DOC AIRBUS | AMM A320FFan Trim Balance - 1 Sensor 3 Speed [CFMI]
TASK 71-00-00-750-001-A
Fan Trim Balance - 1 Sensor 3 Speed
1. Reason for the Job
Self explanatory
2. Job Set-up Information
A. Fixtures, Tools, Test and Support Equipment
B. Referenced Information
3. Job Set-up
Subtask 71-00-00-869-084-C ** ON A/C NOT FOR ALL
Subtask 71-00-00-750-053-A ** ON A/C NOT FOR ALL
[Rev.10 from 2021]
2026.04.01 02:21:35 UTC
Fan Trim Balance - 1 Sensor 3 Speed
1. Reason for the Job
Self explanatory
2. Job Set-up Information
A. Fixtures, Tools, Test and Support Equipment
| REFERENCE | QTY | DESIGNATION |
|---|---|---|
| 856A2678 | 1 | SOFTWARE-FAN TRIM BALANCE PROCEDURE, CFM56 |
| REFERENCE | DESIGNATION |
|---|---|
| TASK 77-31-10-720-002-A | Electrical Test of the No. 1 Bearing Vibration Sensor |
| TASK 77-32-34-750-002-A | Fan Trim Balance with the EVMU (one Shot Method) |
| TASK 77-32-34-750-002-A-01 | Fan Trim Balance with the EVMU (Vectorial Method) |
| TASK 77-32-34-869-042-A | Acquire Unbalance Data During the Flight and Read Unbalance Data |
| TSM 77-00-00-810-862 | Fan Vibration less than 4 Units with Noise / Rumble |
| TSM 77-00-00-810-863 | Fan Vibrations Higher than or Equal to 4 Units and less than 6 Units on Engine 1 or 2 |
| TSM 77-00-00-810-864 | Fan Vibrations Higher than or Equal to 6 Units on Engine 1 or 2 |
Subtask 71-00-00-869-084-C ** ON A/C NOT FOR ALL
A. As an alternative solution, you can use the Fan Trim Balance with the EVMU (Ref. AMM TASK 77-32-34-750-002).
4. ProcedureSubtask 71-00-00-750-053-A ** ON A/C NOT FOR ALL
A. General
(1) Cold Trim Balance
Trim balance is a procedure used to reduce the engine vibration level. This procedure must be applied every time the engine vibration level reaches 6.0 mils, which leads to rapid loss of the EGT margin, or every time the engine vibrations result in significant cabin noise, or after an engine check TSM 77-00-00-810-862, TSM 77-00-00-810-863 or TSM 77-00-00-810-864.
This procedure is known as Cold Trim Balance because it consists in correcting the imbalance on a cold engine without the need for successive ground runs to determine the imbalance to be corrected. The imbalance is determined using the vibration parameters recorded in flight by the aircraft EVMU. This procedure allows the engine vibration level to be maintained continuously, at minimal cost. The same calculation can be performed with the aircraft on the ground, after replacement of fan blades when a static imbalance correction of 400 g.cm or more is necessary.
For imbalance correction calculation, the cold trim balance procedure uses the in-flight recording of the vibrations delivered by the engine No. 1 bearing vibration sensor, at three characteristic speeds which are 62 percent of N1, 84 percent of N1 and Max. of N1, that is between 93 percent and 96 percent.
This procedure is also known as "1 sensor (No. 1 bearing) 3 speeds".
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 and the vibration survey is incorrect. This procedure is therefore performed on ground and allows the vibrations to be reduced to allow take-off and the engine to be balanced through "1 sensor 3 speeds" cold trim balance procedure.
Trim Balance Procedure:
856A2678P01: Multi-Engine Trim Balance Program. CD-Rom WINDOW Compatible
Subtask 71-00-00-750-050-A ** ON A/C NOT FOR ALL (1) Cold Trim Balance
Trim balance is a procedure used to reduce the engine vibration level. This procedure must be applied every time the engine vibration level reaches 6.0 mils, which leads to rapid loss of the EGT margin, or every time the engine vibrations result in significant cabin noise, or after an engine check TSM 77-00-00-810-862, TSM 77-00-00-810-863 or TSM 77-00-00-810-864.
