0071 00TM 1-6625-724-13&P0071 00-38TASK 5 – ISOLATE IRREGULAR VIBRATIONSThe Spectrum used in the “PROBES” or “FFT” flight plan is an asynchronous spectrum. This spectrum provides nophase information as sampling starts at an arbitrary time and is not keyed to anything like a tach signal. Thespectrum displayed is an average of 16 spectra taken consecutively after measure is pressed and shows the vibrationamplitudes 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 pilotknows something is not right, and it is time to turn the aircraft in for maintenance. The drive train has numerousrotating 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 therotating 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 areassociated with the alignment of shafts. An increase in several harmonics of a shaft (e.g. 1/REV, 2/REV, 3/REV, and4/REV) at the same time is an indication of bearing wear. One method of troubleshooting “Hi-Freq” problems can beaccomplished by taking data at identical locations on a known good aircraft and the aircraft under test and comparingthe differences. If there is a problem, comparing these two displays can show which component is the cause.PROCEDURE 1: ACCELEROMETER INSTALLATION NOTEThe AVA is capable of acquiring vibration data from sixaccelerometers: four via DAU ports ACC1 through ACC4and two from the on-board track and lateralaccelerometers via the multi-channel connection. Forthe externally wired aircraft type, ACC1 and ACC2 arethe track and balance lateral and verticalaccelerometers. ACC3 and ACC4 are available forinstallation at any desired location on the aircraft. Theon-board track and balance accelerometers are alsoavailable for vibration spectrum analysis.NOTEVibration measurements need be taken only for theoperating condition of the aircraft during which theirregular vibration is sensed.NOTEIf only the track and balance accelerometers are to beused 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 nearthe 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 labeledACC1, ACC2, ACC3 and/or ACC4. PROCEDURE 2: PROBES MEASUREMENTa. On the CADU, select Flight Plan PROBES within the current aircraft type and tail number by pressing QUIT untilthe Main Menu appears, using the cursor to highlight Flight Plan, pressing DO, using the cursor to highlightPROBES, and pressing DO. b. Establish the operating condition during which the adverse vibration is sensed. Press F1 MEASURE. Thefollowing measurements are available:
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