Now What? Vibration Analysis

The vibration spectrum survey can bring quite a bit of seemingly unrelated data into an organized format that you can use as a valuable troubleshooting tool.

Say the pilot complains of a vibration when the aircraft is accelerating to cruise speed. The vibration analysis equipment could be installed and securely mounted allowing data capture during a maintenance flight simulating the same conditions. A transient survey can be performed and saved inside the analyzer. The data can typically be downloaded to a computer for in-depth analysis. Once the data are saved in the computer, the higher-than-normal peaks can be identified and investigated. Now, how do you make sense of what you’re seeing on the screen?

Comparing Vibration Data to the Manual

Vibration data alone may not be much use. However, the data used in conjunction with the maintenance manual can identify the rotational speeds of all the major components on the aircraft. There is typically a list, a table, or a set of ratios useful in identifying specific components. Keep in mind, most tables are written with the component at 100 percent operating speed. In some cases it will be 100 percent of the actual component rotational speed. In other cases it may be in relation to engine speed, either N1 or N2 speed. When readings are taken at partial power, the rotational speeds will need to be adjusted mathematically to the correct partial operating speed.

For example, look at the starter/generator on an Allison 250-C20. The maintenance manual lists the rotational speed of the starter/generator gear as 12,034 rpm. Further up the gear train, the Gas Producer Turbine Input (N1), that ultimately drives the starter/generator, is listed at 50,970 rpm at 100 percent. This means that both gears are turning the rated rpm at 100 percent N1 speed. Rarely will the N1 power train be turning at 100 percent. Assume the data was gathered with the N1 operating at 87 percent. Multiplying the 100 percent rotational rpm by 87 percent will give us the approximate speed of the starter/generator. This speed is 10,469.58 rpm. Armed with this information, we can examine the vibration spectrum survey for a peak in the 10,500-rpm range. This process can be repeated to confirm that the rotational components are not producing vibration peaks above the manufacturer’s limits. Conversely, it is possible to identify a particular vibration spike and try to identify the component that caused it (Figure 4).

Identifying the Harmonics

Another consideration when trying to isolate vibration or noise problems is looking at a broad enough picture. The problem you are chasing can be in one of the multiples of the primary frequency, known as a harmonic. It is important to remember that every component has a primary frequency and one or more harmonic frequencies. For this reason, the suspicious frequencies need to be identified. Once the primary frequencies are identified, the harmonics can be identified. Then the overall range, or bandwidth, of the survey can be determined.

Corrective Action

Once a determination can be made concerning the component causing the problem, a path of corrective action can be established. Corrections can include balancing the offending component as in the case of a tail rotor or driveshaft. You can replace the suspect component as in the case of a starter/generator or fuel pump. The long-term solution would be to begin a trending program to identify the failure level of the component and remove it before a preset threshold.

This is the theory behind onboard monitoring systems. The data is gathered continuously during the flight. The option exists for an immediate notification presented to the pilot in the form of a caution light or gauge. Another option, if the information is not to be immediately displayed, is to store the data and decipher it later at a maintenance facility. The data will be tracked and the next course of action can be determined.

The ultimate goal is to use the tools at hand to provide effective troubleshooting information. Many times the warning signs for impending problems are noticed early in the failure cycle. If these warning signs can be identified and documented, the potential for catastrophic failure can be reduced. This can result in greater aircraft availability and reduced maintenance costs.

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