TM 1-1520-238-10 2-8 2.9  LANDING GEAR. The  main  landing  gear  (fig  2-2)  supports  the  helicopter during  ground  operation  (taxiing,  take-off,  and  towing). The landing gear system is a three-point system consist- ing of the main landing gear, tail landing gear, and main landing gear brake system. The landing gear system pro- vides for ease of maneuvering when taxiing and towing, has shock struts to absorb normal and high impact land- ings, and kneels to facilitate transport of the helicopter. 2.9.1  Main  Landing  Gear. Each  main  landing  gear support consists of a trailing arm and a nitrogen/oil shock strut. The trailing arms transfer the helicopter landing and static loads to the airframe, and the shock struts absorb vertical loads. The upper ends of the left and right trailing arms attach to a cross tube which passes through the fu- selage and is supported by fuselage-anchored pivot bear- ings. The upper ends of the shock struts are attached to mounts on the fuselage structure. In addition to its normal energy-absorbing function, each shock strut has a one- time  high  impact  absorbing  feature:   shear  rings  are sheared  and  a  rupture  disk  bursts  causing  a  controlled collapse of the strut. 2.9.2  Tail Landing Gear. The tail landing gear consists of two trailing arms, nitrogen/oil shock strut, fork, axle, and wheel. The shock strut has an impact-absorbing capability similar to that of the main landing gear shock strut. The tail wheel is 360° free swiveling for taxiing and ground han- dling. The tail landing gear system incorporates a spring- loaded tail wheel lock. However, the tail landing gear is hy- draulically  unlocked  from  the  pilot  crew  station  or manually unlocked by a ground crewmember using a han- dle attached to the actuator. The tail wheel lock system is actuated by hydraulic pressure from the utility hydraulic system. Pressure is routed to the actuator through a con- trol valve located in the tail boom. The valve is controlled by the tail wheel switch (fig 2-6) at the pilot station. When the tail wheel switch is placed in the UNLOCK position, pressure is applied to the actuator to retract the lock pin. A proximity  switch  will  cause  the  advisory  light  above  the switch to illuminate. When the tail wheel LOCK/UNLOCK switch is placed in the LOCK position, a valve shuts off hy- draulic pressure and opens the line to the actuator. This relieves the pressure on the lock. Spring force will then move the lock pin to the lock position. If the tail wheel is unlocked manually, it can be locked from the pilot crew station by placing the tail wheel switch in the  UNLOCK position, then returning the switch to the LOCK position. The tail wheel shall be locked to: a.   Absorb rotor torque reaction during rotor brake op- eration. b.   Prevent  shimmy  during  rolling  takeoffs  and  land- ings. c.   Prevent  swivel  during  ground  operation  in  high winds. d.   Prevent swivel during operation on slopes. M01-125 Figure  2-6. Tail Wheel Lock Panel 2.9.3  Landing Gear Brakes. NOTE It is necessary to maintain pressure on the brake  until  the  PARK  BRAKE  handle  is pulled out to lock the parking brakes. If the PARK BRAKE handle is pulled out without pressure  applied  to  the  brake  pedals,  the PARK BRAKE handle may remain out and the brakes will not be set. The  brake  system  affects  only  the  main  landing  gear wheels. The main landing gear system consists of two in- dependent  hydromechanical  systems,  one  left  and  one right. Braking action is initiated from either crew station by applying foot pressure at the top portion of the directional control pedals. This activates a master cylinder attached to each brake pedal (fig 2-7 and 2-8). The master cylin- ders pressurize hydraulic fluid in the master cylinder sys- tem components. This pressure is