"Blade trackers measure: (a) rotor blade height and (b) lead-lag for use in a Rotor Track and Balance (RTB) function in a Health and Usage Monitoring System (HUMS). HUMS is a generic term for a system used to measure@ monitor@ process@ and store information relating to the functioning and usage of an aircraft's on-board primary systems@ including the engine(s). Classifications: This specification establishes the interface requirements for blade trackers for use in a helicopter Health and Usage Monitoring System. The main purpose of this document is to standardize the electrical and data interfaces between the blade tracker and the aircraft. We expect that a successful standardization effort will reduce the cost of HUMS blade trackers@ reduce HUMS sensor inventories and increase interchangeability among HUMS sensors and systems. This blade tracker utilizes the SAE AS5394 - HUMS Advanced Multipoint Interface Specification. Sensor characteristics have been specified where they are fixed or limited by the sensing application. The blade tracker may have up to four electrical interfaces with the aircraft: electrical power@ rotational system indexing sensor (SAE AS5392)@ accelerometer (SAE AS5391) (optional)@ and HUMS processor as illustrated in Figure 1. Specification Terminology: The terms ""shall"" and ""should"" within this specification observe the following rules: a. The word ""shall"" expresses a mandatory requirement of the Specification. b. The word ""should"" expresses a recommendation or advice on implementing the specification or actions expected of users of the system."