Chapter 22 - Reliability and Maintainability

The purpose of this Section is to provide an overview of reliability and maintainability (R&M) evaluations conducted during initial flight test. For purposes of this Section reliability is defined as the probability that an item will perform its intended function for a specified interval under stated conditions. Maintainability is a characteristic of design and installation which is expressed as the probability that an item will be retained in or restored to a specified condition within a given period of time, when maintenance is performed in accordance with prescribed procedures and resources. Flight test R&M evaluations are essential because R&M characteristics cannot be predicted with any degree of success. Initial reliability for newly designed equipment, during bench and laboratory testing, is normally 10 to 20 percent of the predicted value. Considerable laboratory and flight test is required before the actual reliability begins to approach the original prediction. Therefore, it is essential that the R&M engineer be involved during the design and early production stages. Otherwise the improvement in R&M during the flight test phase may be insufficient to obtain the desired minimum R&M level without initiating a new design and production schedule. The R&M engineer faces a constant battle to justify his evaluation. It obviously takes time and costs money to run the complete R&M evaluation but the results of a good evaluation may not really be evident until the aircraft is in service use. He must be prepared to utilize data from prototype system and bench tests to provide indicators of potential problem areas and/or validate good characteristics. This Section will concentrate on the flight test portion of the R&M process. However, there are many activities such as ground laboratory and bench tests, studies, maintainability demonstrations, etc., that constitute invaluable portions of this overall evaluation that cannot be ignored and must be considered/utilized when establishing the flight test program.