WASHINGTON -- Under the new airline partnerships known as code shares, passengers with tickets from Northwest and Continental airlines can ride on the same plane and earn frequent flier miles redeemable from either airline.
They may be paying vastly different prices for their tickets, though.
An airfare analyst compared the cost of the same seat on the same plane and found a nearly $600 difference in the price paid for a ticket issued by Continental from the same ticket issued by its code share partner for the past year, Northwest.
Terry Trippler, operator of the Internet-based "Airfare Report," said the same kind of gap can be found with all of the partners in the nation's new string of code shares, including the tandems of United Airlines and Delta Air Lines, or American Airlines and US Airways.
The partner with the higher price depends on discounts or the route flown.
Trippler's warning sounds familiar: Buyer beware.
"Code sharing is so widespread, you better be on your toes. You better ask every single time, every single flight, 'Is this a code share?' And if so, 'With what airline?' and then ask for the price on both of them," he said on Monday.
Airlines began code share partnerships as a means of increasing investor return, pooling resources and smoothing out travel for passengers.
While the airlines generally remain separate entities, they exchange frequent flier privileges and club memberships and also book tickets so travelers from each partner airline can make seamless connections no matter which one issues the tickets.
In doing so, airlines can help fill up each other's planes and also reach areas outside their normal network.
That doesn't mean code share passengers traveling on the same plane pay the same price.
To make his point, Trippler booked six different sets of flights, with one ticket from Continental and one from Northwest for each. While the flights had different numbers, depending on the airline issuing the ticket, they were flown on the same Northwest or Continental planes and had the same arrival and departure times.
The only difference was in the fares.
A Feb. 6-12 round trip from Newark to Singapore cost $847.21 from Northwest, but $1,445.31 from Continental. A Feb. 8-10 round trip from Cleveland to Boise, Idaho, cost $406 from Northwest, but $1,650 from Continental.
A Northwest spokesman said that is exactly how the system is supposed to work.
"We don't coordinate. That would be illegal," said airline spokesman Jon Austin. "We are competing to sell tickets. On some code shares, that means tickets on the same plane, but that doesn't lessen the competition."
Asked to explain the discrepancy between the Northwest and Continental fares, Austin replied: "I have no idea. That would be asking me to read the mind of the Continental marketing folks."
Continental did not immediately return a call seeking comment.
Travel agents and some consumer groups have fought code shares, saying the "virtual merger" of airlines will eventually stifle competition. More immediately, they say they further complicate the already puzzling world of airfares.
Trippler, a former travel agent, said using a travel agent is the best protection against paying different prices for tickets from code share partners. Some online travel services also offer fares from the perspective of both code share partners, he said.
"I seriously doubt that a Northwest agent is going to spend a lot of time checking the same ticket as a Continental fare," Trippler said.
Austin, the Northwest spokesman, said using a travel agent or booking directly with an airline "is a wholly personal decision."
He added: "I think a travel agent is a wonderful thing; I would support people who would want to go that way. On the other hand, there are people who book on their own or through the Internet, and I support that, too."
Trippler's Website can be found at www.rulesoftheair.com.
An example of the different prices paid by Northwest and Continental airlines passengers for travel on the same dates and planes, depending on which code share partner issued the ticket:
Dates: Feb. 6-12
Newark to Tokyo: 11 a.m. - 3:30 p.m. (Continental plane)
Tokyo to Singapore: 6 p.m. - 12:30 p.m. (Northwest plane)
Northwest airfare: $847.31
Continental airfare: $1,445.31
Dates: Feb. 12-18
San Francisco to Tokyo: 12:25 p.m. - 4:26 p.m. (Northwest plane)
Tokyo to Seoul: 6:05 p.m. - 8:50 p.m. (Northwest plane)
Northwest airfare: $660.40
Continental airfare: $1,082.40
SOURCE: Terry Trippler, Airfare-Report.com Inc.