What used to be a battle in the skies over who served the best steak and the finest red wine has become a war over who offers the most films and Nintendo games.

Only a few years ago, passengers trying to while away long hours in the air were lucky to be able to see a single movie and to listen to some scratchy music. Now, 80 percent of the electronics on a new wide-body jet are in the passenger cabin, not the cockpit.Airlines are expected to spend $6 billion over the next five years to equip 4,800 planes with a new generation of systems that will deliver what vendors first promised five years ago: audio and video on demand, an interactive system that enables passengers to start, stop, rewind and fast-forward any one of scores of films and TV programs at will.

Industry executives predict that by early in the next decade the systems will begin showing up in smaller, single-aisle planes flying shorter routes. At the same time, passengers will also be offered live television and Internet connections by satellite, enabling them to watch live football games, send and receive e-mail, surf the World Wide Web and trade stocks.

The systems are expensive to install - as much as $4.5 million a plane. They were at first offered exclusively to first- and business-class passengers. Airlines like Virgin Group's Virgin Atlantic and Singapore Airlines began making them available at every seat on their wide-body planes in the early 1990s, establishing a trend for the industry.

But if offering 500 channels and movies on demand to homes on the ground proved more difficult than many had thought, it was even more challenging in the air.

The video systems on planes place the equivalent of a personal computer under every seat. Each seat is then linked to a central computer over a high-capacity data network.

As many office workers know, such networks are still fairly unreliable. At 35,000 feet, the vibration, heat and lack of humidity in an aircraft make the environment even more hostile to electronic equipment of all types. The personal video systems take added abuse from spilled drinks and outright vandalism.

"Why are the airlines crazy enough to invest in technology that is never 100 percent reliable and extremely expensive?" asked Karl Laasner, the head of in-flight entertainment at Swissair, one of the few airlines that now feature systems with video on demand. "If you want the image of being the world's best," he added, "you have to keep up."