Southwest, which capped off the year with a fourth-quarter profit of $536 million, says 2015 was its best year for earnings ever, thanks to full flights,

more seats and cheap fuel. Delta reported a full-year profit of $4.5 billion, also due in part to the lower price of oil. And cheaper crude helped United post an $823 million profit in the fourth quarter.

Even before airlines posted their end-of-year results, 2015 was a record-buster, with carriers cumulatively posting a nearly $17.9 billion profit, after taxes, in the first nine months.

With fuel typically consuming roughly 1/3 of an airline’s total revenues, there is a strong connection between sinking fuel prices and the industry’s rising profits, though other factors, including the billions of dollars the industry reaps in fees and greater discipline in matching seats to passenger demand, also factor in.

It was record-high oil prices, which peaked at $145.31 on July 3, 2008, that helped spark the rise of checked bag fees and other, extra charges, as the industry tried to stay afloat. Those fees remain. But cheaper fuel "in the intermediate term ... is providing a near-term profit and cash windfall for the carriers,`` Jim Corridore, an S&P Capital IQ equity analyst, writes in a report on the airline industry.

Crude oil reached $102.99 per barrel, on average, in the April quarter of 2014 but has been dropping fairly steadily ever since, according to Tom Kloza, global head of energy analysis for the Oil Price Information Service. It is averaging $31.63 per barrel so far this quarter and in January dipped below $27.

Consequently, the price of jet fuel has also been sliding and is on track to have its cheapest quarter since the final three months of 2003, Kloza says.

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