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