This procedure is known as Cold Trim Balance because it consists in correcting the imbalance on a cold engine without the need for successive ground runs to determine the imbalance to be corrected. The imbalance is determined using the vibration parameters recorded in flight by the aircraft EVMU. This procedure allows the engine vibration level to be maintained continuously, at minimal cost. The same calculation can be performed with the aircraft on the ground, after replacement of fan blades when a static imbalance correction of 400 g.cm or more is necessary.
For imbalance correction calculation, the cold trim balance procedure uses the in-flight recording of the vibrations delivered by the engine No. 1 bearing vibration sensor, at three characteristic speeds which are 62 percent of N1, 84 percent of N1 and Max. of N1, that is between 93 percent and 96 percent.
This procedure is also known as "1 sensor (No. 1 bearing) 3 speeds".
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 and the vibration survey is incorrect. This procedure is therefore performed on ground and allows the vibrations to be reduced to allow take-off and the engine to be balanced through "1 sensor 3 speeds" cold trim balance procedure.
Trim Balance Procedure:
- The correction of the engine vibration level can be obtained after initial correction known as one shot correction. If the results obtained at the next aircraft flight require improvement, a vectorial analysis is performed.
- The one-shot procedure uses the statistical calculation coefficients of the engines. A pre-determined matrix gives the relation of the phase lag to the installed balance weight and the vibration it generates, as well as the relation of the installed amount of weight and the vibration level it generates (sensitivity). This procedure will in most cases solve the engine vibration problems.
- Vectorial procedure.
This procedure compares the vibrations registered during the first aircraft flight with the vibrations obtained for the correction of the weight calculated through the one-shot procedure and installed on the engine for the next flight. A graphic vectorial analysis is used to determine accurately the phase lag and the sensitivity of the treated engine.
A final balance weight is then determined based on these new coefficients. - These calculations can be done by hand, using a polar graph as shown in the example. A computerized calculation program can also be used:
856A2678P01: Multi-Engine Trim Balance Program. CD-Rom WINDOW Compatible
B. One-Shot Plot Procedure
(3) Prior to any correction calculation, translate the negative angles to positive angles.
Example: An angle of - 116 degrees must be used for calculation purposes as 360 - 116 = 244 degrees.
Example: 96 percent of N1 vibrations:
1.9 mils at - 88 degrees.
(5) Take the vibrations corresponding to 84 percent of N1 and 62 percent of N1.
(6) When the recorded angles are negative, convert as follows:
(7) Record these figures. Use the one-shot balancing weight calculation chart.
(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: 1.9 x 287 = 545 g.cm
(9) Calculate the angle of the balancing weight as follows: Phase Lag (Column 4) - Vibration angle (Column 5)"= Balancing weight angle (Column 6).
Example for 96 percent N1: 160 - 236 = - 76 degrees i.e.
(10) The balancing weights A, B and C are obtained.
They correspond, for each speed, to the balancing weight that cancels the engine No. 1 bearing vibration.
(11) On a polar graph, take a g.cm scale and plot the three points.
(12) To determine the balancing weight corresponding to the best compromise, we must search for the intersection of 3 circles, by setting vibration level targets for each speed.
(13) The balancing weight is within triangle A, B, C.
The vibration targets are:
(14) About points A, B and C draw 3 circles with the following radiuses (sensitivity x vibration target):
(a) 287 x 0.5 = 143.5 g.cm
(b) 299 x 1.0 = 299 g.cm
(c) 334 x 1.2 = 400 g.cm
(15) The balancing weight which meets the target requirements is at the intersection of the 3 intersecting circles.
In the example of the figure, weight M is equivalent to 700 g.cm at 289 degrees.
Since this weight is outside the triangle A, B, C, the vibration targets can be improved.
(16) If the circles do not intersect, the targets must be widened by increasing the radius of the circles corresponding to 94 percent N1 and 84 percent N1 while keeping the max value of 1.2 mils at 62 percent.
Example: If you take the same 0.5 mils target for the three points A, B and C, the respective radiuses will be as follows:
(a) 0.5 x 287 = 143.5 g.cm.
(b) 0.5 x 299 = 149.5 g.cm.
(c) 0.5 x 334 = 167 g.cm.
Weight M, common to the three circles, will be 440 g.cm at 295 degrees.
(18) On the Fan spinner rear cone, a spherical mark is machined to show the Fan blade No. 1 location. This first blade is to the left of the mark, forward looking aft. (The blades are counted counterclockwise).
(19) Determine the position of the balancing screws to be installed on the spinner rear cone according to the angle.
