Shock Strut Operation Servicing Shock Struts 13-12 2.  Remove the cap from the air servicing valve.  [Figure 13-25A] 1.  Position the aircraft so that the shock struts are in the  normal ground operating position. Make certain that  personnel, work stands, and other obstacles are clear  of the aircraft. If the maintenance procedures require,  securely jack the aircraft. Figure 13-23.  Fluid flow during shock strut operation is controlled  by the taper of the metering pin in the shock strut orifice.  taxi operations, the air in the tires and the strut combine to  smooth out bumps.  Figure 13-23  illustrates the inner construction of a shock strut.  Arrows show the movement of the fluid during compression  and extension of the strut. The compression stroke of the  shock strut begins as the aircraft wheels touch the ground. As  the center of mass of the aircraft moves downward, the strut  compresses, and the lower cylinder or piston is forced upward  into the upper cylinder. The metering pin is therefore moved  up through the orifice. The taper of the pin controls the rate  of fluid flow from the bottom cylinder to the top cylinder at  all points during the compression stroke. In this manner, the  greatest amount of heat is dissipated through the walls of the  strut. At the end of the downward stroke, the compressed air  in the upper cylinder is further compressed which limits the  compression stroke of the strut with minimal impact. During  Insufficient fluid, or air in the strut, cause the compression  stroke to not be properly limited. The strut could bottom  out, resulting in impact forces to be transferred directly to  the airframe through the metallic structure of the strut. In  a properly serviced strut, the extension stroke of the shock  strut operation occurs at the end of the compression stroke.  Energy stored in the compressed air in the upper cylinder  causes the aircraft to start moving upward in relation to the  ground and lower strut cylinder as the strut tries to rebound  to its normal position. Fluid is forced back down into the  lower cylinder through restrictions and snubbing orifices. The  snubbing of fluid flow during the extension stroke dampens  the strut rebound and reduces oscillation caused by the spring  action of the compressed air. A sleeve, spacer, or bumper  ring incorporated into the strut limits the extension stroke. Efficient operation of the shock struts requires that proper  fluid and air pressure be maintained. To check the fluid  level, most struts need to be deflated and compressed into  the fully compressed position. Deflating a shock strut can be  a dangerous operation. The technician must be thoroughly  familiar with the operation of the high-pressure service valve  found at the top of the strut’s upper cylinder. Refer to the  manufacturer’s instructions for proper deflating technique of  the strut in question and follow all necessary safety precautions. Two common types of high pressure strut servicing valves  are illustrated in  Figure 13-24 . The AN6287-1 valve in  Figure 13-24A  has a valve core assembly and is rated to 3,000  pounds per square inch (psi). However, the core itself is only  rated to 2,000 psi. The MS28889-1 valve in  Figure 13-24B has no valve core. It is rated to 5,000 psi. The swivel nut on  the AN6287-1 valve is smaller than the valve body hex. The  MS28889-1 swivel nut is the same size as the valve body hex.  The swivel nuts on both valves engage threads on an internal  stem that loosens or draws tight the valve stem to a metal seat.  The following procedures are typical of those used in  deflating a shock strut, servicing it with hydraulic fluid, and  re-inflating the strut.  Extension Stroke Compression Stroke Air Hydraulic fluid