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Forum: Reliability & Maintainability Questions and Answers

Topic: Reliability & Maintainability Questions and Answers

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

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Posted by: T.R. Pilling ( )
Organization:HQ ACC/XR
Date posted: Sat Oct 9 22:35:27 US/Eastern 1999
Subject: Fault Isolation Requirement
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


Seymour Morris Subject: Automatic Fault Isolation of Mechanical Systems
Reply Posted by: ( )
Organization: Reliability Analysis Center
Date Posted: Mon Oct 11 12:12:23 US/Eastern 1999
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.


Subject: Fault Isolation Requirement
Reply Posted by: Richard Unkle ( )General Electric Transportation Systems
Date Posted: Wed Oct 13 7:58:57 US/Eastern 1999
Without knowing the specific details of a design, it can be quite difficult to specify fault detection and fault isolation requirements. However, you might try to determine what level of fault detection and isolation are needed to meet other requirements such as maintainability, availability and sparing reqirements. These are typically requirements that will drive your support costs, and the level to which you can fault isolate will surely impact that. Another alternative would be to at least develop a functional model of your system, that is representative of the LRU level. From here, expected (or hypothesized) test points can be created. Such models can then be analyzed with diagnostic analysis tools such as Detex Systems Express. The output would be the percentage of isolation to specific ambiguity group sizes that are possible, given the amount of test visibility. Trade-offs can then be evaluated as to cost of producing automated tests, adding test visibility, etc.

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