Example: 295 degrees correspond to blade positions No. 7 and No. 8.
(20) To determine the number of screws to be installed and their respective positions:
and
Example: 435 g.cm at 295 degrees corresponds to the installation of the following screws:
(22) If there are balancing screws in the determined positions, the weight equivalent to the initial weight installed must be calculated and added as well as the weight determined above.
and
(24) If the tables do not contain the initial configuration of the spinner, a vectorial addition of each screw is necessary.
Example: Assume that the configuration was as follows:
Example:
(27) Select 2 screws and draw 2 circles as follows:
(a) One circle with the end of the first vector as center and the length of the second vector as radius.
(b) On circle with the end of the second vector as center and the length of the first vector as radius.
(28) From the end of the first vector, draw a parallel to the second vector leading to the point of intersection of the two circles.
(29) The vector connecting the origin of the polar graph and the point of intersection of the two circles equals the vectorial sum of the two previous balancing screws.
Example:
The sum of P04 in position No. 3 and P06 in position No. 23 is 200 g.cm at 105 degrees.
(30) Add the found vector to the next balancing screw.
Example 200 g.cm at 105 degrees and a P05 screw in position No. 12 give 250 g.cm at 220 degrees.
(31) Once the total initial weight installed on the engine has been calculated (for example: 250 g.cm at 220 degrees) the correction weight calculated in step (19) must be added.
Example: 440 g.cm at 295 degrees.
(32) Proceeding as above, on a polar graph, add up these two vectors.
Example:
(34) Then determine the screws and positions corresponding to the total weight in following figures:
and
and
Example: 560 g.cm at 268 degrees.
Subtask 71-00-00-750-052-A ** ON A/C NOT FOR ALL 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. AMM TASK 77-31-10-720-002).
(1) Read the unbalance data values that have been aquired during the flight for the following N1 speeds: 62, 84, 94, 95, and 96 percent (Ref. AMM TASK 77-32-34-869-042). NOTE: If the climatic conditions are too cold to reach 96 percent of N1, program 93 percent, 94 percent, 95 percent to obtain recordings for the maximum speed reached in flight. A minimum of 92 percent is required.
(2) Establish a record of the registered vibrations. (3) Prior to any correction calculation, translate the negative angles to positive angles.
Example: An angle of - 116 degrees must be used for calculation purposes as 360 - 116 = 244 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:
1.9 mils at - 88 degrees.
(5) Take the vibrations corresponding to 84 percent of N1 and 62 percent of N1.
| Example: 1.5 mils at - 136 degrees for 84 percent of N1, |
| 0.9 mils at - 116 degrees for 62 percent of N1. |
(6) When the recorded angles are negative, convert as follows:
| 360 - 88 = 272 degrees |
| 360 - 136 = 224 degrees |
| 360 - 116 = 244 degrees |
(7) Record these figures. Use the one-shot balancing weight calculation chart.
(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: 1.9 x 287 = 545 g.cm
(9) Calculate the angle of the balancing weight as follows: Phase Lag (Column 4) - Vibration angle (Column 5)"= Balancing weight angle (Column 6).
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 No. 1 bearing vibration.
| Example: A = 545 g.cm at 284 degrees |
| B = 448 g.cm at 312 degrees |
| C = 300 g.cm at 289 degrees |
(11) On a polar graph, take a g.cm scale and plot the three points.
(12) To determine the balancing weight corresponding to the best compromise, we must search for the intersection of 3 circles, by setting vibration level targets for each speed.
(13) The balancing weight is within triangle A, B, C.
The vibration targets are:
| 0.5 mils at 96 percent of N1 |
| 1.0 mils at 84 percent of N1 |
| 1.2 mils at 62 percent of N1 |
(14) About points A, B and C draw 3 circles with the following radiuses (sensitivity x vibration target):
(a) 287 x 0.5 = 143.5 g.cm
(b) 299 x 1.0 = 299 g.cm
(c) 334 x 1.2 = 400 g.cm
(15) The balancing weight which meets the target requirements is at the intersection of the 3 intersecting circles.
In the example of the figure, weight M is equivalent to 700 g.cm at 289 degrees.
Since this weight is outside the triangle A, B, C, the vibration targets can be improved.
(16) If the circles do not intersect, the targets must be widened by increasing the radius of the circles corresponding to 94 percent N1 and 84 percent N1 while keeping the max value of 1.2 mils at 62 percent.
