TM 1-1510-223-10 Change 3          2-43 or aft, until the locking pin engages in the selected posi- tion. b.   Yaw  Damper  System.  A  yaw  damper  system  is provided to aid the pilot in maintaining directional stability and increase ride comfort. The system may be used at any altitude, but is required for flight above 17,000 feet. It must be deactivated for takeoff and landing. The yaw damper system is a part of the autopilot. Operating in- structions for this system are contained in Chapter 3. The system is controlled by a YAW DAMP switch located on the autopilot control panel. c.   Rudder Boost System. The rudder boost system is a torque sensing, linear actuating system. The system utilizes a pressure transducer on each engine to sense engine torque oil pressure, a stability augmentation com- puter to monitor torque levels, and the rudder servo to apply boost to aid the pilot. The stability augmentation computer monitors torque levels and airspeed to determine if boost is required. The level of boost is proportional to the difference in torque between each engine and inversely proportional to air- speed. Boost commences at approximately 50% torque differential and increases to maximum at 100% torque differential. The level of boost available is inversely pro- portional to airspeed such that maximum rudder boost is obtained   at   100%   differential   and   low   airspeed   (80 knots),  while  no  rudder  boost  is  available  at  high  air- speeds (above 180 knots). The rudder boost system is controlled by a switch placarded RUDDER BOOST - OFF - YAW CONTROL TEST, located on the pedestal extension (fig. 2-12). The rudder  boost  system  is  powered  through  a  5-ampere circuit breaker placarded RUDDER BOOST, located on the overhead circuit breaker panel (fig. 2-7). 2-38.   FLIGHT CONTROL LOCKS. CAUTION Remove control locks before towing the aircraft or starting engines. Serious damage could result in the steering linkage if towed by a tug with the rudder lock installed. Positive  locking  of  the  rudder,  elevator  and  aileron control surfaces, and engine controls (POWER levers, PROP levers, and CONDITION levers) is provided by a removable  lock  assembly  (fig.  2-23)  consisting  of  two pins, and an elongated U-shaped strap interconnected by  a  chain.  Installation  of  the  control  locks  is  accom- plished  by  inserting  the  U-shaped  strap  around  the aligned control levers from the copilot’s side; then the ai- leron/elevator locking pin is inserted through a guide hole in the top of the pilot’s control column assembly. The rud- der is held in a neutral position by an L-shaped pin which is installed through a guide hole in the floor aft of the pilots rudder pedals. The rudder pedals must be centered to align the hole in the rudder bellcrank with the guide hole in the floor. Remove the locks in reverse order (rudder pin, control column pin, and power control clamp). 2-39.   TRIM TABS. Trim tabs are provided for all flight control surfaces. These  tabs  are  manually  actuated,  and  mechanically controlled by a cable-drum and jack-screw actuator sys- tem  except  the  right  aileron  tab  which  is  of  the  fixed, bendable type. Elevator and aileron trim tabs incorporate neutral, non-servo action, i.e., as the elevators or aile- rons are displaced from the neutral position, the trim tab maintains an as-adjusted position. The rudder trim tab in- corporates anti-servo action, i.e., as the rudder is dis- placed from the neutral position the trim tab moves in the same  direction  as  the  control  surface.  This  action  in- creases control pressure as rudder is deflected from the neutral position. a.   Elevator Trim Tab Control. The elevator trim tab control wheel placarded PITCH TRIM, DN - UP, is located on the left side of the control pedestal and controls a trim tab on each elevator (fig. 2-12). The amount of elevator tab deflection, in units from a neutral setting, is indicated by a position arrow. b.   Electric  Elevator  Trim.  The  electric  elevator  trim system is controlled by an ELEV TRIM - OFF/RESET switch located on the pedestal extension (fig. 2-12), dual element thumb switches on the control wheels (fig. 2-22), a trim disconnect switch on each control wheel, and a 5-ampere circuit breaker placarded ELEC TRIM, located on the overhead circuit breaker panel (fig. 2-7). The ELEV TRIM - OFF/RESET switch must be in the ELEV TRIM (on) position to operate the system. The dual element thumb switch is moved forward for trimming nose down, aft for nose up, and when released returns to the center (off) position. Any activation of the trim system through the copilot’s trim switch can be over-ridden by activation of the pilot’s switch. Operating the pilot’s and copilot’s switches in opposing directions simultaneously results in the pilot having priority. An annunciator placarded ELEC TRIM OFF on the caution/advisory annunciator panel in- dicates failure or disconnect of the electric trim system. A  preflight  check  of  the  switches  should  be accomplished before flight by moving the switches indi- vidually on both control wheels. No one switch alone should operate the system; operation of elevator trim should occur only by movement of pairs of switches. The trim system disconnect is a bi-level pushbutton momen- tary-type switch, located on the outboard grip of each