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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: T.R. Pilling (thomas.pilling@langley.af.mil )
Organization:HQ ACC/XR
Date posted: Sat Oct 9 22:35:27 US/Eastern 1999
Subject: Fault Isolation Requirement
Message:
We are currently writing operational requirements for a new fighter aircraft and have received a variety of feedback from contractors regarding a fault isolation requirement. The requirement is currently written as 90% automatic fault isolation to a single ambiguity group (LRU). We've been told that 90% should be easily achieved for avionics but not for hydro-mechanical systems. Is this true? If so, what do you think would be a reasonable requirement given emerging technology over the next 8-10 years? Please respond at your earliest opportunity. Thanks


Reply:

Seymour Morris Subject: Automatic Fault Isolation of Mechanical Systems
Reply Posted by: (smorris@alionscience.com )
Organization: Reliability Analysis Center
Date Posted: Mon Oct 11 12:12:23 US/Eastern 1999
Message:
First, some definitions which you should keep in mind:
1. Fault Detection: A process which discovers the existence of faults.
2. Fault Isolation: Where a fault is known to exist, a process which identifies one or more replaceable units where the fault(s) may be located.
3. False Alarms: An indication of a fault where no fault exists, such as operator error or built-in test (BIT) design deficiency.

90% automatic fault detection may be realistic for hydro-mechanical elements. Detection can be accomplished through monitoring parameters such as pressure, loss of fluid, the position or distance traveled by a mechanical actuator, etc. 90% automatic fault isolation for hydro-mechanical systems is much more difficult. To automatically isolate say, a leak, or a failed structure, would require numerous sensors (dependent on the level of isolation required) and associated processing intelligence, which could sense changes in mechanical behavior characteristics for a given item. This added complexity often introduces false alarms.
Because of the uniqueness of mechanical systems, identifying a realistic automatic fault isolation requirement for 8-10 years down the road really requires addressing program unique issues such as:
Level of repair and associated automatic fault isolation level
Hydro-mechanical assembly types and consideration given to technology available for automatic fault isolation on each
Population of hydro-mechanical line replaceable units

A good paper describing some of the issues involved with automatic fault isolation of mechanical systems is the following:
On-line Diagnostics of a Variable Displacement Pump of a Flight Actuation System, Skormin, V.A.; Apone, J., Watson School of Engineering, Binghamton Univ., NY, USA. Aerospace and Electronics Conference, NAECON 1995, Proceedings (IEEE) 1995 Pages: 503 - 510 vol.1.
This paper describes an on-line diagnostic system for a variable displacement pump, serving as a part of a self-contained flight actuator. A mathematical model describing dynamics of the pump, driving a control surface of an aircraft, is established. Typical pump failures are defined through the model parameters in the form of a failure pattern library. A diagnostic model, intentionally sensitive to particular failures, is defined. A diagnostic procedure, featuring on-line parameter estimation of the mathematical model, failure detection and identification is developed. A failure prediction procedure, detecting and utilizing trends exhibited by parameter estimates is formulated.


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