NOTE: This 1.2 mils target at 62 percent can be reduced if it generates resonance in the cabin.
(17) If all three points A, B and C are within the three circles, the targets must be reduced to determine the balancing weight. Example: If you take the same 0.5 mils target for the three points A, B and C, the respective radiuses will be as follows:
(a) 0.5 x 287 = 143.5 g.cm.
(b) 0.5 x 299 = 149.5 g.cm.
(c) 0.5 x 334 = 167 g.cm.
Weight M, common to the three circles, will be 440 g.cm at 295 degrees.
(18) On the Fan spinner rear cone, a spherical mark is machined to show the Fan blade No. 1 location. This first blade is to the left of the mark, forward looking aft. (The blades are counted counterclockwise).
(19) Determine the position of the balancing screws to be installed on the spinner rear cone according to the angle.
Example: 295 degrees correspond to blade positions No. 7 and No. 8.
(20) To determine the number of screws to be installed and their respective positions:
and
Example: 435 g.cm at 295 degrees corresponds to the installation of the following screws:
- P03 in position No. 5
- P03 in position No. 10.
(22) If there are balancing screws in the determined positions, the weight equivalent to the initial weight installed must be calculated and added as well as the weight determined above.
NOTE: This addition is recommended in order to avoid an excessive number of screws installed on the spinner rear cone after several balancing operations.
(23) Record the initial configuration of the engine. Refer to the table of following figures to determine the installed weight and its position. and
(24) If the tables do not contain the initial configuration of the spinner, a vectorial addition of each screw is necessary.
Example: Assume that the configuration was as follows:
- One screw P06 in position No. 23
- One screw P04 in position No. 3
- One screw P05 in position No. 12.
Example:
- P06 = 441 g.cm.
- P04 = 315 g.cm.
- P05 = 378 g.cm.
(27) Select 2 screws and draw 2 circles as follows:
(a) One circle with the end of the first vector as center and the length of the second vector as radius.
(b) On circle with the end of the second vector as center and the length of the first vector as radius.
(28) From the end of the first vector, draw a parallel to the second vector leading to the point of intersection of the two circles.
(29) The vector connecting the origin of the polar graph and the point of intersection of the two circles equals the vectorial sum of the two previous balancing screws.
Example:
The sum of P04 in position No. 3 and P06 in position No. 23 is 200 g.cm at 105 degrees.
(30) Add the found vector to the next balancing screw.
Example 200 g.cm at 105 degrees and a P05 screw in position No. 12 give 250 g.cm at 220 degrees.
(31) Once the total initial weight installed on the engine has been calculated (for example: 250 g.cm at 220 degrees) the correction weight calculated in step (19) must be added.
Example: 440 g.cm at 295 degrees.
(32) Proceeding as above, on a polar graph, add up these two vectors.
Example:
- Initial weight = 250 g.cm at 220 degrees
- Calculated weight = 440 g.cm at 295 degrees
- Total weight = 560 g.cm at 268 degrees.
(34) Then determine the screws and positions corresponding to the total weight in following figures:
and
and
Example: 560 g.cm at 268 degrees.
- This figures indicates that the 268-degree position is between blades No. 10 and No. 11.
- The 560 g.cm correction weight corresponds to two P04 screws (571 g.cm).
- The spinner therefore requires two P04 screws in positions No. 8 and No. 13, which represents exactly 571 g.cm at 265 degrees.
- The rest of the screws on the spinner will be P07.
C. Vectorial Analysis
Example:
Assume that these vibrations, after addition of 440 g.cm at 295 degrees, are:
(b) If not, do calculation as instructed in the following steps.
(3) Fill in the table. See example:
(4) After installation of the weight obtained through one-shot calculation, the initial vibrations noted AB for the 3 speeds become vibrations AC.
The engine response is determined by measuring vibrations BC on a polar graph.
(5) On a polar graph, using a mils scale, plot the initial and final vibration for the maximum speed.
See example:
(6) Using compasses, do as follows.
(a) Take the length of vector BA and draw a circle with radius BA and center C.
(b) Take the length of vector BC and draw a circle with radius BC and center A.
The vector which passes through the origin of the polar graph and the intersection point of the circles is parallel and equal to BC.
(7) Measure the length and the angle of vector BC.
Example: The engine response at 96 percent N1 is 2.9 mils at 18 degrees.
(8) Record this value in the table.
See example:
(9) Use the same procedure for 84 percent N1 and 62 percent N1.
