allow fluid to flow one way from the gear-down side of the  main gear cylinders back into the hydraulic system return  manifold through the selector valve. Large and high-performance aircraft are equipped with  redundant hydraulic systems. This makes emergency  extension less common since a different source of hydraulic  power can be selected if the gear does not function normally.  If the gear still fails to extend, some sort of unlatching device  is used to release the uplocks and allow the gear to free fall.  [Figure 13-38] In some small aircraft, the design configuration makes  emergency extension of the gear by gravity and air loads  alone impossible or impractical. Force of some kind must  therefore be applied. Manual extension systems, wherein  the pilot mechanically cranks the gear into position, are  common. Consult the aircraft maintenance manual for all  emergency landing gear extension system descriptions of  operation, performance standards, and emergency extension  tests as required. To lower the gear, the selector is put in the gear-down  position. Pressurized hydraulic fluid flows from the hydraulic  manifold to the nose gear uplock, which unlocks the nose  gear. Fluid flows to the gear-down side of the nose gear  actuator and extends it. Fluid also flows to the open side of  the main gear door actuators. As the doors open, sequence  valves A and B block fluid from unlocking the main gear  uplocks and prevent fluid from reaching the down side of  the main gear actuators. When the doors are fully open, the  door actuator engages the plungers of both sequence valves  to open the valves. The main gear uplocks, then receives  fluid pressure and unlock. The main gear cylinder actuators  receive fluid on the down side through the open sequence  valves to extend the gear. Fluid from each main gear cylinder  up-side flows to the hydraulic system return manifold through  restrictors in the orifice check valves. The restrictors slow the  extension of the gear to prevent impact damage. There are numerous landing gear safety devices. The most  common are those that prevent the gear from retracting or  collapsing while on the ground. Gear indicators are another  safety device. They are used to communicate to the pilot the  position status of each individual landing gear at any time.  A further safety device is the nose wheel centering device  mentioned previously in this chapter. There are numerous hydraulic landing gear retraction  system designs. Priority valves are sometimes used instead  of mechanically operated sequence valves. This controls  some gear component activation timing via hydraulic  pressure. Particulars of any gear system are found in the  aircraft maintenance manual. The aircraft technician must  be thoroughly familiar with the operation and maintenance  requirements of this crucial system. The emergency extension system lowers the landing gear if  the main power system fails. There are numerous ways in  which this is done depending on the size and complexity of  the aircraft. Some aircraft have an emergency release handle  in the flight deck that is connected through a mechanical  linkage to the gear uplocks. When the handle is operated, it  releases the uplocks and allows the gear to free-fall to the  extended position under the force created by gravity acting  upon the gear. Other aircraft use a non-mechanical back-up,  such as pneumatic power, to unlatch the gear. The popular small aircraft retraction system shown in  Figures 13-35  and  13-36  uses a free-fall valve for emergency  gear extension. Activated from the flight deck, when the  free-fall valve is opened, hydraulic fluid is allowed to flow  from the gear-up side of the actuators to the gear-down side  of the actuators, independent of the power pack. Pressure  holding the gear up is relieved, and the gear extends due to  its weight. Air moving past the gear aids in the extension  and helps push the gear into the down-and-locked position. Figure 13-38.  These emergency gear extension handles in a Boeing  737 are located under a floor panel on the flight deck. Each handle  releases the gear uplock via a cable system so the gear can freefall  into the extended position.  Landing Gear Safety Devices Emergency Extension Systems 13-22 Manual extension access door Manual gear extension handles