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