AMP's banking operation may be in a rare industry sweet spot, achieving industry leading returns, a low cost ratio and growing above system.AMP Bank yesterday reported growth in operating earnings of 15 per cent, to A$120 million.The cost to income ratio it put at 29 per cent.Margins lifted to 1.67 per cent from 1.59 per cent over one year, but moderated during the second half to 1.63 per cent - down from 1.71 per cent.AMP said the bank's return on capital increased to 16.5 per cent over the year from 16.3 per cent in 2015, placing it among the highest returns in the banking sector.AMP Group presented the bank as "a significant opportunity ... particularly across the aligned advice network."It said it "continues to target total lending growth at or above system, subject to regulatory growth caps."The bank's loan book increased by 13 per cent to $17.1 billion. Sales through the AMP aligned channel were up 24 per cent.