1-16. EQUIPMENT OPERATION (continued).
UPPER REAR DOOR
The upper rear door is hydraulically actuated by the ballistic shield directional control valve on the hydraulic control
panel. When raised, the upper rear door provides ballistic protection between the M992A1 and a supported howitzer.
The door also helps deploy and position the conveyor assembly. From the closed position, it can be raised to any
position by a hydraulic actuator up to 120 degrees. The upper rear door directional control valve can be actuated by
two three-position switches mounted inside the rear door opening on the left side. For inside and outside access, one
is mounted high and the other is mounted low.
In the event of hydraulic system failure, the door is prevented from closing by a pilot check valve that stops hydraulic
fluid backflow within the door hydraulic system. In addition, a mechanical lock engages the vehicle hull in the 45- and
90-degree (from closed) positions.
In the upper rear door, there are two small doors that allow the conveyor to be operated while the upper rear door is
closed. This feature is for use with the M109A6 howitzer.