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made in the following pages as they are identical to the previous ones, which use ground data instead of in-flight recording data. An alternate procedure, known as "1 sensor (No. 1 bearing) 1 speed" allows the engine vibration level to be reduced when the aircraft is not at the main base and the crew is not familiar with imbalance correction, after fan blade replacement when the vibration survey is incorrect. This procedure is therefore performed on ground and allows the vibration to be reduced to allow take-off and the engine to be balanced subsequently through a cold trim balance procedure (Ref. TASK 71-00-00-750-001). This procedure is described in detail in the following pages using a numerical example.
(3) In-flight data EVMU programming Before flight, program EVMU for automatic data acquisition from EVMU "ENGINE UNBALANCE" menu select "LOAD LEFT/RIGHT" sub-menu. Set the following speeds and save:
N1 N TIME 64 2 15 84 2 15 88 2 15 92 2 15 96 2 15
(N1 = speed in percent, N = speed tolerance, time = stabilisation time in seconds).
NOTE : The No.1 bearing vibration sensor must be fully serviceable
____ (its insulation resistance characteristics are most important) to get reliable trim balance results. To make sure of this (Ref. TASK 77-31-10-720-001).
Subtask 71-00-00-750-054
B. One-Shot Plot Procedure
(1) Read the unbalance data values that have been acquired during the flight for the following N1 speeds: 64, 84, 88, 92, and 96 percent.
NOTE : As during EVMU loading speed tolerance N2 is programmed for 2
____ percent speed tolerance, speed recording is eqal to speed target +/- 2 percent. Therefore the observed N1 speed range varies from up to 98 percent N1.
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(2) Establish a record of the registered vibrations.
(Ref. Fig. 509/TASK 71-00-00-991-328)
NOTE : The figure on the following pages give examples to help
____
understanding.
(3) Prior to any correction calculation, translate the negative angles to positive angles. Example: An angle of - 64 degrees must be used for calculation purposes as 360 - 64 = 296 degrees.
NOTE : The positive angles must be used as such.
____
(4) From the vibration level record, take the vibration which corresponds to the maximum speed reached in flight. Example: 96 percent of N1 vibrations: Suppose 2.6 mils at - 124 degrees.
(5) Take the vibrations neared to 84 percent of N1 and 64 percent of N1. Example: 2 mils at - 142 degrees for 84 percent of N1,
0.8 mils at - 104 degrees for 64 percent of N1.
(6) When the recorded angles are negative, convert as follows:
360 - 124 = 236 degrees
360 - 142 = 218 degrees
360 - 104 = 256 degrees
NOTE : Up to 5 speeds and 2 sensors can be taken for calculation.
____
(7) Record these figures. Use the one-shot balancing weight calculation chart. (Ref. Fig. 510/TASK 71-00-00-991-329, 511/TASK 71-00-00-991-330)
(8) The balancing weight, for each speed, is obtained by multiplying the figure of column 1 (vibrations in mils) by the figure of column 2 (sensitivity in g.cm/mils). Example for 96 percent: 2.6 x 205 = 533 g.cm
(9) The angle of the balancing weight is obtained by subtracting the figure of column 4 (phase lag) from the figure of column 5 (vibration angle). Example for 96 percent N1: 160 - 236 = - 76 degrees i.e.
284 degrees
(10) The balancing weights A, B and C are obtained. They correspond, for each speed, to the balancing weight that cancels the engine vibration from bearing No. 1.
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