WASHINGTON (AP) _ Midshipmen involved in the Naval Academy's biggest cheating scandal filed lawsuits in federal and military courts Thursday, seeking to stop the Navy from holding hearings to expel them.

One suit was filed in the U.S. District Court for the District of Columbia by 48 midshipmen accused of cheating on a 1992 engineering exam. It seeks to block a board hearing that would consider dismissal from the academy.

The other suit, filed in the U.S. Court of Military Appeals by three midshipmen, alleges that investigators forced confessions from the midshipmen with threats, screaming, and, in one case, refusing to allow a scheduled cancer treatment.

The Navy has said that 133 midshipmen, all from the 663-member class scheduled to graduate in May, may have had an advance look at the test and that 81 had confessed to cheating. The Naval Academy's superintendent, Rear Adm. Thomas C. Lynch, has already recommended that six be expelled.

Both actions name as defendants Navy Secretary John Dalton; the chief of naval operations, Adm. Frank Kelso; and Rear Adm. Richard Allen, who conducted an honor review board. The federal suit also names Lynch.

The federal court suit asks for a temporary restraining order against an honor review board, described as ''a procedure without statutory or regulatory warrant.'' The Navy is trying to substitute that board for a hearing process that otherwise would consider dismissal, the suit said.

The action alleges ''serious violations of the rights of the plaintiffs that threatens irreparable injury to their reputations and careers.''

The other suit asks the country's highest military court to direct the Navy to stop all proceedings in the case and to forbid the Navy from using ''any coerced statements gained from the unlawful interrogations.''

''From the outset, agents of the inspector general's office did not advise midshipmen of their right to remain silent,'' the suit said. It added that agents ''used a variety of coercive tactics'' that included screaming at the midshipmen, challenging them physically, refusing requests for attorneys and threatening to tell midshipmen's families of possible misconduct.

The suit claimed that Mike J. Steve, one of the petitioners, was initially refused to attend cancer treatment. The other two bringing the suit are Fred J. Regan and Dante J. Marzetta.

Calling the interrogations ''draconian,'' the suit said Steve and Marzetta ''made statement implicating themselves and others.''