TASK 5 ISOLATE IRREGULAR VIBRATIONS
The Spectrum used in the PROBES or FFT flight plan is an asynchronous spectrum. This spectrum provides no
phase information as sampling starts at an arbitrary time and is not keyed to anything like a tach signal. The
spectrum displayed is an average of 16 spectra taken consecutively after measure is pressed and shows the vibration
amplitudes induced at the measuring point by the forces from each rotating part.
MID-FREQ VIBRATIONS (DRIVE TRAIN INDUCED VIBRATION)
Most vibrations above 60 to 80 Hz will be called a Hi-Freq vibration by a pilot. When a Hi-Freq is felt the pilot
knows something is not right, and it is time to turn the aircraft in for maintenance. The drive train has numerous
rotating parts that produce vibration at many different frequencies. Locating the faulty part is not an easy task.
Here are some things that might help. The table at the end of this section contains the frequencies of most of the
rotating parts of the AH-64. 1/REV (rotational speed of the component) vibrations are associated with balance.
Indexing a shaft at its mounting flange will change its balance. 2/REV (twice the rotational speed) vibrations are
associated with the alignment of shafts. An increase in several harmonics of a shaft (e.g. 1/REV, 2/REV, 3/REV, and
4/REV) at the same time is an indication of bearing wear. One method of troubleshooting Hi-Freq problems can be
accomplished by taking data at identical locations on a known good aircraft and the aircraft under test and comparing
the differences. If there is a problem, comparing these two displays can show which component is the cause.
PROCEDURE 1: ACCELEROMETER INSTALLATION
The AVA is capable of acquiring vibration data from six
accelerometers: four via DAU ports ACC1 through ACC4
and two from the on-board track and lateral
accelerometers via the multi-channel connection. For
the externally wired aircraft type, ACC1 and ACC2 are
accelerometers. ACC3 and ACC4 are available for
installation at any desired location on the aircraft. The
on-board track and balance accelerometers are also
available for vibration spectrum analysis.
Vibration measurements need be taken only for the
operating condition of the aircraft during which the
irregular vibration is sensed.
If only the track and balance accelerometers are to be
used for spectrum analysis, skip to procedure 2.
a. Install an accelerometer(s) (28110900) anywhere on the airframe that an irregular vibration is detected, on or near
the vibrating component, using an accelerometer L-bracket from the AVA kit.
b. Connect accelerometer cable (29105600 or 29105601) to the accelerometer and to the DAU receptacle labeled
ACC1, ACC2, ACC3 and/or ACC4.
PROCEDURE 2: PROBES MEASUREMENT
a. On the CADU, select Flight Plan PROBES within the current aircraft type and tail number by pressing QUIT until
the Main Menu appears, using the cursor to highlight Flight Plan, pressing DO, using the cursor to highlight
PROBES, and pressing DO.
b. Establish the operating condition during which the adverse vibration is sensed. Press F1 MEASURE. The
following measurements are available: