Riddle me this, frequent flyer: Why would anyone invest in a legacy airline?

The industry is notoriously volatile, after all, subject to the vagaries of fuel prices as well as the pricing pressure brought to bear by such lean-and-mean low-cost carriers as Motley Fool Stock Advisor pick JetBlue (NASDAQ:JBLU) and Southwest Airlines (NYSE:LUV).

Like me, investors seem to be scratching their heads over the airline industry these days. Bankrupt flyer UAL is hovering near penny-stock status, while Northwest Airlines (NASDAQ:NWAC) has shed more than 56% of its value this year. On the upside, American Airlines (NYSE:AMR) and Continental Airlines (NYSE:CAL) have more than held their own, checking in with double-digit gains on the year thanks in large measure to viable plans for keeping a lid on costs.

Not so Delta Air Lines (NYSE:DAL), whose stock is off nearly 53% thus far in 2005. Delta reported second-quarter results before the market opened yesterday, posting a per-share loss of $2.11, a figure that excludes one-time "special items" charges.

That number is better than analysts had forecast, and the company did show improvement in a couple of key metrics. Operating revenues, for example, ticked up nearly 6% over the comparable 2004 figure, and revenue per passenger showed an increase as well.

Nonetheless, investors still grounded the stock, sending it to the tarmac with a 9.2% loss on the day.

It's hard to blame them: Delta's future is -- pardon the pun -- up in the air.

In addition to fuel costs -- which rose nearly 60% over the year-ago period -- and intense industry-wide competition, pension expenses are a major concern for the once high-flying airline. Indeed, one of the special items charges the firm reported was the result of a lump-sum payout of $104 million to some 350 pilots who left under the company's Pilot Plan retirement program.

In short, it looks as though Delta needs to hit a cost-control trifecta to remain viable. And with that in mind, investors considering a takeoff with the airline should probably consider an alternative flight.

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Shannon Zimmerman runs point on the Motley Fool Champion Funds newsletter service. At publication time, he didn'town any of the securities mentioned above. The Fool has a strictdisclosure policy.