SAE standard aims to protect against servo-actuator contamination

  • 30-Jun-2008 09:55 EDT

ARP5941 (Aerospace Fluid Power–Contamination Sensitivity–Consideration for Establishing Test Procedures for Flight Control Actuators) was generated by SAE International’s A-6 Aerospace Actuation, Control, and Fluid Power Systems Committee to assure continued satisfactory performance of flight-critical servo-actuators under the maximum allowable particulate contamination in airplane hydraulic systems.

The recommended practice is based on the proven experience of the commercial airplane fleet, according to ARP5941 sponsor Manfred Runkel, retired Vice President of Engineering from Smiths Aerospace, now GE-Aviation. “These airplanes have hydraulic flight control actuators incorporating servo-valves designed with certain materials and dimensions,” he explained. “There has never been a case where particulate contamination has caused one of these valves to function improperly. However, it is recognized that future designs—materials, material combinations, clearances, etc.—of flight control actuators may not have as good a fault tolerance as the existing flight control actuator designs and their historical characteristics of sensitivity to contamination.”

The recently approved recommended practice captures important design criteria and establishes a set of tests that should be performed for new valve designs that deviate either in material or clearance from the historical ones. Also included are required acceptance criteria, according to Runkel.

“A key premise of the document is the cooperation between the buyer and seller of the actuator containing the servo-valve,” said Runkel. “It is up to them to agree that the tests are needed—i.e., that the new valve design is sufficiently different than the historical one. They also will have to agree on the specifics of the test setup and test procedures, as well as the final acceptance criteria.”

The U.S. FAA triggered development of this ARP5941 when it requested that the SAE A-6 Committee provide recommendations regarding the maximum in-service contamination levels for hydraulic systems in commercial aircraft. The committee complied, establishing an industry-wide task force and delivering its recommendation to the FAA in 1998. The recommendation was to establish the maximum in-service limit at NAS1638 (now AS 4059), Class 9. This recommendation was based on the historical evidence of satisfactory performance of critical flight-control actuators in existing fleet-wide measured particulate contamination of hydraulic systems. It was accepted by the FAA and forwarded in June of 1999 to the U.S. National Transportation Safety Board, which concurred.

After completion of that task, it was recognized by the committee that a document was needed to assure that this recommendation would also be valid in the future. ARP5941 is the result.

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