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TM 1-6625-724-13&P
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TASK 3 - TRACK AND BALANCE MAIN ROTOR IN FLIGHT
TROUBLESHOOTING
It is always recommended to compare the vibration from one measurement to the next (mainly to ensure that the
aircraft is responding in a normal manner). For the FLIGHT flight plan the VERTICAL readings should be trended. To
trend more than one data set perform the following:
With the latest flight ID picked enter the Display mode by pressing F2.
Select Trend Flights and press DO.
Select the desired channel to trend (Usually interested in vertical readings in flight, 60K, 80K, 100K, 120K, and
140K) and press DO.
Enter the number of Test States to trend (The easiest to understand is just trend the last two flights by entering
2, this will trend the most current flight with the previous flight), then arrow down to Draw Axis and change to
NO by pressing the RIGHT arrow key (this will make the polar chart easier to read), and then press DO.
This will display the vibration trend from the latest flight with previous flights on a polar chart. This process is
displayed in figure 12.
The AVA’s goal is to give corrections that will drive the vibration levels towards the center of the polar chart. The
question when viewing the polar chart is, did the vibration levels move towards the center of the chart or go through
the center of the chart. In some cases the vibration trend will go through the center of the chart and overshoot to the
other side as figure 12 shows. This is a clear indication that the aircraft is responding to the moves in a normal
manner, but the amount of the move was too much. The phase information is very important and these polar charts
should be used to verify the reaction of the aircraft.
Figure 12. AVA Vibration Trend
If the vertical vibration is increasing and moving away from center, check the orientation of the vertical accelerometer.
If the vertical accelerometer is mounted upside down (connector coming off to the top), all tab and pitch link
adjustments will drive vertical vibration in the wrong direction (i.e., increasing).
If the AVA solutions are not effectively reducing the vibration levels flight-to-flight, ensure that the vertical and lateral
accelerometers are wired into correct DAU channels. If the vertical accelerometer is wired to the lateral channel
(ACC1) and the lateral accelerometer is wired to the vertical channel (ACC2), the AVA solutions will be completely
erroneous.
For tachometer errors, check:
a. The blade tracking test port switch is in the track position for the AH-64D.
b. The tachometer connector on the AH-64 to DAU cable is connected to the TACHO 1 DAU port.
c. The tabs on the double and single interrupters are properly bent and that the gap between them and the mag
pick-up is adjusted to a gap of 0.030 to 0.070 inch.
d. The wiring between the mag pick-up and the SPU. The weak link in that wiring is the splice that is approximately
6 inches below the mag pick-up connection.
