Over the past two and a half years, Delta Air Lines, Inc. (NYSE:DAL) has been one of the best stocks in the market. Since September 2012, Delta stock has more than quintupled as the company has steadily improved its profitability and bolstered its balance sheet.

DAL Chart

Delta Air Lines Stock Chart, Sept. 1, 2012 to present; data by YCharts

However, Delta shares have dropped more than 10% since reaching a new all-time high above $50 last month. This dip represents a great buying opportunity for long-term investors.

Rising profitability
Delta earned a pre-tax profit of more than $4.5 billion last year: easily the best in the airline industry and up almost $1.9 billion year over year. Delta exceeded most of its financial performance goals in 2014, including its targets for EPS growth, free cash flow, and return on invested capital.

So far, Delta is projecting an even better performance for 2015, largely thanks to the recent drop in oil prices. For Q1 -- seasonally the weakest period of the year -- Delta expects its operating margin to reach 11%-13%, up from about 7% last year.

One of the catalysts for the recent drop in Delta stock was the company's January financial and operating results report. Delta's unit revenue dropped 3.5% year over year in January on a 6% capacity increase.

These results were in line with management's expectations. Furthermore, Delta disclosed that 1.5 percentage points of the unit revenue decline were caused by a higher completion factor, as it had fewer winter storm cancellations than in January 2014. This will have an offsetting unit cost benefit. Another 1 percentage point of the decline was caused by currency fluctuations.

Delta's unit revenue dropped 3.5 percentage points last month.

Fortunately, due to the decline in jet fuel prices, Delta doesn't need to grow unit revenue to expand its operating margin this year. Delta may be tracking toward the lower end of its Q1 margin guidance range, but that would still be a very impressive result for the weakest quarter of the year.

Strong cash flow trajectory
Thus, Delta is still on track for solid earnings growth in 2015. This will allow it to improve on the $5.8 billion of operating cash flow it produced last year. Depending on the movement of oil prices this year, Delta could produce about $7 billion of operating cash flow in 2015.

Based on Delta's plans to spend $2.8 billion on capex this year, it could generate more than $4 billion of free cash flow. By contrast, Delta rival American Airlines is also generating strong operating cash flow, but it will spend an average of $4.4 billion annually on new aircraft for the next four years. Including non-aircraft capex, American's annual capex will be $5 billion-$6 billion.

Despite keeping annual capital spending at $3 billion or less, Delta is still making critical fleet improvements. Between 2015 and 2019, Delta will buy 150 new planes to replace older models like the Boeing 747 that are ripe for retirement. Delta will also take delivery of 36 used Boeing 717s this year, which it will use on short-haul routes currently flown by regional jets.

Delta has promised that it will return at least $1.5 billion to investors in 2015 through dividends and share buybacks. It will also use its free cash flow to reduce its pension obligations and net debt level. However, there will probably still be enough cash for left over for the company to exceed its $1.5 billion cash return target.

Today, Delta shares trade for less than 10 times free cash flow, which is far below the average for U.S. industrials firms. With free cash flow continuing to rise and net debt levels falling, Delta will be able to return more and more cash to shareholders in the next few years. This should move the stock higher.