A developing country wants to buy new aircraft but has found itself in a quandary.

The planes are needed to hop between islands within the country, so fares will be charged in local currency - meaning no new foreign exchange and thus no hard cash to buy the aircraft.Furthermore, because the aircraft will not be flying internationally, banks are reluctant to finance the deal, seeing no chance of seizing assets should the airline fail to pay.

Enter Alcan Financial Research, a London-based company engaged in countertrade business. Alcan has drawn up a plan whereby the country would realize the necessary money through the export earnings of a cash crop.

It also has rounded up a leasing agent willing to buy the aircraft from the manufacturer and lease the planes to the airline, a local banker ready to guarantee payments to the leasing agent and finance the domestic exporter, a buyer for the crop, and an international bank to handle the necessary external financing arrangements.

The deal has not yet gone through, but Alcan officials are confident they can produce an agreement. In any case, Alcan has performed what it was hired to do: find deals through the rising use of countertrade when normal channels won't work.

Straight barter deals became popular with many cash-strapped countries after World War II, but the need to find alternative methods of payment has escalated dramatically since the start of the Third World debt crisis in 1982.

Alcan Financial Research, a two-man operation tucked inside the French commodities group Safic Alcan, is an example of the kind of shops setting up countertrade deals today.

At any one time, Alcan Financial Research may have between $200 million and $300 million worth of potential business under discussion - but only about 10 percent of that is likely to result in an actual transaction.

You need to be more of a choreographer than a businessman, said Timothy Lewin, Alcan's managing director.

The linked arrangements may take as long as a couple of years to put together.

Interviewed earlier this week, Mr. Lewin said as much as 50 percent of world trade probably is now done on a restrictive basis, with the parties making commitments beyond a simple cash-for-goods deal.

He predicted the volume will keep growing as participants become more familiar with the scope of countertrade transactions and as intermediaries find themselves able to construct increasingly sophisticated deals.

Many of the world's biggest corporations - including such names as Boeing, General Electric and Rolls Royce - undertake countertrade, or offset, arrangements whereby they will agree to purchase goods manufactured in the buyer's country.

And it's not only developing countries that insist on some kind of non- cash compensation. When Boeing won a contract to sell early-warning defense aircraft to Britain two years ago, the company agreed to spend 130 percent of the contract value on British products.

More generally, though, such trading arrangements involve various developing or Eastern bloc countries that do not always have ready access to convertible currencies. In one such transaction arranged by Alcan Financial Research, a country importing merchandise from a West European company agreed to pay for the goods by providing cargo space on the national fleet's vessels.

Mr. Lewin and his colleague Terence Hill, Alcan Financial Research's executive vice president, first became involved in countertrade activities when they worked together in the commodity markets nearly 20 years ago. Initially dealing in London's commodity futures markets, the two men gradually started to trade actual loads of cocoa, coffee and similar commodities, building a worldwide network of business contacts.

As some of their major suppliers and customers found themselves in

financial difficulties in the early 1980s, Mr. Lewin and Mr. Hill recognized that in many cases international transactions could only be undertaken by negotiating some sort of non-cash payment.

It was also self-defense, Mr. Lewin said. By arranging an off-balance- sheet transaction for a hard-up Third World exporter, they hoped to prevent the seller from dumping the product on the world market at rock-bottom prices.

A year ago, the two men took the plunge and set up their own countertrade company, Financial Research Associates. They have since joined forces with Alcan, gaining access to Alcan's considerable resources and 400 employees worldwide.

Although many trading houses, banks and commodity houses have established countertrade departments, the majority are only involved on an ad hoc basis and relatively few have made a real commitment to this type of business, Mr. Lewin said.

Despite efforts to liberalize international trading procedures, he said, circumstances will ensure continued growth in countertrade and related transactions. He also reckoned that commodity houses are best placed to coordinate this type of business because of their depth of experience trading with Third World countries, knowledge of local market conditions and ability to find buyers.