This SAE Aerospace Standard (AS) provides guidelines for the functional, performance, qualification and acceptance testing, and documentation requirements for the components of an airborne engine vibration monitoring (EVM) system which is intended for use as a turbojet engine rotor unbalance indicating system, per FAR 25.1305 (D)(3) on transport category airplanes. For the purpose of this document, this means a system which can provide real-time flight deck displays of engine vibration caused by rotor unbalance, throughout the flight envelope, which are suitable for: a. relating to engine vibration limits (where such limits are specified), b. use in ice shedding procedures, and c. helping the flight crew to determine which engine has the higher level of vibration, following an engine damage event. As a minimum, the functional capability for such a system shall include the ability to compute and display vibration levels specifically related to the rotational speed(s) of the engine rotor(s). Systems which can produce only broadband vibration outputs have been used in the past but they are not considered suitable for the above purposes. Some EVM systems may also generate other outputs, suitable for such functions as engine vibration trend monitoring or trim balance calculation, but these are considered to be outside the scope of the requirements for a minimum system. This document also defines some recommended, optional, functions and features which, while they are outside the scope of minimum requirements, are nevertheless considered to be highly desirable for a practical system. It should be noted that systems incorporating capabilities such as described above are also sometimes referred to by the abbreviation AVM. However, there are, or can be, airborne measurements taken of vibrations from sources other than the engines. For the purpose of this document the more descriptive term, engine vibration monitor (EVM), will be used. The purpose of this document is to provide guidance on the requirements for airborne EVM systems for turbojet engines on transport category airplanes.