TM 1-6625-724-13&P
0071 00
0071 00-37
TASK 5 – ISOLATE IRREGULAR VIBRATIONS
There are two main sources of vibration in helicopters. The rotors produce low frequency vibrations, 200-5000 rpm or
3 to 80 cycles per second (Hz), at multiples of rotor revolution, and the drive train-transmission produces mid, 5000 to
100,000 rpm or 80 to 1700 Hz, to high, above 100,000 rpm or 1700 Hz, frequency vibrations. Unusual low frequency
vibrations are usually a human factors issue and don't indicate a safety of flight problem, but not always. Irregular “Hi-
Freq” vibrations usually indicate an impending failure of some drive train component and so are a safety concern.
Figure 17 is an example of a spectrum and some of the frequencies that you might see. No limits exist for any of
these levels except main rotor 1/REV and tail rotor 1/REV.
Figure 17. Spectrum Example
The AVA is capable of acquiring accelerometer measurements and generating vibration spectra from those
measurements. This measurement mode is referred to as “PROBES”. A vibration spectrum is a plot of vibration
magnitude, usually in inches per second (ips) or in g’s, versus frequency, usually in Hz or revolutions per minute
(rpm). In general, the frequency of a response generated by a rotating component (e.g. rotors, fans, compressors,
engines, etc.) is equal to the frequency or an integer multiple of the frequency of rotation of the component which is
causing the vibration. In this way, the rotating component causing an abnormal response can be identified.
SPECTRUMS
What is a spectrum? If an accelerometer is mounted at a particular location on an airframe, a spectrum will show how
that point is responding to the forces generated by the rotating components of the helicopter. A peak at a particular
frequency associated with a rotating part is the amplitude of the response, the vibration, of that point to the forces
generated by that part. The higher the peak the more forces that part is generating. Low frequency vibrations are
generated by the rotors, Mid frequency vibrations are generated by drive-shafts, generators, pumps, oil cooler
blowers, and engines. High frequency vibrations are generated by gear meshing and fan, turbine and compressor
blade passage. The AVA allows the collection of spectrums at appropriate location by mounting an accelerometer on
the airframe and use the PROBES flight plan or the FFT Aircraft Type. The “PROBES” flight plan allows vibration
data to be taken on accelerometers 1 through 4 for one flight plan, and the range is from 0 to 30,000 rpm (0 to 500
Hz). This range is usually sufficient to trouble shoot most of the vibration problems on the AH-64 series. The FFT
Aircraft Type has many different ranges, but it only allows measurements to be taken on ACC1. The only reason that
the FFT Aircraft Type needs to be utilized is if it is known that the source in question is above 30,000 rpm (500 Hz).
This data can be used for troubleshooting “Hi-Freq” vibration problems. However, there currently are no set limits on
what is good and what is bad. That will be discussed further in this section.
Main Rotor 1/REV
Main Rotor 4P
Tail Rotor 1/REV
Main Rotor 8P
Tail Rotor Drive Shaft 1/REV
Tail Rotor 4P
