TM 1-1520-238-10 Change 9 2-47 NOTE If the GPS system is not keyed or is tracking less than 4 satellites, HAS per- formance may degrade over time. Hover Position Box and Velocity Vector accura- cy will be degraded. Over time the SAS actuators may reach saturation limit if not centered by reinitial- izing. The ATTD/HOVER HOLD features in pitch, roll, or yaw will be lost if satura- tion occurs. The ATTD/HOVER HOLD feature is a limited hands off feature and the controls should always be monitored carefully. Heading hold is a function of HAS. The HAS mode will function only if: (1) the ATTD/HOVER HOLD and all ASE channels are engaged; (2) force trim is on, and (3) air- speed is below 50 KTAS and ground speed is less than 15 knots. Degraded conditions will occur in attitude hold or HAS if individual ASE channel switches are not en- gaged. The ATTD/HOVER HOLD switch will disengage automatically when transitioning between modes; attitude hold will disengage at approximately 55 KTAS when de- celerating. HAS will disengage when 15 knots ground speed or 50 KTAS is exceeded; or when ADSS fails or is turned off. NOTE Backup Control Systems (BUCS) are not in- corporated on aircraft PV-529 (S/N 88–0199) and prior. 2.37.7 Backup Control System (BUCS). CAUTION Engagement of the BUCS system may occur without Shear Pin Actuated De- coupler (SPAD) breakout during crew force fights if a CPG BUCS select switch failure has also occurred. Servoactuator LVDT probe migration may aggravate the force fight condi- tion. If, during a force fight with the CPG BUCS select switch activated, a breakout of the SPAD occurred, the BUCS would not provide a three sec- ond easy on and would subject the air- craft to a large control transient. If a BUCS engagement occurs after a con- trol force fight, attempt to recover the aircraft without severing the mechani- cal controls and follow established procedures for BUCS ON flight. Breakout of the SPAD at the base of the collective control will eliminate the normal mass of the control system and may cause the collective to move slightly in response to rotor system vibration. This slight movement will be detected by the LVDT and can pro- duce an unwanted collective pitch ap- plication. Increasing collective friction can eliminate this characteristic. a. Description. The Back–Up Control System (BUCS) is a single–channel, non–redundant, fly–by–wire control system that can electronically operate all four con- trol axes. The BUCS is modeled to duplicate the mechani- cal flight controls without the Stability Augmentation Sys- tem (SAS) engaged. This provides identical control authority and handling qualities as SAS–OFF flight in any given flight control axis. A shear pin is located on each of the flight control actuators and a Shear Pin Actuated De- coupler (SPAD) is located on each control, in each crewstation. This allows the shear pin to break and the SPAD to decouple in the event of a control jam. This sys- tem allows the controls to decouple regardless of where the jam takes place. The DASEC uses Linear Variable Displacement Transducers (LVDT) control position and