Five Arab defendants pleaded innocent to charges of hijacking and murder in the 1986 seizure of a Pan Am jumbo jet at a Pakistani airport in which 21 people were killed and nearly 200 wounded. It was the first time they had appeared together before a one-man tribunal set up inside Adiyala district prison near Islamabad. The five--three Lebanese, a Syrian and a Libyan--could all hang if convicted. Gunmen wearing Pakistani security uniforms seized the plane with about 400 people on board on Sept. 5, 1986, at Karachi. After 17 hours, the hijacking ended in a shoot-out.