#### § 25.1435 Hydraulic systems.

(a) *Element design.* Each element of the hydraulic system must be designed to:

(1) Withstand the proof pressure without permanent deformation that would prevent it from performing its intended functions, and the ultimate pressure without rupture. The proof and ultimate pressures are defined in terms of the design operating pressure (DOP) as follows:

| Element | Proof (xDOP) | Ultimate (xDOP) |
| --- | --- | --- |
| 1. Tubes and fittings. | 1.5 | 3.0 |
| 2. Pressure vessels containing gas: | | |
| High pressure (e.g., accumulators) | 3.0 | 4.0 |
| Low pressure (e.g., reservoirs) | 1.5 | 3.0 |
| 3. Hoses | 2.0 | 4.0 |
| 4. All other elements | 1.5 | 2.0 |

(2) Withstand, without deformation that would prevent it from performing its intended function, the design operating pressure in combination with limit structural loads that may be imposed;

(3) Withstand, without rupture, the design operating pressure multiplied by a factor of 1.5 in combination with ultimate structural load that can reasonably occur simultaneously;

(4) Withstand the fatigue effects of all cyclic pressures, including transients, and associated externally induced loads, taking into account the consequences of element failure; and

(5) Perform as intended under all environmental conditions for which the airplane is certificated.

(b) *System design.* Each hydraulic system must:

(1) Have means located at a flightcrew station to indicate appropriate system parameters, if

(i) It performs a function necessary for continued safe flight and landing; or

(ii) In the event of hydraulic system malfunction, corrective action by the crew to ensure continued safe flight and landing is necessary;

(2) Have means to ensure that system pressures, including transient pressures and pressures from fluid volumetric changes in elements that are likely to remain closed long enough for such changes to occur, are within the design capabilities of each element, such that they meet the requirements defined in [§ 25.1435(a)(1)](/on/2024-09-19/title-14/section-25.1435#p-25.1435(a)(1)) through [(a)(5)](/on/2024-09-19/title-14/section-25.1435#p-25.1435(a)(5));

(3) Have means to minimize the release of harmful or hazardous concentrations of hydraulic fluid or vapors into the crew and passenger compartments during flight;

(4) Meet the applicable requirements of [§§ 25.863](/on/2024-09-19/title-14/section-25.863), [25.1183](/on/2024-09-19/title-14/section-25.1183), [25.1185](/on/2024-09-19/title-14/section-25.1185), and [25.1189](/on/2024-09-19/title-14/section-25.1189) if a flammable hydraulic fluid is used; and

(5) Be designed to use any suitable hydraulic fluid specified by the airplane manufacturer, which must be identified by appropriate markings as required by [§ 25.1541](/on/2024-09-19/title-14/section-25.1541).

(c) *Tests.* Tests must be conducted on the hydraulic system(s), and/or subsystem(s) and elements, except that analysis may be used in place of or to supplement testing, where the analysis is shown to be reliable and appropriate. All internal and external influences must be taken into account to an extent necessary to evaluate their effects, and to assure reliable system and element functioning and integration. Failure or unacceptable deficiency of an element or system must be corrected and be sufficiently retested, where necessary.

(1) The system(s), subsystem(s), or element(s) must be subjected to performance, fatigue, and endurance tests representative of airplane ground and flight operations.

(2) The complete system must be tested to determine proper functional performance and relation to the other systems, including simulation of relevant failure conditions, and to support or validate element design.

(3) The complete hydraulic system(s) must be functionally tested on the airplane in normal operation over the range of motion of all associated user systems. The test must be conducted at the system relief pressure or 1.25 times the DOP if a system pressure relief device is not part of the system design. Clearances between hydraulic system elements and other systems or structural elements must remain adequate and there must be no detrimental effects.

[Doc. No. 28617, [66 FR 27402](https://www.federalregister.gov/citation/66-FR-27402), May 16, 2001]