The next best thing to flying your own cargo plane is renting someone else's. Increasingly, that's what shippers have been doing lately, fueling a boom in air cargo charters.

"There's been tremendous growth in demand for charters over the past year," said Edwin Laird, managing director of Air Cargo Management Group, a Seattle-based consulting firm. "But there's also been a big shortage of freighter aircraft available. Shippers can't get enough space on charter flights."Mr. Laird estimates that charters generate about 2 percent to 3 percent of revenue for integrated carriers most months, and as much as 5 percent to 7 percent during the late fall. He said airlines flying scheduled operations both day and night have less opportunity to operate charters than express package carriers, whose planes sit idle in the daytime.

While executives at integrated carriers are hesitant to discuss specific charter fees and revenue, major players like Emery Worldwide, Federal Express, UPS and Airborne Express - all of which maintain charter departments - cite

gains in charter business over the past year.

"The more hours we put on our airplanes, the cheaper it is to operate them," said Chuck Greene, director of charter services for Emery Worldwide, which runs about five charters a day.

Shipments range from live animals to clothes, though very rarely perishables. Emery's contracted customers include the U.S. Department of Defense's Air Mobility Command for humanitarian relief operations, as well as scheduled carriers like Swissair; on average, four narrow-body Emery aircraft are devoted solely to this service.

Like Emery, Federal Express has seen an increase in charter requests. Frederick Fink, manager of charter sales, said the improving conomy also has allowed FedEx to operate aircraft solely dedicated to charters.

Charter loads flown by FedEx include hard goods, textiles and livestock

from around the world.

"The perception of FedEx as only an express carrier for documents tends to work against us," Mr. Fink explained. "We have to counter that, because we can carry large loads."

While Mr. Fink prefers contracting long-haul international charters, FedEx also operates on-demand domestic flights on a "hit-or-miss" basis whenever feasible: If there's a hole in its schedule, FedEx is willing to fill it. But the bulk of its charter business is comprised of repeat customers, particularly in the automotive industry.

It is a paradox that the influx of charters comes as the integrated carriers have become busier as well.

"We've been growing at 20 percent annually for the last two years, so there's very little downtime. We don't have extra aircraft sitting around," said Ken Shapero, a UPS spokesman.

Even so, UPS expects to double the average number of charter flights it performed in 1994: from about once a fortnight to once a week. One of the most common charters carries perishables, like cherries from the United States to Japan.

Mr. Shapero said UPS has improved utilization of its crews, allowing it to run charters weekdays as well as weekends, schedule permitting.

Achieving better utilization of both airplanes and crews has become vital to all air carriers, integrated and otherwise. Mike Kuli, vice president of business development for Airborne Express, said the same drive to increase productivity among passenger airlines has carried over to cargo carriers.

"We've got a lot of money in our assets, and it's better to utilize them than not utilize them," he said.

Although Airborne also relies on repeat customers, Mr. Kuli's staff still makes sales calls. It has tapped into the lucrative auto parts market with charters for General Motors and Ford, and operates military missions on behalf of the U.S. Air Force.

Traditionally, the charter issue reverses itself at year-end, when rising demand forces the major carriers to contract charters of their own. But on the whole, the integrated carriers have become less dependent upon seeking out operators to perform charters for them during the holidays. UPS, for example, is "weaning" itself of that need by exercising aircraft options and refitting older planes, Mr. Shapero said.

Emery's Mr. Greene said that his firm uses the same group of charter operators every year, including Arrow Air, Air Transport International, Kitty Hawk and Southern Air Transport; such carriers sometimes call upon Emery for a subservice (a one-off charter) or wet-lease (an ongoing, regular charter).

"If people support us throughout the year, then I support them in the fourth quarter," he said.