Before I started flying I worked as a flight operations officer for a european major. We were instructed to prepare the loading instructions in such a way that the CG was as close to 23% MAC as possible, as this was supposedly the most economic MAC value for short-haul on the 737(-300/-400). We were told that if 50% of the flights were within +/- 1% of 23 it would save the company 1 million dollars worth of fuela year. I don't know how this value of 23% was generated but there are certain considerations as to why you would'nt want to operate with a MAC very close to the aft limits:
- More aft MAC means more,or all luggage and cargo in the aft hold, this means longer turnaround times as only one loading-crew has to load all the stuff in one hold.
- Chances of the a/c being out of trim at the last moment when planned with a aft MAC. (This occasionally happened with the 23% planning aim).
- I'm not a performance expert but, aft CG means more pitch up attitude during cruise and more trim. I think there is a turning point somewhere at wich increased drag of pitch attitude is higher than the gain of reduced drag on the stabilizer.
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