TM 1-6625-724-13&P
0071 00
0071 00-35
TASK 4 – BALANCE TAIL ROTOR
COMPARING MOVES BETWEEN RUNS
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 tail flight plans (tail and tail70) there is only one reading to trend.
To trend this perform the following:
With the latest flight ID picked enter the display mode by pressing F2.
Select Trend Flights and press DO.
Press DO on 1T POLAR.
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 (this will make the polar chart easier to read), and then press DO.
This will display the vibration trend from the latest reading with previous readings on a polar chart. This process
is displayed in figure 16.
Figure 16. Vibration Trend Polar Chart
The AVA’s goal is to give corrections that will drive the vibration levels towards the center of the polar chart. This
example is an almost ideal solution, as you can see on the polar chart with one move the vibration went almost
directly towards the center of the polar chart. The question you ask when you’re 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 you will see
the vibration trend go through the center of the chart and overshoot to the other side. This is a clear indication that
the tail rotor is responding to the moves in a normal manner, but the amount of the move was too much. If the
vibration does not move towards center, something was done wrong. If the vibration trend shows vibration levels
moving in a direction other than towards center, you should stop and try to figure what went wrong. If it cannot be
determined what went wrong than it is recommended to make another adjustment, according to the AVA, but try to
make as few of moves as possible (usually 1) to reduce the vibration levels. The phase information is very important
and these polar charts should be used to verify the reaction of the aircraft. Remember the main thing that you view
the trends for, is to ensure that the move went in the correct direction, (towards the center of the polar chart).
HELPFUL HINTS for flight plans “tail” or “tail70”
a. If the AVA calls for a large move on one blade (i.e., more than 8 grams) with a 1 gram move on another blade,
turn off the 1-gram blade and see if the predicted vibration with the one-blade move is below 0.1 ips. If so, make
only the large move to the one blade.
