Flexible Sustainment (FS) is a concept that provides procedural freedom to optimize Life-Cycle Costs (LCC) through tradeoffs that are accomplished either during initial or follow-on acquisition. FS is a process that encourages the Program Manager (PM) to use performance-based specifications and to develop innovative, cost-effective, life cycle solutions. The principal elements of FS are Reliability-Based Logistics (RBL) techniques and Trigger-Based Item Management (TBIM). 
Guide: DoD Flexibility Sustainment Guide – July 1999
Principal Elements of Flexible Sustainment (FS)
- Reliability-Based Logistics (RBL):Emphasizes the importance of designing Reliability into systems and is an expansion of the process used to determine the support concept for a system, subsystem, and/or component. RBL addresses decisions such as consumable versus repairable, commercial versus organic repair, warranties, technology insertion, and Form-Fit-Function Interface (F3I) specifications as methods for facilitating reliable designs.
- Trigger-Based Item Management (TBIM): A management approach that relies on predetermined indicators (“triggers”) to inform management of the need to take corrective action prior to a situation deteriorating to a crisis point. In addition to RBL and TBAM, other innovative support solutions, such as procurement of Form-Fit-Function-Interface (F3I) spares, performance warranties, and obsolescence assessment are presented as cost-effective support alternatives.
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