The purpose of this SAE Aerospace Information Report (AIR) is to provide management, designers, and operators with information to assist them to decide what type of power train monitoring they desire. This document is to provide assistance in optimizing system complexity, performance and cost effectiveness. This document covers all power train elements from the point at which aircraft propulsion energy in a turbine or reciprocating engine is converted via a gear train to mechanical energy for propulsion purposes. The document covers aircraft engine driven transmission and gearbox components, their interfaces, drivetrain shafting, drive shaft hanger bearings, and associated rotating accessories, propellers, and rotor systems as shown in Figure 1. For guidance on monitoring additional engine components not addressed, herein (e.g., main shaft bearings and compressor/turbine rotors), refer to ARP1839. This document addresses rotary and fixed wing applications for rotor, turboprop, turbofan, prop fan, and lift fan drive trains on both commercial and military aircraft. Information is provided to assist in: a Defining technology maturity and application risk b Cost benefit analysis (Value analysis) c Selection of system components d Selection of technology e Managing interface requirements f Defining information flow requirements The aviation industry uses available and emergent technologies for power train monitoring to affect flight safety, power train and aircraft reliability, availability, maintainability, life cycle cost, and mission effectiveness. This Aerospace Information Report (AIR) provides information and descriptions of common practices based on lessons learned. AIR4174A updates the information contained in the base document.