A typical check valve consists of a spring-loaded ball and  seat inside a housing. The spring compresses to allow fluid  flow in the designed direction. When flow stops, the spring  pushes the ball against the seat which prevents fluid from  flowing in the opposite direction through the valve. An arrow  on the outside of the housing indicated the direction in which  fluid flow is permitted.  [Figure 12-38]  A check valve may  also be constructed with spring loaded flapper or coned shape  piston instead of a ball. Some check valves allow full fluid flow in one direction and  restricted flow in the opposite direction. These are known  as orifice-type check valves, or damping valves. The valve  contains the same spring, ball, and seat combination as a  normal check valve, but the seat area has a calibrated orifice  machined into it. Thus, fluid flow is unrestricted in the  designed direction while the ball is pushed off of its seat. The  downstream actuator operates at full speed. When fluid back  flows into the valve, the spring forces the ball against the seat  which limits fluid flow to the amount that can pass through  the orifice. The reduced flow in this opposite direction slows  the motion, or dampens, the actuator associated with the  check valve.  [Figure 12-38] An orifice check valve may be included in a hydraulic landing  gear actuator system. When the gear is raised, the check  valve allows full fluid flow to lift the heavy gear at maximum  speed. When lowering the gear, the orifice in the check valve  prevents the gear from violently dropping by restricting fluid  flow out of the actuating cylinder. the pilot valve and into the main spool right end chamber.  There, the spring and fluid pressure shift the spool back  to the left into the NEUTRAL or OFF position shown in  Figure 12-36 . To make the actuator move in the opposite direction, the  cockpit switch is moved in the opposite direction. All motion  inside the selector valve is the same as described above but in  the opposite direction. The left solenoid is energized. Pressure  is applied to the actuator through the right port and return  fluid from the left actuator line is connected to the return port  through the motion of the spool to the left. Sequence valves control the sequence of operation between  two branches in a circuit; they enable one unit to automatically  set another unit into motion. An example of the use of a  sequence valve is in an aircraft landing gear actuating system.  In a landing gear actuating system, the landing gear doors must  open before the landing gear starts to extend. Conversely,  the landing gear must be completely retracted before the  doors close. A sequence valve installed in each landing gear  actuating line performs this function. A sequence valve is  somewhat similar to a relief valve except that, after the set  pressure has been reached, the sequence valve diverts the fluid  to a second actuator or motor to do work in another part of  the system. There are various types of sequence valves. Some  are controlled by pressure, some are controlled mechanically,  and some are controlled by electric switches.  Another common flow control valve in aircraft hydraulic  systems is the check valve. A check valve allows fluid to  flow unimpeded in one direction but prevents or restricts  fluid flow in the opposite direction. A check valve may be  an independent component situated in-line somewhere in  the hydraulic system or it may be built-in to a component.  When part of a component, the check valve is said to be an  integral check valve. The operation of a typical pressure-controlled sequence  valve is illustrated in  Figure 12-36.  The opening pressure is  12-25 Pressure-Controlled Sequence Valve Check Valve Sequence Valves  Orifice-Type Check Valve Ret. Left solenoid Right solenoid Press Left actuator line To/from actuator unit Pilot valves Spool Right actuator line Energized Ret. Press Figure 12-37.  Servo control valve right solenoid energized. Figure 12-36.  Servo control valve solenoids not energized.