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:

  • An out of balance rotor, turbine or shaft will produce a vibration at its speed of operation (first order or one per rev vibration).
  • Blade rate aerodynamic effects are produced at the operating speed x number of blades (Blade Rate Vibration).
  • Geared systems will produce vibration at the gear tooth engagement rate of gear speed x number of teeth (Gear Meshing Vibration).
  • Four Cycle reciprocating engines will produce vibration at the firing rate of 1/2 crank speed and at multiples of that speed (Combustion vibration and harmonics).

Variation In Vibration Magnitude

The magnitude (level) of a vibration is often modified by other mechanical characteristics of the machine under test.

For example:

  • The first order, "unbalance" vibration of a rotor will vary proportionally to the amount of unbalance.
  • The first order vibration of a bladed rotor will be affected by moving air (wind).
  • Any of the vibrations described in the section above can be increased in amplitude if they are affected by a mechanical resonance at or near the vibration frequency.

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.

Vibration Measurements

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.

Instrument Settings

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:

  • 0-15K CPM - Ideal for Rotors, Propellers and other drive system.
  • 0-60K CPM - Ideal for Turbines and high speed portions of the drive system.
  • 0-300K CPM - Ideal for analysis of lower frequency Gear Mesh.
  • 0-1200K CPM - Ideal for analysis of higher frequency Gear Mesh.

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:

  • Vibration sensor locations and orientations to be used during the test (seat rail, instrument panel, engine, vertical, lateral, etc.)
  • Conditions of operation and flight under which testing will be performed (120 Kt forward flight, hover, power settings, etc.)
  • Vibration collection settings are to be used (spectrum analysis freq range and resolution, peak/phase measurements, etc.)

Execute the Test Plan

Operate the aircraft at the designated power settings / flight conditions and collect the vibration data specified on the test plan.

Evaluate Vibration

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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