I'm not an accountant and I don't pretend to understand the "why" behind the decision to reverse the valuation allowance on Dec 31, 2013, but UA and AA did the same thing at the end of 2015, so the published results of all three will reflect federal income taxes going forward, even though none of them will pay federal income taxes for a while. The gigantic multi-billion dollar losses from the lost decade will shelter a few billion more in profits at all three. Again, I don't understand it, but I do know that even DL paid no tax, and thus when comparing the legacies, it's more useful to focus on "pre-tax" income.
In 2015, DL earned $5.9 billion of adjusted pre-tax profit, far more than UA, and probably more than AA will report. UA's 2015 profit was $4.5 billion, excluding special items.
For Delta, I compare the "adjusted pre-tax" profit to UA, because the GAAP result is misleading. GAAP required that DL recognize all potential hedge losses (the MTM losses) in 2014, which lowered DL's 2014 GAAP profit to just over $1 billion, which was not anywhere near reflective of DL's outstanding 2014. By assuming that the hedge contracts would lose money in 2015, GAAP artificially lowered the 2014 results. By the same token, the 2015 GAAP results are over-stated, because they don't include the actual hedge losses on those 2015 transactions, because GAAP required those losses to be recognized prematurely in 2014. So the "adjusted pre-tax profit" for 2015 accurately reflects DL's hedge losses that it actually suffered in 2015 (and, of course, keeps 2014 profit where it should have been).
Even better for the DL employees, the DL result above ($5.9 billion) is after payment of $1.5 billion of profit sharing to the employees, while the smaller UA result is after payment of only $698 million of profit sharing. So DL's 2015 pre-tax, pre-profit sharing adjusted income was $7.4 billion, while at UA the comparable figure was just $5.2 billion. Once again, DL's results out-classed UA's results. Well done.