Text in force at 23-63: from 23-49, effective 11 Mar 1996.
(a) Design. Each hydraulic system must be designed as follows:
(1) Each hydraulic system and its elements must withstand, without yielding, the structural loads expected in addition to hydraulic loads.
(2) A means to indicate the pressure in each hydraulic system which supplies two or more primary functions must be provided to the flight crew.
(3) There must be means to ensure that the pressure, including transient (surge) pressure, in any part of the system will not exceed the safe limit above design operating pressure and to prevent excessive pressure resulting from fluid volumetric changes in all lines which are likely to remain closed long enough for such changes to occur.
(4) The minimum design burst pressure must be 2.5 times the operating pressure.
(b) Tests. Each system must be substantiated by proof pressure tests. When proof tested, no part of any system may fail, malfunction, or experience a permanent set. The proof load of each system must be at least 1.5 times the maximum operating pressure of that system.
[(c) Accumulators. A hydraulic accumulator or reservoir may be installed on the engine side of any firewall, if--
(1) It is an integral part of an engine or propeller system, or
(2) The reservoir is nonpressurized and the total capacity of all such nonpressurized reservoirs is one quart or less.]
61 FR 5170 · Source