The San Diego-based Cubic Defense Systems, in the first major action in months in the Ill Wind investigation, pleaded guilty in federal court yesterday to conspiring to bribe an Air Force official to illegally help the company compete for a half-billion dollars in defense contracts.

Guilty pleas entered in Alexandria by CDS and its former chief executive allege that Victor D. Cohen, a special assistant to the assistant secretary of the Air Force for acquisitions, provided inside information and influence that temporarily helped CDS in its failed 1987 bid for a $433 million air reconnaissance system.

The pleas were the first significant development in the three-year Ill Wind investigation into Pentagon procurement fraud since last March when Washington consultant William M. Galvin agreed to a four-count criminal information that also implicated Cohen in influence peddling schemes.

Court documents filed with yesterday's pleas allege that Cohen was bribed with meals, favors and gifts to provide CDS with inside information on defense contracts.

Julian Greenspun, Cohen's attorney, said yesterday that his client "maintains he's innocent of bribery and never agreed to accept bribes from anyone."

CDS, the fifth corporation prosecuted in Ill Wind, agreed to pay $4.65 million in criminal and civil fines for convictions of conspiracy, conversion of government property and filing false claims with the government.

Colvin C. Wellborn, 58, former president of CDS and the 37th individual convicted in Ill Wind, pleaded guilty to conspiracy and conversion of government property and faces a maximum possible prison term of 15 years.