Maintainability is key part of RAMS estimation and prediction in complex assets. Indeed, availability calculation comprises accurate estimation of maintainability and many times, it is just a time stamp for MTTR estimations. However, maintainability is a human related figure where the skill, capabilities, tools and the design of the asset play key role in its performance. The aim of this article is to describe the effects of ergonomist contribution during maintainability process for system/products design. System designer thinking in system and its subsystem in a way of technical functionality. On the other hand, ergonomist are expertise in human capability and limitation. If human, become a part of system than their interface and interaction become crucial factors in a success of system performance and its sustainability. In this paper, it has discussed four main issues that help the process of maintainability design. These issues are safety (Safety I and Safety II), task analysis (Hierarchical Task Analysis (HTA) as tool) and risk analysis (using William Fine method). It has also touched reliability engineer’s task in order to increase Overall Equipment Effectiveness (OEE).