Crack Your Flight's Complicated Code-Share

If you buy a ticket from one airline for a flight operated by another, which checked-baggage fees must you pay? (Usually those from the airline that sold the ticket.) United has no weight limits on carry-ons in coach. But partner

Air China

has an 11-pound limit, partner

Air New Zealand

has a 15-pound limit and partner

Lufthansa

18 pounds. If you buy a United ticket for a flight on those airlines, do you need to break out the scale? (Yes.)

U.S. airlines say about 25% of their international passengers use partner airlines under code-sharing arrangements. Policies differ on awarding frequent-flier miles, carrying pets in cabins, perks for elite-level frequent fliers and a host of other issues. Unless you study carefully in advance, you may get tripped up.

“We’ve been saying for 20 years it should be seamless, so customers reasonably expect that it should be seamless to them,” says a former airline executive involved in code-sharing for many years. “The major functionalities are functioning quite well. Some of the finer points are not yet.”

One example: Advance seat assignments are still difficult to get on a code-share flight at many airlines. Airlines say they are working on it.

“The occasional travelers are somewhat baffled by it,” the former executive says. “Frequent travelers know they have to make a separate call to get that seat assignment.”

Travelers are also baffled when two airlines offer different prices for the same seat.

Jeff Rothman,

a Los Angeles accountant, searched for direct flights to Montreal and saw a $513 round-trip price on

Air Canada

and $651 on United—for the same Air Canada flights. He assumed United was overcharging regular customers who don’t bother to shop for cheaper fares.

“Any way they can get another buck out of you, that’s what they are going to do,” Mr. Rothman says.

United declines to comment on pricing, and says it is spending more to resolve other disconnects. Other airlines say the higher prices on their code-share flights aren’t gouging. Unless they have joint ventures with antitrust immunity, they have to set prices independently. Prices reflect supply and demand for seats on particular flights, airlines say. The operating airline has more seats to sell; the code-share partner may only have a limited number of seats.

In most cases, airlines prefer you fly on their own planes. American and

Cathay Pacific

fly between Los Angeles and Hong Kong, and both are Oneworld alliance partners. For a trip March 7 to 14, Cathay’s price on American’s flights was $1,327, twice as much as American’s $647 fare. For Cathay flights the same day, Cathay offered a $508 fare while American priced the same code-share flights at $1,709.

Why buy the higher fare? An official at American and other airlines say one reason is corporate travel departments agree to give airlines a large percentage of their business in exchange for fare discounts. Booking one airline over another may make sense even if the published price is higher.

Checked baggage fees and allowances promise their own code-share chaos. The U.S. and Canadian governments issued rules a few years ago that for tickets into or out of their countries, the checked baggage rules of the airline that sold the ticket apply to the whole journey. In the rest of the world, the allowance and charges of “the most significant carrier” for that day’s trip apply. Different parts of a trip could have different allowances and fees.

Airlines must show checked baggage allowances on tickets when flights are operated by partners.

Code-share confusion even causes travelers to miss flights. Sometimes partners are located in different terminals and passengers may skip over the small print about which airline is actually operating the flight and show up at the wrong place.

After several years of significant profits, carriers finally are spending more to fix problems in code-sharing. Tech advances also make it easier to get airline computer systems to work together.

Delta

is working on systems that will recognize its frequent-flier levels at partner airlines, and so Delta can recognize the different tiers of customers from its partners. Currently customers with elite-level status are lumped together, whether top-tier or bottom-tier, in code-sharing situations.

Another change coming: better flow of information when flights get canceled. Delta now automatically rebooks customers on its flights, including code-share passengers. But the partner airlines don’t know that and customer contact information may not have transferred to Delta, leaving customers in the dark. Typically partner airlines tell customers to call Delta or to wait in a long airport line.

The closest airline connections are joint ventures with antitrust immunity that let airlines set fares together and share costs. Next step down: Airlines say partners within one of the three big airline alliances—Oneworld, SkyTeam or Star—try to align policies and co-locate gates to make connections easier. Sometimes you can get an advance seat assignment on an alliance partner, sometimes not.

Then there are looser partnerships outside alliances where airlines sell seats on each other’s flights to broaden their networks. Not counting regional affiliates, American has 12 code-sharing partners, United 10 and Delta nine.

Write to Scott McCartney at middleseat@wsj.com