Two Marine aviators were arraigned Monday on manslaughter and negligent homicide charges, exactly six months after their jet clipped a ski lift cable in the Italian Alps, killing 20 people.

Capt. Richard J. Ashby, the pilot of the EA-6B Prowler, and Capt. Joseph P. Schweitzer, the navigator, did not enter pleas in this first step of their courts-martial.They have until the start of their trials to enter pleas. Ashby's trial is tentatively scheduled for December, and Schweitzer's for January.

The two men have denied that they were flying the Prowler recklessly on Feb. 3, and their lawyers said maps did not show the cable, strung 370 feet above the ground. But prosecutors said they were flying too fast and well below altitude limits when their jet hit the cable, dropping a crowded ski lift gondola to the ground.

Ashby, 30, of Mission Viejo, Calif., and Schweitzer, 30, of Westbury, N.Y., face 20 counts of involuntary manslaughter and 20 counts of negligent homicide. They also are charged with damage to military and civilian property and dereliction of duty.

If convicted of all the charges against them, they could spent the rest of their lives in prison.

The government has said its witnesses will include experts on data recordings, aircraft movements and optical illusions, as well as psychologists.

The next hearing is scheduled for Sept. 8. After that, lawyers from both sides plan to make a 10-day trip to Italy.