FAQ: Vibration Troubleshooting
What is Vibration Troubleshooting?
Flight crew will sometimes report that an aircraft is producing unusual vibration during operation and request that vibration troubleshooting be performed to determine the cause of the vibration. Unusual vibrations may be produced any time the aircraft is operation or may only be produced at certain power and/or flight regimes.
Maintenance personnel may suspect unusual vibrations are contributing to accelerated wear or early failure of aircraft components and want to perform vibration troubleshooting to determine the cause of these issues.
Any time the vibration characteristics of an aircraft change unexpectedly, it is wise to perform vibration troubleshooting in order to avoid costly repairs or unscheduled downtime later on.
Basic Theory Of Vibration Troubleshooting
Each rotating component, mechanical or aerodynamic effect produces vibration at its characteristic frequency or speed (RPM). The vibration troubleshooter's job is to identify problem vibrations and identify their root cause by analyzing their characteristic frequency
Examples of vibration problems and their characteristics:
Variation In Vibration Magnitude
The magnitude (level) of a vibration is often modified by other mechanical characteristics of the machine under test.
Obtain Detailed Information About The Complaint
Flight crew should provide details about the conditions under which the unusual vibration is felt. Make sure to provide power settings, speed range, altitude, flight configuration and any other factors which might affect vibration readings.
Maintenance personnel should provide details about any suspected component distress or failure which may be caused by vibration.
When vibration troubleshooting, the most useful vibration measurement modes are Spectrum Analysis and Peak Phase (balance).
Spectrum analysis collects a broad range of vibrations from near zero up to the maximum frequency specified and at the specified resolution. This mode is very useful for identifying what vibration frequencies are present and of those, which are producing the problem complaint.
Peak Phase (balance) readings collect narrow band vibration at the incoming tach rate. These measurements, in conjunction with a balancing process are very useful to determine if a rotating assembly has good mechanical integrity or if wear and corresponding looseness makes balancing difficult.
Investigating Crew Complaints
If investigating crew complaints of excess vibration, one should choose sensor locations which are in the vicinity of the cockpit and/or other crew areas. The aircraft would then be operated at the flight conditions which generated the crew complaint, and vibration readings collected for analysis.
Investigating Mechanical Distress
If investigating mechanical distress, one should choose a sensor location which measures the vibration on or near the problem structure itself. The aircraft should be operated and vibration data collected at a variety of different conditions in order to capture the particular condition which adversely affects the problem structure.
When investigating crew complaints, the recommended data collection settings are Spectrum Analysis 0-15K CPM freq range, resolution 400 Lines, and vibration units in Velocity (IN/S).
For investigation of vibration suspected of producing mechanical stress, wider frequency ranges should be used. The following Spectrum Analysis frequency ranges are available with the MicroVib II:
Spectrum Resolution of 400 Lines or higher is recommended for vibration troubleshooting. The higher the spectrum resolution, the more detail will be available in the test data.
Velocity (IN/S) vibration units are recommended for evaluation of vibration frequencies up to 100K CPM. For vibration frequencies 100K CPM and higher, Acceleration (G) vibration units are recommended.
Formulate a Test Plan
Use the information provided by the flight crew and/ or maintenance personnel to formulate a test plan. The test plan should include the following:
Execute the Test Plan
Operate the aircraft at the designated power settings / flight conditions and collect the vibration data specified on the test plan.
Download and identify the vibration data in MicroBase Pro, then use the Machinery Identification feature to determine what components and/or effects are producing the vibration in question. See Application Note AN-MBP-MACHID for detailed instructions of this process.
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