TM 1-1500-204-23-2 (b) Cushioning during taxiing.  The liquid spring controls the bouncing motion of the aircraft in much the same way as the shock strut does.  As the piston in the liquid spring moves inward and outward In relation to its housing, there is a back-and-forth move- ment of fluid through an orifice in the piston.  This fluid flow restrains the rate at which the piston moves, thus dampening the up-and-down movements of the aircraft fuselage. (c) Reducing shock during landing. In the liquid spring, the resistance to fluid flow necessary to convert motion energy to heat energy is effected by means of a check valve in the piston.  As the liquid spring compresses, fluid flows through the check valve In addition to flowing through the orifice.  During extension, the check valve closes and fluid then passes through the orifice only.  The additional restraint that is set up to the movement of fluid from one side of the piston to the other provides rebound control during landing. (2) Servicing.   The liquid spring can be ser- viced as follows' (a) The exposed portion of the liquid spring shaft should be cleaned by using a clean, lint-free cloth moistened with hydraulic fluid. (b) When the liquid spring is serviced to the correct level with the proper type of hydraulic fluid, the fluid is under pressure A special gun, which looks like a hand grease gun, is used to force the fluid Inside the spring. NOTE Specific  servicing  instructions  are  stamped on the nameplate of the spring and are also included In the manual for the aircraft. 4-20.  Use of Test Stands.  The purpose of the aircraft hydraulic test stand is to perform the following test operations on aircraft hydraulic systems flush or fill the system with micronically filtered hydraulic fluid, provide a source of hydraulic pressure for testing aircraft hydraulic systems without the necessity of operating the aircraft engine, and test all components of the aircraft hydraulic systems, including aircraft with pressurized reservoir systems.  The appropriate aircraft and equip- ment technical manuals must be consulted prior to use of any test stand This manual contains only general guidelines for their use. a.  Safety.  Prior to use of a test stand, it is essen- tial that the personnel responsible for its operation are completely familiar with procedures and operation of the equipment. WARNING   Always wear ear protection when noise produced by test stand engines exceeds established limits.  Failure to comply will result in serious injury to personnel.   Do not connect or disconnect any hoses with the fluid bypass valve closed and the test stand inoperation.  High pressure contained in the hose may injure person- nel b.    Procedures.    General  operating  procedures  are  as follows: (1) Set the parking brake to secure the test stand in position. (2) Perform the operator inspection of the test stand (3) Connect the test stand coupling halves to the corresponding outlets on the aircraft.  Keep unused cou- plings capped at all times. (4) Open the fluid bypass valve. (5) Start the engine in accordance with ap- plicable operating procedures. (6) Adjust the pump delivery for requirements of the aircraft under test. WARNING Do  not  connect  or  disconnect  any  hoses with the fluid bypass valve closed and the test   stand   In   operation.      High   pressure contained in the hose may injure personnel. (7) Slowly close the fluid bypass valve. 4-166