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This Aerospace Information Report (AIR) is a general overview of typical airborne engine vibration monitoring (EVM)
systems applicable to fixed or rotary wing aircraft applications, with an emphasis on system design considerations. It
describes EVM systems currently in use and future trends in EVM development. The broader scope of Health and Usage
Monitoring Systems, (HUMS ) is covered in SAE documents AS5391, AS5392, AS5393, AS5394, AS5395, AIR4174.
1.1 Purpose
The purpose of this AIR is to provide information and guidance for the selection, installation, and use of EVM systems and
their elements. This AIR is not intended as a legal document but only as a technical guide.
2. INTRODUCTION
A complete EVM system includes all the equipment, data, and procedures used for monitoring and analyzing aircraft
turbine engine vibration and engine driven equipment. A complete comprehensive EVM system is shown in Figure 1 and
a simplified system in Figure 2. EVM may be one part of an engine condition monitoring system that monitors a number
of engine parameters, or it may be a stand-alone system. A distinction is usually made between that part of the system
dedicated to monitoring engine functions on board an aircraft and that part used for ground based analysis and
monitoring. The on-board portion is commonly called an airborne engine vibration monitoring (EVM) system, and it is this
part of the complete system that is described in this AIR. The ground based portion is described in SAE document
AIR4175A.
The primary moving parts of all turbine engines are the rotors, shafts and their bearings. When operating, the engine
rotors spin at relatively high speed within the engine case. Elements of these rotors, particularly fan, compressor, and
turbine blades, are subject to wear and damage, some types of which may unbalance the affected rotor. Increased rotor
unbalance causes increased cyclic stress on the structure and on the associated rotor bearings. In addition, the cyclic
forces due to unbalance may induce destructive vibration in other engine parts and accessories. Small amounts of rotor
unbalance are always present; large amounts usually cannot be tolerated for extended periods of operation without risk of
fatigue by certain engine components. In extreme unbalance events, continued operation may not be safely possible.
Most of the EVM systems now in use were developed to comply with FAR Part 25.1305(d)(3). These systems were
intended to provide the flight crew with a relative indication of engine imbalance. In most applications, specific operating
instructions were not tied to specific display levels, but rather the display was provided to give the crew some relative
indication of imbalance related vibration. With improvements in EVM system reliability and accuracy, increased
understanding of diagnostic capability, and the potential for prognostic use, the industry is recognizing the potential for
reducing operating costs, and potential safety benefit, from vibration signal analysis as part of engine health monitoring.
This benefit is not limited to Part 25 certified transport aircraft as evidenced by the growth in availability of systems for
turbine powered rotorcraft and other classes of turbine powered fixed wing aircraft.
EVM systems have also been developed for monitoring the vibration of other powerplant elements including afterburners,
reduction gears, bearings, transmissions, accessories and propellers. The recent availability of high speed, digital signal
processing integrated circuits has made it practical to provide very sophisticated on-board vibration analysis in today's
systems. Similarly, identification and rectification of rotor/propeller unbalance can minimize degradation of the engine and
other aircraft systems.
Many specific engine problems are detectable by an EVM system and the technical application is readily applicable to any
fixed or rotary wing gas turbine engine installation.
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