SAE AIR6334

Time in Service (TIS), or flight hours, logged in maintenance records against an installed rotorcraft transmission is normally used as the “official” time on wing metric for the transmission’s component wear out inspection interval requirement and, in some instances, retirement change on life limited parts. This AIR addresses traditional methods of transmission TBO extensions and introduces rotorcraft transmission monitoring usage metrics that could be used to modify TIS inspections by tracking torque to determine both loads on life limited parts and component wear. This is a document of the SAE HM-1 Committee intended to be used as a technical information source and is not intended as a legal document or standard. This AIR does not provide detailed implementation steps, but does address general implementation, past experience, concerns and potential benefits. This SAE Aerospace Information Report (AIR) provides insight and guidance into both proven and proposed practices for extending Times Between Overhaul (TBO) on Rotorcraft Drive Trains through the use of monitoring data. Manufacturers of commercial transport rotorcraft are currently required by Federal Aviation Administration (FAA) Advisory Circular (AC) 121-22C and other international equivalents to develop maintenance programs that are guided by the tenets of the Maintenance Steering Group (MSG)-3, Volume 2. The maintenance processes of MSG-3 were refined over many years by the international aviation industry with the objective of reducing costs while increasing reliability and enhancing safety. Time-in-Service (TIS) monitoring data and associated metrics enable maintenance processes to meet the objectives of MSG-3. Future Health and Usage Monitoring System (HUMS) equipment is anticipated to produce monitoring data that may also be used to establish extensions to inspection intervals of power train assemblies for rotorcraft fleet operators while also meeting the objectives of MSG-3.