274 Cost — Benefit of Risk Advised Maintenance for Wiring during the Space Shuttle Major Maintenance Period (PSAM-0042) Available to Purchase

The risk of conducting improper maintenance that requires rework to correct inadequacies or collateral damage is addressed below. The focus is on preventive and corrective maintenance during the overhaul period between Shuttle flights. A methodology has been developed for predicting potential maintenance errors, including their impact. The methodology utilizes recorded maintenance data at NASA Kennedy Space Center (KSC) and other locations. Cost-benefit metrics are used to evaluate the effectiveness of candidate changes for risk reduction. Collateral damage to wiring is a significant contributor to wire failures, as well as defective workmanship, operational degradation and improper vendor installation practices. Aging phenomena are less significant and are difficult to identify. A wire risk maintenance risk model was developed to determine the importance of these causes and to provide the basis for the quantitative risk input to the cost-benefit metrics.

The project goals and decision criteria required the demonstration of the feasibility of a maintenance risk model that quantitatively evaluates maintenance procedures in terms of effectiveness. A cost-benefit evaluation of proposed maintenance improvements, within a risk—advised framework, was a criterion to be met. Cost-benefit analysis methods include Net Present Value (NPV) of procedural change cost savings from reduction of maintenance risk impacts, and comparison of NPV with cost of implementing the change. The basic decisions are related to identifying the maintenance risk drivers for the subject maintenance procedures, and the evaluation of candidate improvements. Maintenance work flow diagrams were used as a basis for identifying the improvements. Methodology development was measured against specific Shuttle wiring decision requirements so that the final product has demonstrated utility. Supporting data analysis for modeling using the NASA PRACA, Ref [1] and SMART, Ref [2] databases also served to meet some of the requirements directly. The Wire Risk Assessment structure and analysis flow was established. It is based on a probabilistic risk assessment (PRA) framework using event trees and supporting fault trees. The QRAS PRA software Ref [3] was utilized for logic representation and risk computations. A wire risk assessment tool (WRAT) and supporting methodology was developed and implemented. Supporting data sources were identified and data was compiled and analyzed for input to the QRAS models. A supporting human error plan was implemented which utilized human factors domain area interviews that were conducted by the USA Corp. at NASA KSC.

Procedures selected for maintenance risk evaluation and cost benefit analysis are:

▪ OEL-4061 Multiconductor Butt Splice Standard Procedure

▪ APU 2 EGT 1&2 Removal and Replacement-PR APU-2-27-0599, 2/21/02-Transducer Thermocouple Failed Nominal S/B Reading- Procedure OV 109V-0381

▪ LRU ORBITER OI Multiplexer/Demultiplexer OA3 (Removal and Replacement)

The details of this work are presented in the Wire Risk Assessment Interim Report, Ref [1] dated August 1, 2004.