You win some, you lose some: United is cutting the fee for a second checked bag, while Delta is adding a fee for the first checked bag. And if you think carry-ons are business-as-usual, look out…

United was going to charge $50 for a second bag, but decided to curtail that and roll it back to $25. Good on ‘em.

Then there’s Delta, which recently merged with Northwest and is trying to align policies and procedures. In this case, they’re taking the lower road plowed by Northwest and imposing a fee for the first checked bag.

Readers may recall that I praised Delta’s logic in charging no fee for the first bag, but charging heftily for the second bag. The former policy kept the occasional checked bag part of the base fee, while heftily penalizing those who travel with the kitchen sink. Seemed reasonable then, still seems so now.

Delta suggests it didn’t see any increase in bookings as a result of the free-first-bag rule, so it deduces that customers weren’t “differentiating Delta as the only major airline not charging for a first checked bag,” according to their press release. (Ignore for the moment that they leave out Southwest, which doesn’t charge ANY baggage fees…) In any case, this is too bad.

Note also that paying a luggage fee doesn’t prevent your suitcase from being inspected by zoo animals: A Delta baggage handler recently opened the cargo doors to find a cheetah roaming around inside. The animal had escaped from its cage. No luggage was harmed, though it was almost certainly sniffed. Anyway…

Then there’s this change, governing carry-on bag size:

Carry-on rules also are changing. Continental recently reduced the allowable linear inches (length, plus height, plus width) from 51 to 45. That puts it in line with the linear inches allowed by American, Delta and Northwest. AirTran allows up to 55 linear inches, US Airways’ limit is 51 linear inches and Southwest’s limit is 50 linear inches. Many airlines also limit the weight of carry-ons to 40 pounds.

Expect these numbers to shrink, too, and expect airlines to start adding a fee for “oversize” carry-ons. They know that people are avoiding checked luggage because of the fee, and they’ll want their money one way or the other. That day is coming.

And just for the record, with oil trading around $60 per barrel today, the fuel-expense argument rings hollow for these luggage expenses.