By Allan Chernoff
CNN Sr. Correspondent

(CNN) - Federal investigators are concerned a potential danger persists because of the simultaneous use of intersecting runways at Newark Liberty International Airport, one of the nation's busiest and a gateway to the New York metro area.

[cnn-photo-caption image= http://i2.cdn.turner.com/cnn/2009/images/11/09/faa.gi.art.jpg caption="The FAA says it intends to have new technology fully operational at Newark by mid-December."]

The alert comes after repeated instances in which planes above the Newark airport flew too close to each other in violation of safety standards. There were four such instances last year and at least four this year, according to the U.S. Department of Transportation inspector general.

In one case, on January 16, 2008, two Continental planes - a Boeing B-737 and an Embraer 145 - missed each other by 600 feet, according to a DOT inspector general's report.

"That was very scary. I was there for that one personally in the control tower, and it scared the heck out of everybody up there," said Ray Adams, a Newark air traffic controller.

Potential danger arises when approaching planes need to abort their landings, which happens about every 700 flights at Newark, according to a Federal Aviation Administration analysis.

In what the FAA calls "go-arounds," the diverted plane approaching Newark has to make a sharp right turn through the flight path of planes landing and taking off from an intersecting runway, allowing little margin for error.

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