First there was online ticketing. Then check-in by smartphone. Now airlines are looking at rolling out machines that check boarding passes at the gate.

Delta Air Lines recently tested a self-service turnstile that lets fliers scan their own boarding passes. If implemented, the device could make it possible to bypass all interaction with airline employees, from the time travelers enter the airport until they're on the plane.

"We're into the age of speed. We're into the age of trusting technology," said Mary Tabacchi, a professor who teaches airline management at Cornell University. "Airlines will find it a very useful tool."

At least 17 airlines in Europe and Asia use self-boarding machines and several U.S. carriers are testing the devices, according to the International Air Transport Association. The experimentation comes as airlines search for ways to cut down on labor costs, a large part of their operations.

Some airlines that have tried the machines say they free staffers to assist customers with pressing problems. But unions representing airline workers see nothing more than a cost-saving measure.

"We're concerned about the jobs that could be lost and displaced as a result of it," said spokesman Frank Larkin of the International Association of Machinists and Aerospace Workers, which represents United Airlines and US Airways employees. Some workers at Delta are trying to reorganize under IAM.

Delta has tested the device in Atlanta and Las Vegas, with agents on hand to help passengers if necessary. Delta has no plans to expand the test to other airports, said spokesman Morgan Durrant, who declined to discuss customer feedback or other details of the test.

Officials at McCarran International Airport in Las Vegas said Delta's test was positive. The airport plans to make 14 gates with self-boarding machines available to interested airlines by the end of summer, but did not disclose which airlines will be using the gates.

Advocates say the devices give agents more time to upgrade passengers, police the size of carry-on bags and sell add-ons, such as seats with more legroom. One agent can watch over more than one self-boarding gate.

"Virtually all air carriers that have tried them have realized an operational and economic benefit from them," said Samuel Ingalls, assistant director of information systems at the Las Vegas airport. "Additionally, the customers like to be empowered over all of the various processes."

Brisk sales

The German manufacturer behind the airport's self-boarding gates, Kaba Gallenschutz GmbH, has seen a sharp increase in sales. Last year, Kaba sold 500 of the self-boarding gates, up from 200 in 2010, said Lars Rosenberger, a business manager who deals with airport solutions.

German airline Lufthansa has purchased Kaba's self-boarding gates, which cost roughly $19,500 to $26,000 each. Lufthansa was a pioneer of the technology, first using the machine in the late 1990s.

Customers were able to board planes faster and the machines paid for themselves over time because there were fewer employees required at the gate, said Michael Kuenzel, manager of product management passenger services. The number of agents was cut by about half, Kuenzel said.

U.S. airlines are eager to give self-boarding machines a try. United Airlines and American Airlines have tested the self-boarding gates. US Airways has tested letting fliers scan their own boarding passes using the existing equipment but opted against self-boarding devices because it was too costly.

Still, analysts said there could be a huge financial benefit. Labor was historically the largest single cost of an airline's operations until fuel prices soared, analysts said. It still represents about 23 percent of an airline's operational expenses, according to trade group Airlines for America.

Gate agents earn $25,000 to $50,000 a year in wages and benefits, according to AirlineFinancials.com. Unlike workers, a machine can get through the whole day, said Bob Herbst, the firm's founder. The savings could cover a machine's cost in six months, he said.

It may take awhile for U.S. consumers to embrace the devices. Some travelers still struggle with check-in kiosks today.

Airlines also risk alienating their elite travelers, who may enjoy the personal greeting when they hand an agent their boarding pass.

"A lot of first-class passengers have gone to flying private jets," Tabacchi said. That's partly because of the lack of customer service, she added.

A security risk?

Some airline employees have expressed concern that the machines could lead to security problems, but advocates said that hasn't been an issue.