As we continue our discussion on the engine components  and  operating  procedures  in  this chapter, the only reference to the modifications Mod  2  or  Mod  3  will  be  to  point  out  where  a particular  component  or  operating  procedure  differs in  one  Mod  from  the  other. Engine  Structure The   engine   structure   is   a   framework   for supporting the engine and most of its components and for securing the entire assembly to the ship’s structure. It is composed of a welded steel base made in two longitudinal box sections with the necessary  ties,  plates,  and  other  structural  members. The  two  sections  are  bolted  together  near  the center.  See  figure  3-1. Two pairs of saddles are mounted on the base for supporting the engine cylinder. Vertical stands are  welded  on  these  saddles  to  support  the  saddles for  the  accumulator.  Between  these  two  stands  is  a frame of welded channels, angles, and gusset plates to  provide  trusses  and  ties  for  the  frame. On  the  crosshead  end  of  the  welded  base support plates, webs and gussets support the rails for  the  crosshead.  On  this  end  of  the  base  are welded  longitudinal  guides  for  the  accumulator assembly.  Near  the  end  of  this  frame  and  bolted  to it is the crosshead stop, which is removed when the crosshead  is  installed  or  removed. Constant  Runout  (CRO) Control Valve Assembly The constant runout control valve (CRO valve) is installed at the fixed sheave end of the Mk 7 arresting engine, as illustrated in figure 3-1. It  is  designed  to  stop  all  aircraft  with  the  same amount of runout regardless of the aircraft’s weight and speed (within the limits specified in current recovery bulletins). The CRO valves of both the Mk 7 Mod 2 and the Mk 7 Mod 3 arresting engines have  the  same  major  parts–arresting  valve,  aircraft weight  selector,  drive  system,  and  control  system. The CRO valve is the heart of the equipment. It controls the flow of fluid from the cylinder of the arresting engine to the accumulator. The other components of the valve are used either to adjust the  initial  opening  of  this  valve  for  aircraft  of different  weight  or  to  activate  the  valve  during  the arresting   stroke. The CRO Drive System When  a  landing  aircraft  engages  a  deck  pendant, or  barricade,  it  withdraws  purchase  cable  from  the arresting  engine.  This  action  causes  the  crosshead to  move  toward  the  fixed  sheave  end  of  the  engine. In addition to causing fluid displacement from the engine cylinder, the movement of the crosshead causes the CRO valve drive system (fig. 3-1) to rotate the CRO valve cam. Rotation of this cam forces a plunger down onto a set of levers (fig. 3-4), which in turn forces a valve sleeve and valve stem down to mate with a valve seat to close the valve, shutting  off  the  flow  of  fluid  from  the  engine cylinder to the engine accumulator, bringing the aircraft to a stop. As  stated  earlier,  the  CRO  valve  is  designed  to bring  all  aircraft,    regardless   of   weight,   to   a controlled  stop  while  using  approximately  the  same amount  of  flight  deck  landing  area. This  is accomplished  by  adjusting  the  allowable  opening  of the  CRO  valve,  a  smaller,  more  restrictive  opening to arrest a heavy aircraft or a large valve opening to arrest a light aircraft. Control Valve Weight Selector The aircraft weight selector makes it possible to adjust  the  valve  for  aircraft  of  different  weights  by varying the valve opening. See figure 3-4. The  size  of  the  initial  valve  opening  is  adjusted while  the  arresting  engine  is  in  the  BATTERY position. The lead screw receives rotary motion from the motor unit or handwheel and converts it into  Iinear  motion.  This  linear  motion  positions  the upper  lever  and  drives  the  local  and  remote indicators. In each of the two levers (upper and lower), the distance between the fulcrum and roller is constant. On  the  upper  lever,  the  distance  between  the fulcrum and the point of application of force from the  cam  is  variable,  its  greatest  length  being  twice that  of  the  lower  lever.  The  lever  arm  ratio  of  each lever,  therefore,  is  variable  between  1:1  and  2:1. When  the  upper  lever  is  fully  extended,  the  ratio of  each  lever  is  1:1. In this setting the initial opening of the control valve upon engagement of an 3-6