Give JetBlue (NASDAQ:JBLU) credit. Even though the beleaguered discount carrier is still bleeding money on the bottom line, its first-quarter results were plenty encouraging.

But first, the bad news. A 24% improvement on the top line only trimmed losses on the bottom line. JetBlue booked a $22 million loss in the quarter, down from $32 million the year before.

The carrier also burned through $65 million in cash and added more than $500 million in new debt. JetBlue has more than $2.9 billion in debt -- more than three times the equity stockholders have in the business.

Feeling airsick? Hey, I don't blame you. Turbulence comes with investing in airlines, as JetBlue found out in February.

And yet the airline is improving in many ways. Available seat miles rose by 12% year over year, which suggests that JetBlue is flying more aircraft more frequently. Revenue per seat mile was also up by double digits. Cost per mile rose, but at a slower pace.

Then there's the load factor, which declined by nearly four percentage points. This is good news, believe it or not. Airlines don't really want to fly every plane packed to the gills. They'd rather charge more and carry a lighter load. JetBlue is doing exactly that -- its average fare was up 3.7%.

So, would I buy this stock? No. I own shares of Southwest (NYSE:LUV) because it boasts a more streamlined, cost-efficient business. And, in my experience, Southwest's employees are at least as loyal and passionate as their true-blue counterparts.

I don't say that lightly. JetBlue, for my money, is a wonderful airline. But the investor in me wonders how much we really know about the business. Has it really been tested? What happens when its new customer's bill of rights forces JetBlue to pay for service interruptions that, in many cases, are likely to be beyond its control? I've no idea. Nor do any of us -- and that, for a Fool, is entirely unacceptable.

So, congratulations on a good quarter, JetBlue. Keep 'em flying. Just don't expect those of us here on the ground to invest till we see what business is like under the thumb of a customer's bill of rights -- and how well your financials respond to the extra baggage.

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Fool contributor Tim Beyers, ranked 3,251 out of more than 27,800 in CAPS, usually flies coach, but will take first class any time he can get it. Tim owns shares of Southwest. All of his portfolio holdings can be found at Tim's Fool profile. His thoughts on Foolishness and investing in general may be found in his blog. The Motley Fool's disclosure policy is always on time for departure.