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Forum: Reliability & Maintainability Questions and Answers

Topic: Reliability & Maintainability Questions and Answers

Topic Posted by: Reliability & Maintainability Forum (src_forum@alionscience.com )
Organization: System Reliability Center
Date Posted: Mon Aug 31 12:47:36 US/Eastern 1998

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Original Message:

Posted by: Julien NICO (ARZ_St_ELB@compuserve.com )
Organization:Aérazur - Zodiac Group
Date posted: Mon Aug 2 11:45:52 US/Eastern 1999
Subject: Reliability Prediction
Message:
I'm looking for information to calculate reliability of a component. I'm using the NPRD-95 and the FMD-97 to establish some analysis. Unfortunately, for one of our component, there is one part that we cannot find on this document. I try to determine MTBF for a frangible self sealing valve (for fuel application on an Helicopter). Component is composed of two opposite check valve (reliability given by the NPRD) and a metallic part to maintain both check valves open. This metallic part has a predifined level of rupture and in case of crash, shall broke to allow both valves to close and separate the fuel line. I find two failure mode for the part. One, it will fail with a level of stress under the predifine level (rupture of the line without crash), the second, it will not break when needed (fuel line will not separate in case of crash). I don't know how to calculate MTBF of this metallic part and also define the percentage of occurrence of each failure mode. I will really appreciate if someone have method to study reliability of this component (and in fact, reliability of all new component)


Reply:

Subject: Reliability of Mechanical Parts
Reply Posted by: Seymour Morris (smorris@alionscience.com )
Organization: Reliability Analysis Center
Date Posted: Mon Aug 2 14:47:32 US/Eastern 1999
Message:
I would recommend that you not calculate an MTBF for this part, but rather estimate a design life (especially for the metallic part you refer to). To do this you need to perform a classical mechanical engineering analysis and/or testing of the component to ensure structural robustness over the expected aircraft life (yet still function as a fuel cut-off in a crash situation). You must ensure that all structural and moving elements that make up this assembly are of sufficient strength to withstand normal operational stresses over the aircraft lifetime, or over a given preventive maintenance replacement period. Simply determining a handbook MTBF for such a critical component does not sufficiently take into account the unique environmental input parameters associated with the application.


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