Apr. 15 - An aircraft on approach to Newark Liberty International Airport tried this week to land on a runway where a plane was waiting to take off, marking the second such incident in little more than a month.

The planes in both episodes set off a radar warning system that alerted air traffic controllers and allowed them to reroute the incoming planes. Both occurred on days when the airport was socked in by cloudy, rainy weather.

"We never saw him," Newark air traffic controller Russ Halleran said of the most recent incident, which took place Tuesday. "We visually never saw him. The equipment worked. Thank God we had it."

On Tuesday morning, bad weather had lowered the ceiling to about 400 feet and visibility was about one mile. Around 7 a.m., Air Transport International Flight 816 dropped through the clouds and lined up for a landing on Runway 4-left.

"I was standing behind the controller working and all of a sudden the ... zero alert blows off; it says runway 4-left is occupied," Halleran said. "The pilot was basically chasing for the wrong runway. He was definitely going to land on 4-left if we didn't get him to come around."

And that could have been potentially deadly for the passengers on a Continental Boeing 737 that was sitting on the runway waiting to take off.

Instead, controllers had the Air Transport International DC-8 circle and land on another runway.

Halleran said the planes were within a quarter-mile of each other and about 200 feet from each other vertically. Federal Aviation Administration officials said the planes did not come close enough to warrant classifying the incident as an "incursion," in which planes come too close to planes or vehicles on the runway.

Newark had three incursions in 2003 and two in 2002, according to FAA figures. Nationwide, incursions dropped to 324 in 2003 from 339 in 2002.

However, the National Transportation Safety Board has kept the runway incursions on its "most wanted" safety improvement list, saying the numbers nationwide are still too high.

The FAA has worked to modernize its equipment to provide a greater measure of runway safety, but controllers say proposed cuts in the agency's budget could hurt that progress.

"Our funds are going down, and we still have equipment like this we need to put on line," Halleran said. "Not having money for new controllers, staffing or equipment, it's an ugly scenario."

On days such as Tuesday, when bad weather has enveloped the airport, controllers lose the ability to visually spot planes from their 335-foot-high perch in the control tower. Instead, they rely on a bank of radar systems to tell them what is coming and what is going.

The system that alerted controllers to Tuesday's situation is called AMASS, or Airport Movement Area Safety System.

The system works by taking data from the airport's various radar systems and using it to determine potential conflicts based on the position, velocity, and acceleration of aircraft in the sky and on the ground.

"AMASS was installed to enhance safety, particularly in weather like we had [Tuesday]," said FAA spokeswoman Arlene Salac. "It's a safety enhancement. Nothing can replace the job the controllers do, but it's a tool that enhances the level of safety."

The system also proved its mettle on March 6 at Newark Liberty, when it helped controllers reroute an inbound cargo plane that also was lined up with the incorrect runway where a British Airways plane stood waiting to take off.

The FAA began deploying the AMASS system in 2001, and it is now at Newark Liberty, Kennedy International, and La Guardia airports. However, not all airports nationwide have the system, Salac said.