Future Airplane Hydraulic Distribution System Design Developments and Testing Criteria

Advanced tensile, impulse, and flexure testing techniques are evaluated and compared with actual airplane combined load environments encountered by hydraulic systems on large jet aircraft.

Based on extensive analyses, an advanced all-welded approach and a conventional distribution system were fabricated and combined environment tested, as individual systems, to the most severe flight environments for the life of the airplane on an accelerated basis. Results of the system test program and separate tests conducted per Military specification are reviewed in detail. Noted are the vast differences between the combined stress levels measured using the two testing techniques. Maximum combined stress levels measured in the simulated airplane systems only reached one-third of those actually required if the systems were tested per Military specification.

Investigation showed that installation practices tended to explain why excessive stresses and/or wear conditions caused tube failures in service in spite of the fact that the connections had already passed Military specification requirements. Whether or not the resultant installation build-in stress levels were excessive only tends to point out the limited applicability of the present specification techniques; and gives another reason to establish a new basis for tubing and fitting testing. An evaluation of hydraulic impulse surges by computer and test verification in Boeing aircraft was conducted. A revised impulse wave form is considered as a recommended standard for impulse testing of tubing and fittings.

New criteria are recommended for both the installation and the testing of hydraulic tubing systems. Advanced installation techniques are suggested as a means to circumvent most of the service failures once the tube connectors have passed the advanced testing methods.