Location of the Balancing Weights Figure 516/TASK 71-00-00-991-335- 12 (SHEET 1)
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 Location of the Balancing Weights Figure 516/TASK 71-00-00-991-335- 22 (SHEET 2)
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 Balancing Weight Centered on One Screw Figure 517/TASK 71-00-00-991-337- 13 (SHEET 1)
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 Balancing Weight Centered on One Screw Figure 517/TASK 71-00-00-991-337- 23 (SHEET 2)
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 Balancing Weight Centered on One Screw Figure 517/TASK 71-00-00-991-337- 33 (SHEET 3)
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 Subtask 71-00-00-750-057

 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 (Ref. TSM 770000810862), (Ref. TSM 770000810863) or (Ref. TSM 770000810864). 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. Two alternate procedures may be applied giving similar results:
 (a) "One sensor (No.1 bearing) 3 speeds" procedures: In this procedure, it is easy to do the calculation by hand. This is explained in detail in the following pages by the use of a numerical example. 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 64 percent of N1, 84 percent of N1 and Max. of N1, that is between 93 percent and 96 percent.
 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

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 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:
 This program is available under three references. 856A2650G010: Multi-Engine Trim Balance Program. 5 1/4 Hard Disk 856A2650G011: Multi-Engine Trim Balance Program. 3 1/2 PC without Hard Disk 856A2650G012: Multi-Engine Trim Balance Program. 3 1/2 Hard Disk 856A2678P01 : Multi-Engine Trim Balance Program. CD-Rom WINDOW Compatible
 NOTE : The above programs are available whatever the trim balance
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 procedure selected.

 (b) "Two sensors (No. 1 bearing and TRF) 5 speeds" procedure This procedure uses the capacity of the EVMU, to record in flight vibration data on engine both sensors up to 5 predetermined N1 speeds. The calculation algorithms are the same as the in above procedure, but as up to 10 points of calculation may be performed, it is recommended to use the CFMI programs 856A2650 listed above. The advantage of this procedure is to manage the TRF vibration level using the one-shot calculation. As the imbalance correction calculation is done the same way as in the previous procedure, but with 2 sensors instead of one, the description of this procedure is not made in the following pages. Refer to the programme user's guide (GSEM): Ground Support Equipment Manual for detailed explanation on how to use the program, when using two sensors. Recommended speeds are: 64, 84, 88, 92, 96 percent of N1.
　
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