Meanwhile, American, Alaska and Virgin have no plans to filter sites based on their content. At most, an airline may manage traffic and delay large downloads, or in Virgin's case give passengers the option of enabling controls for their kids.

"We think decency and good sense and normal behavior" will prevail, said Jack Blumenstein, chief executive of Aircell LLC, which is launching service on some American and Virgin flights in 2008.

Alaska, which plans to start offering service on some flights in the spring, said the same guidelines apply whether a passenger is flipping through a magazine, watching a DVD on a laptop or surfing the Web.

"Occasionally we do have conversations with customers about content," Alaska spokeswoman Amanda Tobin Bielawski said.

In many ways, airlines are facing issues similar to those encountered by Wi-Fi networks on the ground — at airports, coffee shops and other public places.

Glenn Fleishman, editor of the Wi-Fi Networking News site, said operators of public networks generally do not filter because users are conscious that others can see what they surf. A coffee shop employee might occasionally ask a customer to leave, Fleishman said, "but those stories tend to be pretty far between."

Airplanes, however, are different because customers are in closer quarters and are more likely to include kids.

Allowing pornography could subject an airline to harassment complaints much like an employer that refuses to clamp down, said John Palfrey, a Harvard Law School professor.

"I think they have a right to (filter), but I come up short of saying they have the responsibility," Palfrey said. "I'd rather have the responsibility in the hands of passengers and require them to be accountable for what they do on laptops and airplanes."

Airborne Internet activities — such as hacking and piracy — could raise new questions about which country's laws apply.

The in-flight services also could exacerbate long-standing grievances.

What if the passenger in front of you wants to recline, making it difficult to surf comfortably on your laptop? What if you're finishing a crucial e-mail on deadline and an adjacent passenger needs to leave for the bathroom? What if the person next to you keeps peering over while you're trying to review a confidential Web site?

Steve Jones, a University of Illinois at Chicago professor who specializes in Internet studies, said passengers and flight crews would need to undergo "the kinds of learning the ropes and learning the etiquette anytime we put new technology in new settings."

Just as most people have come to set boundaries for cell phone use in public settings, he said, "we will develop social norms for using the Internet in flight."

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