A U.S. destroyer aimed its weapons at a helicopter chartered by a television news crew after two warnings to keep clear apparently were lost in the din of Persian Gulf radio traffic. The incident occurred Wednesday, three days after the American cruiser Vincennes shot down an Iran Air Airbus A300, killing all 290 people aboard. The exchange by radio between the destroyer John Hancock and the pilot of the helicopter team was monitored by this reporter with another airborne press group. The United States says the Vincennes, which had just finished a skirmish with Iranian gunboats, mistook the jetliner for an Iranian F-14 fighter. On Wednesday, the John Hancock had a routine radio exchange with an Iranian jet fighter but did not contact an Iran Air passenger jet flying a few miles away. An Iran Air spokesman confirmed the arrival in Dubai of Flight 657 on a scheduled flight late Wednesday morning from Shiraz and said it returned to the central Iranian city after refueling and taking on passengers. It flew within several miles of the John Hancock, which was off the United Arab Emirates and in radio contact with many commercial aircraft.