(b) Trailing Arm Assembly.

TM 1-1520-238-T-4 3–15 3–7. SYSTEM DESCRIPTION (cont) 3–7 (b) Trailing Arm Assembly. When the wheels contact the ground during landing, the trailing arms pivot (rotate) down, transferring the vertical load inputs to the shock struts. As the weight of the fuselage is supported by the MLG, the squat switch target on the left MLG moves out of proximity with the squat switch (proximity switch). The proximity switch de–energizes a set of relays which in turn: (When the helicopter becomes airborne, the shock struts extend causing the trailing arms to rotate up and move the squat switch target back into proximity of the squat switch. This causes the relays to become energized and returns to its previous condition.) � Enables the wing–mounted intercommunication system (ICS) receptacles. � Activates the generator under–frequency protection circuit. � Commands the pylon ejector racks to the ground stow position. � Enables the fault detection/location system (FD/LS) ground test. � Disables the laser, gun, rockets, and missiles. � Disables the yaw function of the digital automatic stabilization equipment (DASE). � Removes electrical power from the TADS/PNVS anti–ice circuit. � Erases the code in the TSEC/Kit–1A if IFF mode 4 has been selected. (c) Shock Strut Assembly. The lower chamber of the cylinder and the lower piston are filled with hydraulic fluid. The upper strut assembly is filled with dry nitrogen. During operation, the internal floating diaphragm rides on top of this hydraulic fluid and compresses the nitrogen for a dampening effect of shocks received to the fuselage. When the helicopter is in flight (or on jacks), nitrogen pressure forces the floating diaphragm toward the lower end of the piston driving the piston to the extended position. The rate of extension is controlled by an internal poppet valve and orifices at the lower end of the piston. Servicing of the strut is accomplished through the hydraulic fill/bleed port, and a nitrogen fill/bleed port. Two vent ports at the lower chamber, either side of the kneeling coupling, are used during servicing. Several markings are stenciled on the strut to indicate location of valves, plugs, ports, and locking positions. A data plate gives shock strut inflation, instructions, and servicing procedures. During takeoff, as the upper piston extends the controlled rate of fluid flow prevents the MLG from dropping too fast. An elastomer spring assembly, installed internally in the top portion of the upper cylinder, provides damping during extension. During landing, the weight of the helicopter causes the shock struts to compress. The upper piston is forced toward the bottom of the cylinder. The poppet valve and orifice, in the upper chamber valve assembly, controls the rate of fluid flow between the piston and cylinder as the upper piston moves in and out. The floating diaphragm is forced upward by fluid pressure increasing the nitrogen pressure against the upper side of the diaphragm. The compression of the nitrogen acts as a spring to cushion the landing. A fuse collar mates with the machined flanges on the exterior surface of the lower piston. A quick release pin is used to secure the fuse collar in the lock position. Three retaining pins (screws) prevent the fuse collar from pulling away from the cylinder when the collar is disengaged from the internal flanges. The external operating surface of the piston is chrome plated, with a red band around the upper portion of the lower piston housing which provides visual indication of proper piston extension for fuse collar to flange engagement. The lower end of the piston attaches to the trailing arm through a spherical self–lubricating bearing installed in the lower piston. (d) Kneeling/Erecting. The kneeling/erecting operation of the helicopter can be started by connecting external hydraulic pressure to the kneeling couplings on both struts (TM 1-1520-238-23). When the fluid in the cylinder is externally pressurized, the shear collars are unlocked. Fluid pressure is then slowly reduced to allow the lower pistons to move into the lower cylinder chambers. This controlled method of lowering the helicopter avoids damage to the aircraft. To raise the helicopter, external pressure is applied to the cylinders until the pistons are fully extended. The shear collars are then locked.