Airline ticket sales online are really taking off, helping boost a sagging industry still struggling to recover from the Sept. 11 terrorist attacks.

Nearly $14 billion was spent online in 2001 for airline tickets, according to Forrester Research of Cambridge, Mass. And while that is still less than 10 percent of all airline ticket sales, the major carriers now realize how vital their Web sites have become.

"Airline Web sites have done extremely well considering airlines had such a lousy year," said Lorraine Sileo, an analyst at PhoCusWright, an online travel strategy and research company. "Airline-branded Web sites brought in $6.4 billion in 2001 – an increase of 28 percent over 2000."

Two of the most profitable airline Web sites, operated by Delta and Southwest, attribute their success to customer feedback that has helped shape the sites and improve their features.

"Delta.com allows customers to have control over the entire travel experience. This provides them with self-sufficiency that our customers appreciate," said spokeswoman Katie Connell.

And the customers are logging on to spend: More than 4.2 million Delta tickets, which generated more than $1 billion in revenue, were purchased on delta.com in 2001, representing a 60 percent growth over the previous year. January was a record-breaking month, with 545,000 tickets producing some $145 million in revenue.

New technology allows all Delta SkyMiles members to check in and print same-day flight boarding passes for their e-tickets on delta.com. The feature is a dream for travelers in a rush who, as Connell put it, "don’t want to interface with anyone at the airport."

Perks go beyond technological treats. Delta.com customers on transcontinental flights were surprised earlier this year with free Broadway tickets as a reward for purchasing their air tickets online.

Customers aren’t the only ones who benefit. Melanie Stillings, manager of interactive marketing for Southwest, said online sales are extremely important for the airline.

"Our online distribution is the lowest cost of distribution in terms of tickets," she said. "Online purchases costs us about one dollar, verses $7 to $8 for a travel agent, and we give these savings back to our customers in the form of low fares."

Their customers have responded by making Southwest.com the most successful airline Web site. More than $2 billion in Southwest tickets were sold online in 2001, representing more than 40 percent of all passenger revenue.

"The Web site is an excellent communication tool especially since 9/11 because it gives us the ability to get information up quickly about security, flight times, airport information," said Stillings. "We consider our Web site the customer’s Web site. Everything we do online is a result of a request from our customers."

Almost all airlines have seen double-digit growth in Internet sales through their sites. Sileo says "sticky features" that keep customers coming back are key. Among the benefits: redeeming frequent flier miles online and allowing customers to sign up for e-mail or pager alerts to notify them when their flight is delayed.

Southwest customers who purchase online are also rewarded with double the normal credit (each one-way trip is a credit), meaning passengers can qualify for a free ticket for every four round trip tickets they purchase.

"You don’t have to get out of your chair to buy your ticket. On airline Web sites you choose your seat, plan your travel, and more — it’s not such an event anymore. You can do it on your lunch break," Sileo said.

Ease of use and savings are two features of online travel planning that have convinced some that they’ll never pick up the phone again to buy airline tickets.

"It’s easier and quicker than doing it over the phone or through an agency and there are better rates," said William Brownlow, a Washington, D.C., analyst who likes to buy tickets on Airtran.com. "They offer a discount if you buy through their Web site. I save $15-20 per flight, and when you’ve got a girlfriend in Boston, it makes a big difference."

Terry Martin, a television executive in New York City, said he prefers purchasing tickets online because he can monitor fare changes day-to-day. "And there’s no question I get cheaper fares than if I called in and asked an agent — as much as $200 less per round-trip ticket."

With customer satisfaction running high and innovative features making the travel experience a bit easier, the future for airline-branded Web sites looks bright.

"Sites continue to be a place where airlines invest — 10 percent of all travel is booked online, so there’s a long way to go," said Sileo. "It’s a young industry, but it will continue to grow at double-digit pace, even if travel as a whole is flat."