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Since the centre of gravity must lay in a certain location, I'm wondering if it would save fuel if it lays exactly in the right spot?

Let's assume that you have an A320 and weight distribution is fine (within tolerance). If ballast is shifted until COG is located at the perfect location, would you save fuel? How much would that be?

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  • $\begingroup$ Shifting the fuel during ascent descent and level flight was an active task on the Concorde $\endgroup$
    – Antzi
    Commented Jul 30, 2016 at 3:23

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For precisely this reason the A310-300 had a fuel tank in the horizontal tail. However, the pilots I spoke with (Pan Am) did not understand the purpose and insisted that the tank is empty all the time. The A330 and A340 also have a tail fuel tank, and here its fuel level is automatically controlled.

A330 tank arrangement

A330 tank arrangement (picture source). Note the trim tank in the horizontal tail.

Shifting the c.g. a little back reduces drag slightly (less than 2% reduction in fuel consumption, depending on initial c.g. position) but also static stability. This is no problem when the FCS adds enough stability, but in case of a system failure you should make sure that the rear tank can be emptied soon. Hand flying an aircraft with relaxed static stability is tiring.

The A350 did away with this feature because it was felt that the added complexity would outweigh the benefits.

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    $\begingroup$ the pilots I spoke with (Pan Am) did not understand the purpose Are you saying they were not aware that it was for CG purposes, or that the benefit was so miniscule they didn't see a point in having it there? $\endgroup$
    – TomMcW
    Commented Jul 29, 2016 at 17:57
  • $\begingroup$ How would 2% less drag reduce fuel consumption by 2%? $\endgroup$
    – dani
    Commented Jul 29, 2016 at 18:02
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    $\begingroup$ @TomMcW: They were convinced that the aircraft would be totally unstable as soon as fuel would be transferred to the rear tank. This was at a time when you could still walk up and ask to visit them in the cockpit. Which I did. I did not, however, try to explain the benefits to them, seeing how convinced they were of their wisdom. $\endgroup$
    – Peter Kämpf
    Commented Jul 29, 2016 at 18:44
  • $\begingroup$ @dani: Thanks, this was sloppy of me. At those small values the error is not very large, however. $\endgroup$
    – Peter Kämpf
    Commented Jul 29, 2016 at 18:46
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    $\begingroup$ Sad to say, I found through the years prior to my retirement in 1999 that many pilots did not understand weight and balance. I also remember running across two FAA POIs (Principal Operations Inspectors) that were in charge of monitoring airline compliance with weight and balance standards that lacked basic knowledge of the subject. One even expressed the opinion that he was good at weight and balance. The other was more realistic, asking me personally at the end of a 2 hour presentation if I could explain the "teeter-totter problem." $\endgroup$
    – Terry
    Commented Jul 29, 2016 at 18:52
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Short answer: yes it can. Long answer: Yes, but this may not always be practical for a given passenger manifest and cargo. On a dynamically stable airplane, a aft CG is more beneficial, since it reduces lift generated by the tailplane, thereby reducing induced drag on the wings.

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