Sen. Strom Thurmond (R-S.C.), conceding he violated Senate ethics rules in accepting free transportation from Beech Aircraft Corp., says he has sent a check for $5,669 to reimburse the company.

In documents filed this week with the Senate, Thurmond said, " . . . it appears from information recently brought to my attention that I unknowingly accepted and allowed members of my family to accept gifts of transportation from a source" prohibited by Senate ethics rules.

"I regret that these errors occurred due to a lack of complete and accurate information available to me at the time," said Thurmond.

In a letter to Beech, Thurmond said he originally "was under the impression that Page Avjet was the provider of these planes and pilots." He said Page Avjet, the Washington distributor for Beech, was not a prohibited source because it did not have a registered lobbyist or a political action committee.

Thurmond said he had since learned that Beech, a defense contractor with a political action committee, paid Page Avjet for the flights. Between 1982 and 1984, Thurmond said he and his family accepted 18 free trips from Page Avjet and that he and his family have also flown free this year. He reimbursed Beech at commercial rates.

Last month, the House Ethics Committee voted to conduct a preliminary inquiry into the acceptance by Rep. Dan Daniel (D-Va.) of free airplane rides from Beech. Daniel said he had misunderstood House rules and has reimbursed Beech for 23 free flights.

In recent years, Daniel and Thurmond have supported the purchase of Beech planes by the Pentagon. Daniel took a leading role this year in urging Congress to authorize the Pentagon to buy $56.9 million worth of Beech C-12 aircraft.

Both Daniel and Thurmond have said their support of Beech was in no way connected to the free rides.