The chute assemblies are interlocking segments. They provide a flexible path through which the conveyor elements transport live rounds and expended cases around an aircraft structure. The bypass chute provides a path from the entrance unit to the exit unit, through which the conveyor elements pass. The feed chute provides a path from the exit unit to the adapter assembly, through which the conveyor transports live rounds or unexpended cases. The return chute provides a path from the transfer unit to the entrance unit, through which the conveyor transports expended cases or unfired rounds.
The construction of the chute assemblies lets the conveyor elements pass through in only one direction. If the system is rotated in the wrong direction, the element tabs will jam in the chute segments and damage the system. The chute ends are color-coded red and green to key the right connection to other components. Additionally, each end is clearly marked with a metal labeling plate. This identifies the component to which a particular chute end must be connected. The ends of the chutes are equipped with quick-release latches for ease of removal and installation.
The conveyor assembly consists of individual conveyor elements shaped to cradle a 20-mm case. The elements are joined together by removable hinge pins to form an endless conveyor assembly. (See figure 6-13.) 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. Positive control of the conveyor elements is maintained by tabs on the conveyor elements, which engage guides in the chutes, exit unit, entrance unit, adapter assembly, and transfer unit. The total number
Figure 6-13.-Typical conveyor element assembly.
of elements required for a system varies according to aircraft application.
GUN DRIVE AND DRUM DRIVE SYSTEMS
The M61A1 gun and the ammunition handling system is simultaneously driven by a hydraulic drive unit run by the aircraft's hydraulic 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. This is done by actuating a manual control on the hydraulic drive unit when the aircraft's hydraulic system is operating.
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 after gun firing to ensure that all gases are cooled and purged.
The F/A-18 gun gas purge system uses engine bleed air and has an additional gas control provided by a hydraulically actuated ram-air scavenge door that opens automatically during gun-firing operations. The gun gasses are vented through louvers in the lower mold line of the aircraft fuselage.
M61A1 GUN SYSTEM INSTALLATIONS
LEARNING OBJECTIVE: Identify M61A1 gun system installations to include those used on the F/A-18 and F-14 aircraft.
M6lA1 gun and ammunition handling system installations have the same basic components. However, some components are peculiar to specific aircraft. These differences are discussed in the following paragraphs.