Air cargo charters can indeed rescue those in need. They're also expensive, but shippers who shop wisely can cut costs.

''Charters are considered premium transportation, and no one wants to use a charter unless it's absolutely necessary,'' said John Churchill, vice president of sales and marketing for Reliant Airlines Inc., a charter operator based in Ypsilanti, Mich. When machinery goes down at a plant, automakers can suffer millions of dollars in lost production. Hence they are prime customers for Reliant and other charter carriers.

A charter may also be the only option when heavyweight equipment must be moved to a Third World country. Dangerous goods and high-value goods may fly charter as a matter of course.

Charters command some of the globe's stiffest transportation rates, whether for intra-U.S. hops or long-haul international moves. A ballpark estimate for the cost of a charter is double the cost of scheduled service.

''If an average rate is $1 per kilo on scheduled service, on an emergency charter service, the rate may be $2 per kilo,'' said Harry Steiner, vice president of Chapman Freeborn America Inc., a charter broker in Atlanta.

Charters are usually contracted by the mile or hour. Depending on the route and/or the aircraft involved, these per-unit costs can vary widely. On the one hand, the carrier may have to make a long flight just to pick up the goods, thus raising costs for the customer. On the other hand, the customer can enjoy big savings if the carrier can avoid empty flight segments. The same goes for backhauls.

A good charter company will poll its peers for additional cargo when space permits or to fill an empty segment. A case in point was provided by a multinational high-tech firm that needed a charter last year for an urgent shipment from San Jose, Calif., to Amsterdam.

The company had enough freight to fill three quarters of a planeload. It contacted an all-cargo carrier's charter division directly, paying upwards of $150,000 for the round trip. But if it had contacted a reputable forwarder or charter company, it may have shaved its costs in half.

Knowing who to call and when is critical in finding a charter. There's a range of players: air carriers of all types, forwarders, professional charter management companies and brokers.

The players vying for charter loads are often chasing the same business. Word travels fast among airlines, express carriers, charter management companies, brokers and forwarders.

The luxury of time is unusual. ''The average lead time to arrange for a charter is often six to eight hours,'' Mr. Steiner said. Nevertheless, he advises those in the market for charters to give as much advance notice as possible.

''We can be more creative with more advance notice and save money for the client,'' he said. Charter companies can then consolidate freight movements and fill backhauls that might otherwise go empty.

As a dedicated charter airline, Reliant typically flies for express carriers and forwarders, as well as shippers shopping direct. Those include the Big Three automakers, which have their own charter managers.

''Automakers call us and say 'now', '' said Mr. Churchill, adding, ''We offer them wheels up within 45 minutes from their call.''

Mr. Churchill advises shippers to have all the shipment details ready when they call: size, weight and the time when the shipment will be ready.

Reliant's fleet of 13 Falcon 20 small jet freighters rescues production lines by shipping emergency replacement parts. They also move explosives and live animals. Sometimes they even carry packages for the likes of Federal Express, United Parcel Service and DHL Worldwide Express. If its fleet isn't available or appropriate, Reliant might broker the shipment to another carrier or refer the customer elsewhere.

Charter brokers such as Chapman Freeborn are specialists in accessing capacity worldwide and coordinating payloads. Chapman Freeborn works exclusively with forwarders and taps into all types of aircraft, from small domestic jets to Antonov heavy freighters for long international flights.

The moral of the story? Consider a company with expertise in chartering if your forwarder doesn't have the experience. Direct shippers should be discreet when shopping for a charter even if time isn't on your side. Have shipment requirements readily at hand.

Charters continue to make up a growing portion of the air cargo pie. This year, charters will constitute more than 14 percent of all the air cargo capacity between the United States and international destinations, an increase from just under 13 percent last year, according to MergeGlobal Inc., a transportation consulting firm in Arlington, Va.