What happened
At 1710:53 EST, the local tower controller at Newark International Airport cleared Continental Flight 321, a DC-9-82 with registration N806NY, for takeoff on runway 22L. Eleven seconds later, the crew of Continental Flight 688, a Boeing 727 with registration N577PE, reported being on the left downwind leg for runway 29. The controller instructed Flight 688 to maintain an altitude of 1,500 feet while crossing over the airport field.
Runway 22L, which was 9,300 feet long, is located south of runway 29. Consequently, the left downwind position for runway 29 placed Flight 688 near the departure end of runway 22L. At 1712:12, the controller informed Flight 321 that Flight 688 was passing overhead and behind them at an altitude of 1,800 feet.
At approximately the same time, the crew of Flight 321 noted they were climbing through 1,000 feet when they visually identified Flight 688 passing directly overhead. The captain of Flight 688 observed Flight 321 taking off and estimated that separation remained in excess of 1,000 feet. However, the captain of Flight 321 expressed concern regarding the close proximity and contacted the tower after landing.
Radar data analysis revealed that the two aircraft passed with approximately 400 feet of vertical separation and 1,000 feet of lateral separation, located approximately one-quarter mile south of the airport. Both flights were operating under instrument flight rules. The controller did not advise either crew of the other traffic until moments before their flight paths crossed. The airport was experiencing heavy traffic volumes at the time.