Processes & Considerations in Extensions to Time Between Overhauls and Paths to On-Condition for US Army Rotorcraft Propulsion Systems

To ensure airworthiness, the Aviation Engineering Directorate (AED) of the US Army Aviation and Missile Research, Development, and Engineering Center (AMRDEC) is responsible for evaluating and approving proposed extensions to time/cycle based overhaul intervals of legacy, US Army aircraft. This paper provides the current AED processes and considerations in extending Time Between Overhaul (TBO) intervals for US Army legacy rotorcraft propulsion drive systems and engines. It explains how US Army legacy rotorcraft, drive systems and engines are initially assigned TBOs as a result of qualification specifications, testing, tear down analyses, and customer maintenance requirements. The discussion then addresses US Army recommended processes to extend original rotorcraft drive system/engine TBOs in accordance with (IAW) Aeronautical Design Standards while considering the applicable platform maintenance program. A review of beneficial relationships between Condition Based Maintenance (CBM) initiatives and field data to facilitate TBO extension approvals is also included. Subsequently, current pursuits of On Condition maintenance paths for propulsion drive/engine systems through the use of vibratory CBM are described. Since scarce documentation exists for recommending component sample sizes in the verification and validation of vibratory CBM algorithms for On Condition maintenance, this paper addresses requisite sample sizes, statistical processes, and considerations in verifying and validating vibratory CBM algorithms prior to approval of US Army On Condition maintenance as a replacement to legacy TBO maintenance.