Travel search engines try to find every possible airfare or hotel rate for their customers, but they always come up short.
Like every other travel website, they have content holes.
Southwest Airlines, for example, wants customers to come to its site, not others. It refuses to let its fares be listed on any third-party website, so the flights of the nation's largest discount airline are missing from any travel search.
JetBlue Airways won't let its fares be sold by the online travel agencies Expedia, Orbitz, and Travelocity because it wants to deal with customers direct. But the fast-growing discount airline is experimenting with the travel search engines because they bring consumers directly to the airline.
None of the travel search engines have access to the travel inventory of Travelocity or Expedia. Orbitz is available to Sidestep and Farechase, but not to Kayak. Several airlines and hotel chains have refused to sign compensation arrangements with the travel search engines, forcing them to find alternative ways to list their fares and room rates.
Sidestep, for example, sends customers wanting to buy a ticket on Northwest Airlines to Orbitz, where they have to pay a service fee. Kayak lists the Four Seasons Hotel among its Boston hotels but tells users they have to call the hotel to book or check rates.
David Cush, vice president and general sales manager at American Airlines, said travel search engines are fairly comprehensive on domestic airfares, but less so abroad because there are so many more airlines with whom they need to negotiate deals.
In a limited Globe test on flights to Paris, Orlando, and Las Vegas, the travel search engines fared well against Orbitz, Travelocity, and Priceline.
There were no $1,000 savings, but the search engines in several cases had cheaper flights than the big online travel agents, particularly to Las Vegas, where JetBlue flies.
Krista Pappas, senior vice president at Smarter Travel Inc. in Cambridge, said her company's BookingBuddy.com offers the only truly comprehensive way to search for travel.
BookingBuddy users plug in their travel parameters once and then proceed through every applicable website looking for the best deals. ''What you see is what you get here," Pappas said.
Carpenter at Sidestep says the problem with BookingBuddy is that it forces the consumer to do all the search work. Hafner at Kayak says the goal of travel search should be to show every travel option on one integrated display. ''That's what the consumer wants," he said.
The travel search engines have been making some progress lately. Airlines and hotels that initially balked when the search engines tried to poach their fares without permission are making peace with them now and seem intrigued by their low referral costs, which are well below those of online travel agencies.
''It's still to be seen whether travel search is going to take off and be embraced by consumers, but it could be at the tipping point," said Noreen Courtney-Wilds, director of sales for JetBlue.
The search engines have also linked with bigger players with deeper pockets. Farechase sold itself to Yahoo last year, America Online acquired a stake in Kayak late last year, and Sidestep said last month that it would power Amazon.com's travel store.
Michael Cannizzaro, director of information services at PhoCusWright Inc., a Connecticut Internet research firm, said the partnerships with the larger companies are a way for travel search engines to raise their profile.
''It's hard to see how the stand-alone metasearch companies can survive on their own," Cannizzaro said. ''They don't have enough marketing muscle, but packaged with a travel portal it should work."
Bruce Mohl can be reached at email@example.com.