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TM 1-6625-724-13&P
0071 00-36
TASK 4 – BALANCE TAIL ROTOR
b. Most of the optical rpm sensor brackets will cause the sensor to view the blade on the blade shank, rather than on
the blade proper. Therefore, the amount of reflective tape required is usually only 6-8 inches running along the
shank itself. Avoid excessively long tape applications. Apply tape according to figure 13.
c. The horizontal stabilator is a good indicator for determining the magnitude of tail rotor out-of-balance. Keep an
eye on the stabilator during the first run-up and use it as a gauge as to whether or not it is safe to run all the way
up to 100% (101% for AH-64D). The vertical tail whip antenna is also a good indicator. Excessive vibration can
cause the whip antenna to break off. If severe out-of-balance is suspected, remove the whip antenna prior to first
run-up.
d. The AH-64 tail rotor sensitivity is measurably non-linear. This means that the sensitivity at 1.0 ips is measurably
different than the sensitivity at 0.3 ips. Measured data indicates that the aircraft is roughly 1/3 more sensitive
when vibration levels are in excess of 0.7 ips than for levels below that point. However, the AVA will not allow
programming for this non-linearity (i.e., the script file can contain only one set of sensitivities). The AVA is
programmed with the sensitivity of the aircraft for tail imbalance below 0.7 ips. If a tail measurement at 100%
(101% for AH-64D) Nr is in excess of 0.7 ips, cut the AVA-recommended adjustment in half for both blades until
the measured levels drop below 0.7 ips. Once below 0.7 ips use the AVA solution. If over-shooting still persists,
the solution can be adjusted, but be careful to ratio the inboard blade adjustment and the outboard blade
adjustment by the same amount, Do not change them by the same number of grams. For the tail70 flight plan, do
not adjust the AVA-recommended moves, regardless of ips level, unless a clear over-shoot at 70% has already
occurred.
e. If a weight adjustment is not split evenly between the two tip pockets, this will change the resulting phase angle,
by which, the vibration measurement will change. Large moves in excess of 10 grams can significantly alter the
result and cause the vibration trend to not go directly through center and therefore not go directly to zero. Split all
moves as evenly as possible (only 1 gram difference allowed) between pockets.
TROUBLESHOOTING
a. If vibration increases after an adjustment, it is important to view the polar chart and plot the vibration trend
between the current measurement and the prior one (Display Trend Flights). If the move was made correctly, it is
possible that the vibration did in fact increase, but went through the polar chart center to the other side of the
chart. It is important to distinguish an increase due to an overshoot from an increase due to an incorrect move for
which the measurement went away from center.
1. If the vibration moves away from center, the adjustment was not made to the correct blade or blades. A
correctly made adjustment will never cause the vibration to move away from the center of the polar chart.
2. If the vibration shot through center, and the adjustment was made correctly, the aircraft is significantly more
sensitive than the script file coefficients reflect. Cut the AVA solution in half for both blades to prevent
overshooting on the subsequent run. Return to full AVA moves if subsequent undershooting is evident.
b. For tachometer errors, ensure that the gray optical rpm sensor cable is connected to the DAU “TACHO2” port.
Follow the line of sight of the sensor to the blade to ensure that the optical beam is hitting the reflective tape.
1. Verify that the red LED on the back of the optical tachometer sensor is operating with the DAU turned on and
supplied power. With optical tachometer sensor in place rotate tail rotor so that blade with reflective tape is in
front of the optical sensor. With tape in front of sensor red LED on back of sensor should be illuminated. A
sensor with a pulsating LED indicates that the signal is strong.
2. After installation of the reflective tape, wipe the tape with alcohol, contact cleaner etc. This will ensure
maximum reflectivity as oils from your fingers can reduce reflection.
3. Some sensors have not had the gain adjustment set to maximum. To set the gain to maximum, remove the
clear plastic cover from the back of the sensor. Using a small screwdriver, turn the “gain” adjustment
clockwise until a click is heard. If no click can be heard or felt then turn the screw 15 turns clockwise.
