TM 1-1500-204-23-1 CHAPTER 9 GENERAL AIRFRAME MAINTENANCE 9-1.      General.      The   maintenance   instructions   and procedures  contained  in  this  chapter  are  general  and applicable   to   airframe   maintenance   of   Army   aircraft. Maintenance  instructions  that  are  specific  and  peculiar to  particular  airframes  are  contained  in  the  applicable maintenance  manuals  and  will  be  used  in  conjunction with general information contained in this chapter. 9-2.  Landing Gear Systems.  The landing gear of an aircraft  consists  of  main  and  auxiliary  units,  either  of which may or may not be retractable.  The main landing gear forms the principle support of the aircraft on land or water   and   may   include   any   combination   of   wheels, floats, skis, shock-absorbing equipment, brakes, retracting mechanism with controls and warning devices,     cowling,     fairing,     and     structural     members necessary to attach any of the foregoing to the primary structure.    The  auxiliary  landing  gear  consists  of  tall  or nose  wheel  Installations,  outboard  pontoons,  skids,  etc, with necessary cowling and reinforcement 9-3.  Retractable Landing Gear Systems.  Retractable landing   gear   systems   are   explained   in   the   following paragraphs. a. Types.  The electric and hydraulic landing gear retraction    systems    are    explained    in    the    following paragraphs. (1)   Electric landing gear retraction system.  The electric  landing  gear  retraction  system  is  an  electrically driven jack for raising or lowering the gear, as shown in figure 9-1.  When a switch in the cockpit is moved to the UP  position,  the  electric  motor  operates.    Through  a system  of  shafts,  gears,  adapters,  an  actuator  screw, and a torque tube, a force is transmitted to the drag strut linkages.  Thus, the gear retracts and locks If the switch is moved to the DOWN position, the motor reverses and the gear moves down and locks. (2)    Hydraulic  landing  gear  retraction  system. Devices used In a typical hydraulically operated landing gear    retraction    system    include    actuating    cylinders, selector  valves,  uplocks,  downlocks,  sequence  valves, turbine,  and  other  conventional  hydraulic  components. These   units   are   interconnected   so   that   they   permit properly   sequenced   retraction   and   extension   of   the landing  gear  and  the  landing  gear  doors.    Figure  9-2 shows a typical hydraulic landing gear retraction system. b. Emergency Extension Systems. The emergency extension  system  lowers  the  landing  gear  if  the  main power  system  fails.    Some  aircraft  have  an  emergency release    handle    in    the    cockpit,    which    is    connected through   a   mechanical   linkage   to   the   gear   uplocks. When  the  handle  is  operated,  it  releases  the  uplocks and  allows  the  gear  to  free-fall,  or  extend,  under  their own  weight.    On  other  aircraft,  release  of  the  uplock  is accomplished using compressed air which IS directed to uplock    release    cylinders.        Some    installations    are designed so that either hydraulic fluid or compressed air provides   the   necessary   pressure,   while   others   use   a manual  system  for  extending  the  landing  gears  under emergency    conditions.        Hydraulic        pressure        for emergency    operation    of    the    landing    gear    may    be provided by an auxiliary hand pump, an accumulator, or an electrically powered hydraulic pump, depending upon the design of the aircraft. c. Safety    Devices.    Accidental    retraction    of    a landing  gear  may  be  prevented  by  such  safety  devices as  mechanical  downlocks,  safety  switches,  and  ground locks.      Mechanical   downlocks   are   built-in   parts   of   a gear-retraction  system  and  are  operated  automatically by  the  gear-retraction  system.    To  prevent  accidental operation  of  the  downlocks,  electrically  operated  safety switches are Installed. (1) Safety switch.  A landing gear safety switch, as shown In figure 9-3, in the landing gear safety circuit is usually mounted in a bracket on one of the main gear shock   struts.      This   switch   is   actuated   by   a   linkage through the landing gear torque links.  The torque links spread apart or move together as the shock strut piston extends  or  retracts  in  its  cylinder.    When  the  strut  is compressed (aircraft on the ground), the torque links are close  together,  causing  the  adjusting  links  to  open  the safety   switch.      During   takeoff,   as   the   weight   of   the aircraft   leaves   the   struts,   the   struts   and   torque   links extend,  causing  the  adjusting  links  to  close  the  safety switch.    As  shown  in  figure  9-3,  a  ground  is  completed when   the   safety   switch   closes.      The   solenoid   then energizes and 9-1