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B737NG FMC TRIM VS LOADSHEET TRIM

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B737NG FMC TRIM VS LOADSHEET TRIM

Old 10th Nov 2016, 13:23
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B737NG FMC TRIM VS LOADSHEET TRIM

what would be the maximum discrepancy allowed between the TRIM value generated by the FMC when compared to Computerised / manual Load sheet?
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Old 10th Nov 2016, 14:46
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Hmm, do you get a calculated trim value with your loadsheet? Ours just had a CG value which we entered into the FMC and then we used the trim value from the FMC. So there was no discrepancy at all.
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Old 10th Nov 2016, 15:12
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We get a calculated value on the load sheet. Which we compare with the value calculated by the FMC. Just wanted to check, if there is recommended max discrepancy value mentioned by Boeing.
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Old 10th Nov 2016, 17:37
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There should be only a minor difference.However it also depends upon the software of the Computerised LS.Some versions calculate the SLIGHTLY INCREASED nose up trim correction for reduced thrust settings (assumed temp), which can be around 1 unit nose up correction for a fully derated thrust setting.Others dont make this correction leading to a difference in STAB TRIM setting, in this case you should apply the correction yourself, and the only place you can find a reference to this might be a table in the Weight and Balance Manual.Obviously if you use full thrust then no correction might be required.....Peter.
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Old 10th Nov 2016, 17:50
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If i recall correctly there was a difference in trim settings depending on chosen flap setting. Do you tell your loadsheet provider which flap setting you use? The OPT can chose the optimal flap setting for you as far as i know. The software we used before the OPT certainly did, so we knew which flap setting we were going to use only after the final performance calculation was done.
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Old 10th Nov 2016, 19:17
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I'm not sure how this value was determined to be satisfactory, but our operation has the following guidance:

"The FMS computed value may be used in lieu of the
WDR value if the difference is 0.75 or less."
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Old 10th Nov 2016, 20:55
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Originally Posted by Denti View Post
If i recall correctly there was a difference in trim settings depending on chosen flap setting. Do you tell your loadsheet provider which flap setting you use? The OPT can chose the optimal flap setting for you as far as i know. The software we used before the OPT certainly did, so we knew which flap setting we were going to use only after the final performance calculation was done.
Our loadsheets cover all flap settings. There's a trim setting for 26K Flap 1-5, then another from 10-25 (from memory there's only two divisions), and then the same for 24K. This is then compared to the FMC generated stab trim. All our SOP say is to "check for reasonableness".

It's rare to see it more than 0.1 units out, but near impossible to trim to that accuracy anyway.
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Old 11th Nov 2016, 02:08
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To be absolutely 100% correct in all cases, the full CG calculation should always be made of course, however on two types I have conducted extensive base training/circuits on , using a default figure of 2 units NU stab trim (767) or 4 units NU stab trim (757) has always produced an in-trim airplane for the take off phase 99.9% of the time.Actually 100% of the time.PETER
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Old 11th Nov 2016, 14:09
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Our loadsheets cover all flap settings. There's a trim setting for 26K Flap 1-5, then another from 10-25 (from memory there's only two divisions), and then the same for 24K.
FMC trim for a given CG changes with thrust setting 20/22/24/26/27 and flap selection, but not when an assumed temperature is being entered.

Manual load sheets in FCOM tables give corrections to a uncorrected trim in a more basic manner, thus are not as accurate but well within tolerance limits, thus used, a simplified method.

It would be much more "messy" for Boeing to publish in AFM a large correction table which is then to be used by airlines listing each thrust and flap setting combination rather then to average several similar approximations into a single value when possible thus minimising pilot correction values listed on loadsheet for simplicity.
The FMC and OPT however are not restricted by such limitation and thus provide excellent indication of trim usually within 0.1 of each other as they use a digital file containing all corrective values.
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