Although barred from buying U.S. aircraft because of support for terrorists. Libya has quietly obtained at least one and possibly more used Lockheed L100 transport planes through cash-and-carry purchases from shadowy brokers who apparently violated U.S. export controls, American officials say.

L100s, a civilian version of the workhorse, are a valuable addition to any air force interested in long-range troop and equipment airlift capability. But U.S. officials say Libya already has acquired other modern military transport aircraft from the Soviet Union and Italy and is buying the Americans planes more out of spite than need.

"I think that for the Libyans it's more a symbolic thing than anything else," said one U.S. official. "The Libyans have been trying every back door they can find."

An American used-aircraft dealer said he was recently outbid on one L100 by a Luxembourg-registered company acting on behalf of the Libyans and dealing with quick cash. Asked why the Libyans' purchasing agents were able to obtain the plane when he, too, was willing to pay cash, he replied:

"I have a felling [their deal] was cash cash. You know, the kind in a black bag."

Libya has been trying for some time -- and in various ways -- to obtain delivery of eight C130s purchased in the United States but blocked on the runway by the Carter administration because of a U.S. government finding that Col. Muammar Qaddafi's government lends support to terrorist organizations.

James Day, a former Texas legislator, pleaded guilty two months ago to a charge he misled Libya into believing he could use influence with Carter officials to obtain delivery of the aircraft. He plea capped a year's investigations by a federal grand jury that also included testimony from Billy Carter, former president Jimmy Carter's brother linked to Libya through friendship trips and a loan.

The Libyans, meanwhile, also have been trying for more than a year to obtain C130s or L180s by purchasing them abroad from foreign owners to get around the American restrictions.

U.S. law in principle requires that any sale of a U.S.-made aircraft -- even in a foreign country by a foreign owner -- be approved just as the original export was. But, is knowedgeable official explained, the only way the United States can really enforce that law is to hold up export requests for spare parts or other airplanes.

For large companies certain to want to do business with U.S. aircraft manufacturing firms again, this is a fairly powerful enforcement tool, he said, but not for fly-by-night brokerage agencies sometimes formed and disbanded for one deal.

For instance, on L100 that U.S. officials are nearly certain has ended up in the Libyan Air Force was used rented to Liby by an American company on a "wet lease" -- with a pilot included in the deal -- and then ferried to Tripoli and left there in what turned out in substance to be a purchase. When U.S. officials tried to track down the company involved, they found it was a storefront in Miami registered in Delaware at a "mail drop" address with a number of other firms.

So far, there has been no official determination whether U.S. law was formally violated in the case, in part because of a lack of investigators. In addition, some countries view the U.S. re-export restrictions as an infringement on their own sovereignty and refuse to help enforce them.

Because Qaddafi's Air Force already has more than adequate airlift ability, including recent Soviet-made I176s and Italian-made G222s, U.S. officials are not overly worried by his acquisition of the American aircraft. This despite Libyan intervention in Chad and fears that Qaddafi may plan more intervention in other Moslem African nations. At the same time, they are irritated because Libya has managed to flout U.S. export law even while Washington is making a principle of withholding delivery of the C130s already purchased here.

Libya had bought eight other C130s before Qaddafi took over in a military coup in 1969, and they are part of the Libyan Air Force. But one was lost at Entebbe, Uganda, in 1979 when Qaddafi sent Libyan troops to help the then-falling Ugandan dictator, Idi Amin. Ironically, the United States has received reports that an L100 believed delivered about six months ago under the table was painted in the colors of the Libyan Air Force -- with the same numbers as those of the aircraft lost at Entebbe. a