modes—director, disturbed, and cage. The pilot can select any one of the A/G or A/A modes while in flight. The  ammunition  handling  system  holds  a maximum of 578 rounds of ammunition. A round limiter, located in the gun compartment, can be preset to limit the total number of rounds the pilot can fire. The round limiter is used during training missions, and permits two or three gunnery missions from one gun load-out. For example, ground maintenance personnel set the round limiter at 200 rounds. When the pilot has fired 200 rounds, the gun's electrical system automatically initiates the gun clearing cycle. This prevents further firing until ground maintenance personnel manually reset the round limiter. While in flight, the pilot has the option of selecting unrestricted  firing  or  presetting  the  number  of  rounds per  burst.  If  the  pilot  selects  unrestricted  firing,  the gunfire’s    continuously    as    long    as    the    trigger    is depressed and ammunition is available. For example, if the pilot presets 50 rounds, the gunfire’s  bursts  of  50  rounds  each  time  the trigger is pulled and released. A display panel in    the    cockpit    continuously    indicates    the number of rounds remaining. The clearing sector retainer assembly (fig. 6-14) is used  to  manually  clear  the  gun.  When  the  manual clearing  handle  is  in  the  cleared  position,  a  wire  rope assembly  depresses  the  gun  clearing  sector  assembly against  the  gun  housing.  This  directs  the  breech-bolt assemblies into the clearing cam path when the gun is manually rotated. The manual clearing handle is held in the   clearing   position   by   a   locking   tab.   For safety reasons, the manual clearing handle should remain in the cleared position until you are actually performing gun-arming procedures.  When  the  gun access door is closed, you can determine the position of the   manual   clearing   handle   by   the   position   of   the indicator  located  on  the  door.  If  the  indicator  is  flush with the door surface, the manual clearing handle is in the  firing  position.  If  the  indicator  protrudes  from  the door  surface,  the  manual  clearing  handle  is  in  the cleared position. The   entire   gun   system   is   handled   as   a   single palletized  unit.  This  includes  the  M61A1  gun,  drum unit assembly, ammunition chutes, element chutes, and hydraulic   motor.   The   system   bolts   directly   to   the aircraft structure with four bolts, and does not require any other boresighting or alignment. Other than minor adjustments in the aircraft, all maintenance is performed  at  the  intermediate-maintenance  level.  The system is removed from the aircraft as a unit by using gun-handling adapters, a weapon skid or trailer, and an Aero 14C bomb-hoisting unit. The bomb-hoisting unit is  used  to  raise  or  lower  the  gun  system  as  it  is  being removed   or   installed.   A   gun   system   hoist   adapter, designed to support the hoist boom, is attached to the aircraft  during  the  raising  or  lowering  operation.  A gun-handling  adapter  attached  to  a  weapons  skid  or trailer, supports the gun system after it is removed from the aircraft. For   further   information   concerning   the   F/A-18 M61A1 gun installation, you should refer to Description and Principles of Operation, A1-F18AA-750-100;    Testing and Troubleshooting, A1-F18AA-750-200;   Maintenance,   A1-F18AA-750- 300; and System Schematics, A1-F18AA-750-500. REVIEW NUMBER 4 ANSWERS A1. The  feed  chute  provides  a  path  from  the  exit unit to the adapter assembly. A2. The  return  chute  provides  a  path  from  the transfer unit to the entrance unit of the drum. A3. The  gun  gas  purge  system  cools  the  barrels and  purges  gas  from  the  gun  compartment during gun-firing operations. A4. The gun gas purge systems of F/A-18 aircraft use engine bleed air and have additional gas control provided by a hydraulically actuated, ram-air scavenge door that opens automatically during gun-firing operation. F-14 GUN SYSTEM INSTALLATION The   F-14   M61A1   gun   system   (fig.   6-15)   is mounted in the forward fuselage on the left side of the aircraft.  Depending  upon  the  mission  objective,  this gun  system  can  be  operated  in  an  A/G  mode,  A/A mode,  or  an  air  combat  maneuver  (ACM)  encounter mode. The computer pilot attack mode (ACM encounter   mode)   operates   in   conjunction   with   the weapon control system, computer signal data converter (CSDC),  and  the  vertical  display  indicator  system  to display   target   data.   The   manual   attack   mode   is normally used in the A/G mode. It is also used as the primary backup for the ACM encounter mode in case of system malfunction. The M61A1 gun system accommodates a maximum of 676 rounds of 20-mm ammunition. There 6-16