(10) On a polar graph, determine the actual weight added between the two flights as follows.
(a) Plot the initial weight (for example: 250 g.cm at 220 degrees).
(b) Plot the weight actually installed on the spinner for the other flight (for example: 571 g.cm at 265 degrees).
(11) Draw two arcs as follows:
(a) One arc with the end of the second vector (second weight) as center and the length of the first vector (first weight) as radius.
(b) One arc with the origin of the graph as center and the length of the line connecting the 2 vectors as radius.
Then note the value and the angle of the weight actually added (Example: 405 g.cm at 291 degrees).
(12) The engine sensitivity at each speed is obtained by dividing the weight actually added by the vibration response at each speed.
Example:
(13) The engine phase lag is obtained by adding the following angles:
Angle of the added weight + angle of the vibration response + 180 degrees.
Example:
(14) Record the calculated values in the table
See example:
Calculation Chart of the Vectorial Analysis Balancing Weight - Calculation Example ** ON A/C NOT FOR ALL
(15) In the table of figure, record the initial vibrations registered during the first flight.
See example:
Calculation Chart of the Vectorial Analysis Balancing Weight - Calculation Example ** ON A/C NOT FOR ALL
(16) The improved balancing weights are obtained by multiplying the figures of column 1 by the figures of column 2.
Example for 96 percent N1: 1.9 x 139 = 264 g.cm.
(17) The angles of the improved balancing weights are obtained by subtracting the figures of column 5 from the figures of column 4.
Example for 96 percent N1: 129 - 272 = -143, i.e. 217 degrees.
(18) Three balancing weights A, B and C are obtained.
They correspond to the weights which cancel the No. 1 bearing vibration at the three speeds.
(19) On a polar graph using a g.cm scale, plot these three points.
(20) To determine the balancing weight corresponding to the best compromise, search for the intersection of three circles by setting vibration level targets for each speed.
(21) The balancing is within triangle ABC.
(22) These vibration targets are:
Since the balancing weight for 96 percent N1 (264 g.cm at 217 degrees) is located within the 0.5 mils circles of the 84 percent N1 and 62 percent N1, it is adequate for balancing the engine.
(24) The initial weight (for example: 250 g.cm at 220 degrees) must be added to the determined balancing weight (example: 264 g.cm at 217 degrees).
To do so, a vectorial addition of these two vectors must be made, using the polar graph,
Example: The two angles are almost equal. The weight to be installed on the spinner is therefore equal to 264 + 250 = 514 g.cm at 220 degrees.
(25) Determine the screws to be used and their position:
and
Example: 220 degrees corresponds to position No. 15.
The weights are therefore centered on one screw. The screws to be installed on the spinner are the following:
NOTE: If the vibrations registered during the next flight, after the one-shot plot procedure, remain unsatisfactory the vibration level can be improved by calculating the correction factors (sensitivity, phase lag) specific to the engine to be balanced.
(1) Record the flight registered vibrations. Example:
Assume that these vibrations, after addition of 440 g.cm at 295 degrees, are:
| at 96 percent of N1 3.0 mils at 340 degrees |
| at 98 percent of N1 2.0 mils at 320 degrees |
| at 62 percent of N1 1.0 mils at 330 degrees |
CAUTION:
WE RECOMMEND TO USE GSEM No.11 PRIOR TO USING THE DISKETTES IN ORDER TO PREVENT ANY HANDLING ERROR LEADING TO DESTRUCTION OF THE DATA.
CAUTION:
THE PROGRAM WAS DESIGNED TO OPERATE ON IBM PC TYPE OR COMPATIBLE COMPUTERS, USING IBM DOS AND EQUIPPED WITH AN EGA OR VGA GRAPHIC CARD.
(2) Compute or calculate the sensitivity and phase lag as well as the new balancing weight to be installed as follows: NOTE: A Ground Support Equipment Manual (GESM) is published by CFMI to give the customers the details they need forthe calculation program.
(a) If computation is possible, use the CFM trim balance program 856A2678 SOFTWARE-FAN TRIM BALANCE PROCEDURE, CFM56 (856A2678P01). (b) If not, do calculation as instructed in the following steps.
(3) Fill in the table. See example:
(4) After installation of the weight obtained through one-shot calculation, the initial vibrations noted AB for the 3 speeds become vibrations AC.
The engine response is determined by measuring vibrations BC on a polar graph.
