Purpose For Engine Monitoring Systems to meet their potential for improved safety and reduced operation and support costs@ significant attention must be focused on their reliability and validity throughout the life cycle This AIR will provide program managers@ designers@ developers and customers a concise reference of the activities@ approaches and considerations for the development and verification of a highly reliable engine monitoring system When applying the guidelines of this AIR it should be noted that engine monitoring systems physically or functionally integrated with the engine control system and/or performing functions that affect engine safety or are used to effect continued operation or return to service decisions shall be subject to the Type Investigation of the product in which they'll be incorporated and have to show compliance with the applicable airworthiness requirements as defined by the responsible Aviation Authority. This is not limited to but includes the application of software levels consistent with the critically of the performed functions. For instance@ low cycle fatigue (LCF) cycle counters for Engine Critical Parts would be included in the Type Investigation but most trend monitors and devices providing information for maintenance would not Introduction An engine Monitoring System (EMS) adds value by providing real time or near time information on the functional and physical condition of gas turbine engines. This information is used to alert operators to conditions that could impact safe operation@ schedule inspections and repairs to improve functional performance@ forecast spares requirements@ and manage warranties. The elements that comprise an EMS are discussed in ARP1587. When discussing the reliability and validity of an EMS@ the entire system needs to be considered@ (i.e.@ design philosophy@ software@ hardware@ sensors@ operating procedures@ training@ service introduction and field support). The reliability and validity of the EMS is only as strong as its weakest sub-element Prior to the advent of EMS@ engine operational information was obtained from flight crew and maintenance personnel observations gathered during operation or inspection. While there is no known qualification by a regulatory authority of the reliability and validity of these manually based approaches@ they have evolved into the procedures and regulations that are currently considered the reliability baseline. Today@ most EMS automate and enhance some or all the manual processes to increase equipment safety and reduce operating cost Many early@ attempts to introduce EMS have lacked total success for a number of reasons One of the major factors was a general lack of reliability. This reliability shortfall has come@ in part@ from the inherent unreliability of the system hardware elements. More importantly@ however@ reduced reliability was often due to the unreliability of theinformation provided by the system. History is replete with examples of false alarms@ ranging from false chip detector lights to more sophisticated systems producing many false fault codes per flight. The inability of early EMS to provide reliable information has caused many to be extremely cautions in deciding how or when to implement a system