Two news helicopters covering a police chase on live television collided and crashed to the ground today, killing all four people on board in a plunge that viewers saw as a jumble of spinning, broken images. Both helicopters went down in a park in central Phoenix and caught fire. No one on the ground was hurt. TV viewers did not actually witness the accident because cameras aboard both aircraft were pointed at the ground. But they saw video from one of the helicopters break up and begin to spin before the station abruptly switched to the studio.