuator) will result in a 12% aft control authority from the point of the jam. Forward cyclic can break the shear pin  and  gain  full  control.  the  yaw  axis may be limited to +/– 25% control author- ity from the point of jam. If flight control authority  is  insufficient  for  a  hover,  at- tempt a roll–on landing. CAUTION After a BUCS engagement and transition to  BUCS  controlled  flight,  a  flight  con- trols controllability check (small control inputs in each axis to check for correct response)  should  be  conducted  to  es- tablish if any other control axis has been affected. c.   Control  System  Engagement  Logic  (Pilot crewstation – pilot flying the aircraft). (1)   Jams. A jam can occur anywhere in the flight control  system  through  malfunction  or  battle  damage. Should the pilot be unable to move the controls in one or more axes, he should aggressively decouple into the ap- propriate axis. The pilot can expect the control to displace to  full  throw  when  the  SPAD  releases. The DASEC will wash in full authority over a period of approximately three (3) seconds allowing the pilot to center the controls and transition to BUCS controlled flight. The  BUCS ON and ASE (ASE not applicable for collective) caution/warning lights will illuminate. SAS will be OFF in the engaged axis. Force trim will be available in all axes. Attitude/Hover hold will be available in the axes that SAS is available. (2)   Severances. (a)   Severance  aft  of  the  pilot’s  control.  A severance can occur due to malfunction or battle damage between the pilot’s controls and the actuator. In this case the  BUCS  will  automatically  engage  when  the  pilot achieves  the  sufficient  mistrack  with  respect  to  RAM