Another technical issue in addition to the flat surface (which only really makes CG computation more difficult) is the fact that such systems tend to work base upon undercarriage oleo compression to work out how much mass is pressing down at each location. However the compressibility of an oleo is affected by ambient pressure and temperature, whilst these figures are readily available from other systems on the aircraft, the resulting calculation could only ever be a comparison with the load sheet. If there was a significant disparity between the two the resulting extra effort would outweigh (no pun intended) the cost of gaining certification to use the self-weighing device.
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