Hey Onkle Karl,
For the vast majority of situations, a maritime patrol aircraft on an ASW mission would not be fully Loaded - ≥80% load. This is why we didn't include actual sonobuoy weights.
Not sure why you were using 27 kg/sonobuoy - that's way to big for standard A-size sonobuoys and is more representative of a B-size.
For a S-3A with a typical ASW load, you are looking at:
40 SSQ-41A/B x 7.5 kg = 300 kg
16 SSQ-53A/B x 10.5 kg = 168 kg
4 SSQ-62A/B x 18 kg = 72 kg
4 Mk 46 Mod 2 x 231 kg = 924 kg
2 300 gal drop tank x 920 kg = 1,840 kg
Total payload: 3,304 kg (about 73% of full load)
R/
Chris