(5) On a polar graph, using a mils scale, plot the initial and final vibration for the maximum speed.
See example:
(6) Using compasses, do as follows.
(a) Take the length of vector BA and draw a circle with radius BA and center C.
(b) Take the length of vector BC and draw a circle with radius BC and center A.
The vector which passes through the origin of the polar graph and the intersection point of the circles is parallel and equal to BC.
(7) Measure the length and the angle of vector BC.
Example: The engine response at 96 percent N1 is 2.9 mils at 18 degrees.
(8) Record this value in the table.
See example:
(9) Use the same procedure for 84 percent N1 and 62 percent N1.
(10) On a polar graph, determine the actual weight added between the two flights as follows.
(a) Plot the initial weight (for example: 250 g.cm at 220 degrees).
(b) Plot the weight actually installed on the spinner for the other flight (for example: 571 g.cm at 265 degrees).
(11) Draw two arcs as follows:
(a) One arc with the end of the second vector (second weight) as center and the length of the first vector (first weight) as radius.
(b) One arc with the origin of the graph as center and the length of the line connecting the 2 vectors as radius.
Then note the value and the angle of the weight actually added (Example: 405 g.cm at 291 degrees).
(12) The engine sensitivity at each speed is obtained by dividing the weight actually added by the vibration response at each speed.
Example:
| 96 percent of N1 S = 405 g.cm/2.9 mils = 139 g.cm/mils |
| 84 percent of N1 S = 405 g.cm/2.6 mils = 156 g.cm/mils |
| 62 percent of N1 S = 405 g.cm/1.3 mils = 311 g.cm/mils |
(13) The engine phase lag is obtained by adding the following angles:
Angle of the added weight + angle of the vibration response + 180 degrees.
Example:
| 96 percent = PL = 291 + 18 + 180 = 489 i.e. 129 degrees |
| 84 percent = PL = 291 + 355 + 180 = 826 i.e. 106 degrees |
| 62 percent = PL = 291 + 12 + 180 = 483 i.e. 123 degrees |
(14) Record the calculated values in the table
See example:
Calculation Chart of the Vectorial Analysis Balancing Weight - Calculation Example ** ON A/C NOT FOR ALL See example:
Calculation Chart of the Vectorial Analysis Balancing Weight - Calculation Example ** ON A/C NOT FOR ALL Example for 96 percent N1: 1.9 x 139 = 264 g.cm.
(17) The angles of the improved balancing weights are obtained by subtracting the figures of column 5 from the figures of column 4.
Example for 96 percent N1: 129 - 272 = -143, i.e. 217 degrees.
(18) Three balancing weights A, B and C are obtained.
They correspond to the weights which cancel the No. 1 bearing vibration at the three speeds.
| Example: A = 264 g.cm at 217 degrees |
| B = 234 g.cm at 242 degrees |
| C = 280 g.cm at 239 degrees |
(19) On a polar graph using a g.cm scale, plot these three points.
(20) To determine the balancing weight corresponding to the best compromise, search for the intersection of three circles by setting vibration level targets for each speed.
(21) The balancing is within triangle ABC.
(22) These vibration targets are:
- 0.5 mils at 96 percent of N1
- 1.0 mils at 84 percent of N1
- 1.2 mils at 62 percent of N1.
| Example: 96 percent N1 = 0.5 x 139 = 69.5 g.cm |
| 84 percent N1 = 1.0 x 156 = 156 g.cm |
| 62 percent N1 = 1.2 x 311 = 372 g.cm |
Since the balancing weight for 96 percent N1 (264 g.cm at 217 degrees) is located within the 0.5 mils circles of the 84 percent N1 and 62 percent N1, it is adequate for balancing the engine.
(24) The initial weight (for example: 250 g.cm at 220 degrees) must be added to the determined balancing weight (example: 264 g.cm at 217 degrees).
To do so, a vectorial addition of these two vectors must be made, using the polar graph,
Example: The two angles are almost equal. The weight to be installed on the spinner is therefore equal to 264 + 250 = 514 g.cm at 220 degrees.
(25) Determine the screws to be used and their position:
and
Example: 220 degrees corresponds to position No. 15.
The weights are therefore centered on one screw. The screws to be installed on the spinner are the following:
- P01 screw in position No. 15
- P03 screw in position No. 12
- P03 screw in position No. 18.
All the other positions are equipped with P07 screws.