During system operation, the conveyor receives rounds of ammunition from the exit unit and delivers them through the feed chute to the transfer unit. The conveyor also receives expended cases and unfired rounds from the transfer unit and transports them through the return chute to the entrance unit. After receipt by the entrance unit, the expended cases and unfired rounds are removed from the conveyor elements and stored in the ammunition drum. The empty conveyor passes from the entrance unit to the exit unit through the bypass chute. Tabs on the conveyor elements, which engage guides in the chutes, exit unit, entrance unit, adapter assembly, and transfer unit, maintain positive control of the conveyor elements. The total number of elements required for a system varies according to aircraft application. GUN DRIVE AND DRUM DRIVE SYSTEMS A   hydraulic   drive   unit   run   by   the   aircraft’s hydraulic  system  simultaneously  drives  the  M61A1 gun    and    the    ammunition    handling    system.    The hydraulic pressure is supplied through a hydraulic fluid manifold electrically controlled by a dual-rate solenoid valve.   This   solenoid   valve   is   controlled   from   the cockpit  through  the  gun  control  unit  (GCU),  which results in the gun firing at 6,000 (GUN HIGH) or 4,000 (GUN   LOW)   rounds   per   minute.   Attached   to   the hydraulic  drive  unit  is  a  mechanical  drive  unit  that consists of a gear train with one input shaft (from the hydraulic  drive  unit)  and,  depending  upon  the  type  of aircraft, one or two output shafts. F-14 aircraft. The F-14 aircraft uses a mechanical drive unit with two output shafts. The mechanical drive unit causes a telescoping shaft to drive the gun and the drum unit assembly. F/A-18    aircraft.    The    F/A-18    aircraft    uses    a mechanical  drive  unit  with  one  output  shaft.  Because the ammunition drum is near the mechanical drive unit, a gear on the output shaft of the mechanical drive unit meshes   directly   with   the   drum   drive.   A   two-piece telescoping shaft transmits power from the same output shaft of the mechanical drive unit to the gun drive. This, in turn, drives the gun rotor. Both aircraft have provisions to manually rotate the gun  system  by  using  a  manual  hand  crank.  During ground  maintenance,  the  gun  system  may  be  rotated hydraulically.   Actuating   a   manual   control   on   the hydraulic   drive   unit   when   the   aircraft’s   hydraulic system is operating does this. REVIEW NUMBER 3 ANSWERS A1. The major parts of the drum unit assembly are the drum unit, entrance cover, exit cover, and scoop disk. A2. The  inner  drum  helix  of  the  drum  assembly looks like the threads of a screw and produces a  screwing-type  motion,  causing  rounds  to slide  along  partitions  from  one  end  to  the other end of the outer drum. A3. The  purpose  of  the  retainer  gear  and  steel ball bearings contained in the entrance cover is  to  support  the  scoop  disks  and  the  inner drum helix. A4. A  spring-loaded  timing  pin  on  the  entrance cover is used to index the drum for installation of the exit unit. A5. The two sets of sprocket spur gears located on the scoop disk mesh with the retainer gear in the   entrance   and   exit   covers   that   provide rotating support for the inner drum helix. A6. The last round switch in the exit unit prevents expended rounds from being fed into the gun and jamming it. A7. The sprocket assemblies in the entrance unit remove  live  rounds  or  expended  cases  from the conveyor elements and place them in the entrance cover retainer partitions. A8. The adapter assembly of the transfer unit and adapter assembly interfaces with the linkless ammunition loading system (LALS). GUN GAS PURGE SYSTEM The   M61A1   gun   is   internally   mounted   in   the aircraft's    fuselage.    When    the    gun    is    fired,    the temperature of the gun barrels increases rapidly, and the gun compartment is filled with gun gas from the fired rounds.   If   the   barrels   are   not   properly   cooled,   the rounds may cook-off due to excessive barrel temperatures.  Gun  gas,  when  confined  to  an  enclosed area  such  as  a  gun  compartment,  is  highly  explosive. The gun gas purge system cools the barrels and purges gas from the compartment during gun firing operations. The gun gas purge system in the F-14 aircraft uses cold air from the refrigeration system to cool and purge residual gun gases from the ammunition drum and gun compartment.   The   subsystem,   activated   when   gun firing is initiated, remains active for a 30-second period 6-14