The broadcast networks are flying without a script.

Unveiling their 2004-2005 primetime schedules to Madison Ave. last week, the nets told advertisers they won’t premiere some of their best stuff in fall.

They’re eliminating a huge chunk of reruns — and spending millions more to spread original programming throughout the year; their schedules will remain in a constant state of flux, as shows replace one another at regular intervals. Also, they still believe in comedy — just not right at this moment.

And, most emphatically, they’ll not only tighten their embrace of reality programming, they’re launching the season with gobs of it.

Indeed, with so much reality on their skeds, the broadcast networks are literally flying without a script this fall.

For one thing, replacing repeats with original fare is going to add tens of millions to the nets’ programming budgets. But webheads figure they have no choice: If they don’t do something to stop audience erosion, they’ll ultimately lose far more due to declining ad revenue.

All of this comes as more bad news for TV’s beleaguered scribe tribe, particularly those who toil on sitcoms.

It’s become a cliche to bemoan the fall of the funny, but the numbers tell the story: For the upcoming, the nets have scheduled just 36 sitcoms — down nearly one-third from 50 last fall.

“Comedy is in a challenged state,” admits NBC Entertainment topper Kevin Reilly. “The best way to get comedy on the schedule right now is to keep it off in the short term.”

Combine that with a rise in reality skeins — as well as several new primetime improv laffers — and it’s a safe bet that agents all over town this week will be scrambling to find work for a slew of scribes. By one estimate, there’ll be 100 fewer staff positions available for sitcom writers.

And given the enthusiastic reception advertisers had to reality skeins such as Mark Burnett’s “The Contender,” a WGA strike seems even more of a suicide mission.

For the primetime TV biz, where change usually comes at a glacial pace (how long did it take the nets to wake up and compete with cable?), last week looked like a revolution.

Sure, TV’s Chicken Littles have been forecasting the end of the TV season as we know it for eons. But it took the explosive popularity of reality for execs to realize there are other ways to program.

The midseason dominance of shows like “American Idol” and “The Apprentice” reminded nets that it’s not when you launch a show, it’s what you launch.

“It’s an entirely new way of doing business: 52 weeks of original programming,” NBC Universal TV topper Jeff Zucker says. “It reflects a sharing of time periods, pairing up shows to minimize repeats. Several of our biggest projects will hit well into the season.”

Fox took the most aggressive stance, announcing not one, but three schedules for next season, timed around the net’s two tentpoles, Major League Baseball playoffs/the World Series and”American Idol.”

With baseball disrupting Fox’s fall, and “Idol” completely altering its lineup in January, the net hasn’t had a cohesive schedule in years.

And the successful launch of “The OC” last summer as well as the popularity of short-order reality skeins like “My Big Fat Obnoxious Fiance,” showed the net that viewers are up for sampling new shows throughout the year, not just in September and March.

“This is no longer an experiment,” says Fox Entertainment prexy Gail Berman. “Fox is altering the way this industry does business. As a way to prevent network erosion, it was an inevitability. We saw that coming, which is why we got on this plan.”

But Fox wasn’t the only network banging the “52-week schedule” drum last week. ABC, NBC and the WB also spelled out plans to cut down on repeats and stick with more originals by swapping out time periods throughout the year.

ABC, for example, will premiere Sunday night drama “Desperate Housewives” in the fall, before switching that show out for the return of “Alias” in midseason.

At the Frog, net says it might hold back “Everwood” until November, then air 10 original segs in a row before resting it with a midseason show like “Global Frequency” or “Rocky Point.”

“Audiences are getting trained to watch TV differently,” says WB CEO Jordan Levin. “Young audiences didn’t grow up with a summer of repeats. What the industry has to get over is having to get outside their heads the way they perceive that a logical schedule should work. You’ve got to try very hard to throw out these rules. If you don’t adapt, you’ll die.”

Not everyone is convinced. CBS CEO Leslie Moonves dismisses much of the year-round spin as hype, pointing out that it’s impossible to predict what your needs might be in January, a full eight months from now.

“Things change, and to announce what’s going in a slot after something else, you don’t know,” he says. “We try to play it looser than that.”

Moonves also says he doesn’t think the economics make sense to schedule original episodes of series all year long– although he admits Fox has a different problem due to baseball.

And, referring to NBC, he adds, “When someone orders five new shows and seven backups, it means someone doesn’t have the confidence in the shows they’re putting on.”.

The decision to launch fewer shows in the fall, in addition to picking up more short-order series, has also had an impact on the studios, which say they now must find new ways to streamline their operations.

For Warner Bros. TV topper Peter Roth, who heads up the industry’s No. 1 primetime supplier, that may mean making fewer pilots next season.

“We’re going to begin to pull back and be more targeted and more calculated,” he says.

But Roth says it’s about time the webs went away from their rigid launch patterns.

“If back in 1980 we had done as some suggested and programmed more year round, and worried less on immediate investment, then perhaps the business would not be in as challenged a state as it is right now,” he says.

Year-round proponents at the networks say they’re amortizing costs via same-week repeats in some cases, and by announcing shows now in order to maximize CPMs (cost per thousand) during upfront sales.

But they’re also able to go heavy on originals thanks to the never-ending pipeline of unscripted fare. This was the year the nets had to eliminate any lingering prejudices they had against scheduling reality shows.

For this coming fall, the networks slated a whopping 22 reality skeins — up from 10 in 2003.

That was an especially big step for execs trained in the world of scripted TV, who have learned to adjust to the new reality of reality. Skedding five unscripted series for fall, Steve McPherson says he didn’t discriminate by genre. If the show’s good, it’s in.

“I don’t separate out shows in terms of reality, drama or comedy,” he says. “(I care about) what shows people will get passionate about. Great reality shows have great storytelling, just like great drama. That’s what gets me excited.”

It’s also been a wake-up of sorts for Levin, whose network hadn’t aggressively stepped up to the reality plate until recently.

“I will not make that same mistake again,” Levin adds. “Reality is a reality for our core demo.”

Part of the problem came via mixed messages from the media-buying world, where reality TV has in the past been considered a second-class citizen.

“We heard last year that advertisers did not want reality,” he says. “We listened to what we thought the advertisers wanted. But then reality scored, and advertisers flocked to them in a big way. It’s definitely a genre you have to take a lot of at bats with. It’s sort of like the Wild West.”

Levin says the Frog’s earlier decision to stick with more “positive and life-affirming” series (“Pop Stars”) also limited the net’s success.

“It was naive on our part, given the competition,” he says. “No doubt the shows in reality with edge and underlying tension perform better than shows that are brighter in spirit.”

Nets went so hog wild on reality, they even scheduled shows in the fall that had originally been targeted for summer (“The Benefactor,” for instance).

But that same kind of enthusiasm couldn’t be mustered for the comedy genre this year.

The comedy crisis was particularly pronounced at NBC, where the net boasted 18 sitcoms in 1997. This fall? Four.

NBC Universal TV Studios co-prexy David Kissinger also takes a glass-half-full view of the comedy conundrum, noting that fewer sitcoms means more comedy scribes are available to perhaps create the next big thing.

“I remain, idiotically perhaps, optimistic about the genre,” Kissinger says. “It is so overdue for a rebirth. There’s probably more A-list comedy talent available to develop in the coming year than ever before. Hopefully something extraordinary will come out of that.”

Josef Adalian contributed to this report.