TM 1-1500-204-23-2 (8) Adjust the pressure relief valve to relieve pressure at approximately 200 psi above the system pressure of the system under test. (9) Open the flow control valves and proceed with the test In accordance with the applicable aircraft technical manual (10) Following the test procedure, open the bypass valve (11) Close the flow control valves (12) Allow the engine to cool as required by the specific operating procedures (13) Shut down test stand In accordance with applicable operating procedures (14) Disconnect couplings from aircraft and cap Immediately to prevent contamination. (15) Release parking brake and tow test stand from work area 4-21.  Conversion of Aircraft to Fire-Resistant Hydraulic Fluid .  Following are general guidelines for conversion of Army aircraft from MIL-H-5606 to MIL-H- 83282 hydraulic fluid. a.  Hydraulic Fluid MIL-H-5606.  MIL-H-5606 is a petroleum base hydraulic fluid and IS presently specified for use In some Army rotary wing and fixed wing aircraft Because of its relatively low flashpoint of 200 °F (93   C), It has been considered a safety hazard In an effort to remove the vulnerability of aircraft systems and related support equipment to accidental or combat-incurred fires, hydraulic fluid MIL-H-83282 has been developed as a directly interchangeable fluid which is significantly less susceptible to fire, with a flashpoint of 400 °F (204°C). b.  Hydraulic Fluid MIL-H-83282.  MIL-H-83282 consists of a synthetic hydrocarbon base and contains additives which provide superior anti-wear charac- teristics and inhibit oxidation and corrosion.  MIL-H- 83282 has an operational high temperature limit of 400°F (204    C) as compared to 275°F degrees (135   C) for MIL-H-5606.  These factors should provide sig- nificantly improved reliability.  Flashpoint, fire point, and spontaneous ignition temperatures of MIL-H-83282 ex- ceed that of MIL-H-5606 by greater than 200 °F (93    C), and tests show that MIL-H-83282 extinguishes itself when the external source of flame or heat is removed MIL-H-83282 is compatible with all materials used in systems presently employing hydraulic fluid MIL-H- 5606 and may be combined with the latter fluid with no adverse effect except a degradation of Its fire resistant properties.  However, the presence of MIL-H-5606 in amounts exceeding 3 percent by volume will com- promise the fire resistant performance of MIL-H-83282. Although MIL-H-83282 exceeds the performance of MIL-H-5606 at normal temperatures, the viscosity of MIL-H-83282 increases at low temperatures, therefore, use of MIL-H-83282 must be within the limitations set forth in paragraph 4-21f. c.  Applications of MIL-H-83282.  The fire hazards of MIL-H-5606 are greatest in flight control systems which are under high pressure and which have a rela- tively large volume of fluid The aircraft hydraulic sys- tems and components shown in table 4-43 will be converted from MIL-H-5606 to MIL-H-83282. d.  Non-Applicability of MIL-H-83282.  The com- ponents shown In table 4-44 will retain MIL-H-5606 hydraulic fluid.  These components cannot be readily serviced by field personnel or have not been tested for MIL-H-83282.  In the event that the temperature limita- tions in paragraph b above are exceeded, field person- nel will be unable to change these components from MIL-H-83282 back to MIL-H-5606. e.  Conversion Procedures.  Procedures will be performed at Direct Support or higher levels of main- tenance unless otherwise directed by reference to a specific maintenance manual Refer to the applicable aircraft Technical Manual or to TB 55-1500-334-25 for specific conversion procedures f.  Operating Temperature Limitations.  Although MIL-H-83282 operates well under normal temperatures, the viscosity of the fluid increases as the temperature decreases Therefore, the minimum ambient operating temperatures shown In table 4-45 have been estab- lished for the use of MIL-H-83282.  These temperatures are based on a cloid soak of the hydraulic fluid at low temperatures.  Aircraft that are sheltered or pre-heated prior to engine start should not have any problems with stiff or sluggish flight controls.  Unless the sheltered areas are at or below the temperatures in table 4-45, the viscosity of MIL-H-83282 will not create any difficulty. Once the aircraft is started and warm, the hydraulic fluid should stabilize within normal operating limits. 4-167