The resolution of an FFT analysis determines the amount of detail which may be extracted from the accelerometer signal. This is done by selecting the complete frequency range and dividing it into a number of equal parts. The more parts which the frequency range is divided into the greater will be the detail of the analysis.

Take for example a typical 0Hz to 500Hz analysis. At a resolution of 200 the 500 Hz sample range will be split into 200 individual lines, sometimes termed bins. In this case each line or bin will be will be 2.5Hz wide (500 / 200 = 2.5).

Therefore if an aircraft generates actual vibrations which are closer together than 2.5 Hz then a 200 line FFT will not be able to separate them into individual components. Instead it will take their combined value for display.

This may be satisfactory for some circumstances. However if it is known that in the helicopter there are gears, shafts or other components which are generating frequencies closer than 2.5 Hz then it may be impossible to identify them individually. This will necessitate using a higher 400, 800 or even 6,400 'line' resolution. See the illustration above.

To illustrate this point using Figure 7-2 as an example. We wish to know the amplitude of the vibration generated from a particular component which from the Vibration Order Sheet we know runs at 5.4 Hz at 100% Nr.

Using the 200 line FFT, inspect the 5.0 Hz to 7.5 Hz bin. This bin contains a significant spectral line indicating that there is in fact vibration being generated at frequencies between 5.0 Hz to 7.5 Hz. We may well be lead to assume this to be wholly attributable to the 5.4 Hz frequency and make decisions with regard to its acceptability. We might even consider removing the component for investigation.

If however we investigate using, as it were a more powerful ‘magnifying glass’ - an 800 line FFT - we find that the 5.4 Hz is now clearly the smallest of three closely spaced frequencies in that range. From this analysis we may not have drawn the same conclusion, as before.