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All is quiet as air traffic controllers at Salt Lake City International Airport's tower work what is known as the "evening bank" toward the end of a recent summer day. There's no idle chatter. No goofing off. Just the silence of concentration.

For good reason. During the next hour - from 6:30 to 7:30 p.m. - the tower controllers will handle 114 takeoffs and landings, almost two flights per minute. It's a high-wire, high-tech ballet, bathed in the darkness of the basement control room and the green hues of the radar screens.

Down in their cubbyhole, the shift of eight "approach" controllers - who handle all flights within a 60-mile radius of the airport - choreographs the arrivals and departures of commercial and regional jets, smaller commuter turbo-props and single-engine planes into a cohesive, nonstop parade. Outside, the results are easily seen and heard, from the regular rumble of takeoffs to an intricately spaced line of approaching aircraft, their landing lights visible for miles.

"It takes you right to the edge, but after you've been doing it awhile, you kind of look forward to it," says Drew Crampton, a 15-year air traffic controller. "I enjoy being pushed. It's like an athlete playing against a tough competitor; it only improves your skills. At the same time, there are not a lot of options. There's not a lot of margin for error."

No, there is not. Salt Lake City International's air traffic controllers work a relatively small and unique airspace compared with their counterparts at other major U.S. airports. The towering Wasatch Mountains to the east and Oquirrh Mountains to the west essentially preclude a wider distribution of aircraft when landing to the north and approaching the airport over the west side of the Salt Lake Valley. That airspace is further constricted by rules limiting arriving aircraft to a corridor between Interstate 215 and the Oquirrh ridgeline. It is through this 6-mile-wide box that controllers must maneuver incoming planes.

When the winds shift, the approach to the south, coming from over the Great Salt Lake, provides more room for controllers to move aircraft. But even with that, when one combines all of the geographic barriers, the limited airspace and the sheer number of flights - with more than 400,000 takeoffs and landings annually, Salt Lake City International is North America's 15th busiest commercial airport - the challenges for air traffic controllers are significant. This is particularly true during peak traffic periods, and especially in bad weather, when visibility is limited and aircraft are operating on instrument (not visual) flight rules.

"The complexity issue is extreme," says Clark Desing, the Federal Aviation Administration's tower manager at Salt Lake City International. "We don't have much fudge room. Right now we're putting 1,248 flights a day through this little airspace. And when you get a lot of arrivals and departures close together, it gets very tricky."

Close calls: And occasionally, stuff happens - as FAA records reveal.

On the afternoon of Aug. 11, 2002, a departing northbound SkyWest regional jet was looped west, under the stream of arriving aircraft at 11,000 feet. When the controller turned the plane again to begin another loop through the arrival stream and start its journey east, it suddenly found itself in the path of a smaller, southbound turbo-prop, also traveling at 11,000 feet. A second controller spotted the convergence and immediately ordered the SkyWest flight to dive to 10,000. The two planes missed each other by just 400 feet vertically and a half-mile laterally - just seconds, in other words; the two controllers handling the planes had failed to properly communicate with each other.

Then there was the evening of Feb. 4, 2003. During the airport's peak traffic period, with the weather deteriorating and the tower shifting from visual to instrument flight rules, a SkyWest regional jet made a swooping turn over South Jordan to line up for a final approach to the north at 8,000 feet. Suddenly, it found itself nose-to-nose with a southbound Delta jet - which had been trailing the SkyWest flight at the same altitude but drifted toward the final approach path because of high winds. With its collision alarm sounding, the SkyWest plane was ordered to quickly climb to 9,000 feet - but not before the planes got within 400 feet vertically and two miles laterally of each other.

Before the next 90 seconds passed, two other planes in that final approach flow would violate the FAA aircraft separation standard of 3 miles laterally and 1,000 feet vertically, as the controller fell behind the traffic curve and struggled to get things back on track. The convergence of close calls still sends shivers down the backs of local FAA officials.

"It took 10 years off my life," says tower manager Desing. "I'll never forget it."

Those incidents represent four of the 11 "operational errors" Salt Lake City International's approach controllers committed between May 2002 and May 2003, according to FAA records, obtained through an open records request. But that is out of a half-million flights, and most of the errors involved just slight violations in the separation standard, which passengers, and in many cases pilots, would never notice or be aware of.

"We don't like airing our dirty laundry - nobody does," says Brady Allred, a tower approach controller and local spokesman for the National Air Traffic Controllers Association. "We're human. We make mistakes. But we're still 99.99 percent error-free, which is something we're very proud of."

Pushing the plan: Still, in what is a zero-tolerance business, the FAA and the controllers union - including those in the top of the tower who handle the on-the-ground airport traffic and those at the nearby Salt Lake Center, who guide en route flights throughout the region - are pointing to their too-close encounters and what they call an increasing number of flight delays in pushing for the controversial Northern Utah Airspace Initiative. The initiative, a large-scale redesign of the airspace around Salt Lake City International, is currently in the midst of an environmental study.

The FAA proposal calls for a "four-corner post" system, currently in use at virtually all other major U.S. airports, to route incoming flights. It also proposes a second "downwind" approach over the east side of the Salt Lake Valley, one that would complement the current west downwind near the Oquirrhs.

Proponents say the four-corner post and east downwind would allow controllers to better segregate aircraft types - which travel at different speeds - and minimize crossing over between arriving and departing flights. The east downwind also would allow them to split the incoming flow of aircraft and shorten their sequencing of the aircraft as they line them up for final approach.

The changes will "lessen delays and increase the margin of safety," says controller Brian Johnson, the Salt Lake tower's design lead for the Northern Utah Airspace Initiative. "By moving everything farther apart, it gives the controller and the pilot more time to react to situations and make last-minute changes. In our current configuration, there are very few outs. It's like taking eight lanes of traffic on I-15 and condensing them into one."

Critics - including Salt Lake City officials, the airport board, U.S. Reps. Jim Matheson and Rob Bishop, and the environmental group Save Our Canyons - are not convinced. They call the plan unacceptable, arguing that the proposed east downwind would negatively impact hundreds of thousands of east-side residents, while the southeast entry of the four-corner post would put incoming flights right over wilderness areas and ski resorts of the Cottonwood canyons.

Delta and Northwest airlines have called the current airspace configuration adequate for their needs.

Alternatives: Salt Lake City International Airport Executive Director Tim Campbell says he sympathizes with the controllers, and believes some sort of airspace redesign is needed. But he is adamant that the current proposal would create at least as many problems as it solves.

Campbell, who has hired a pair of airspace design consultants to offer alternatives, says the FAA could more easily sell the Northern Utah Airspace Initiative with some modifications - such as a far east downwind that would route planes on the other side of the Wasatch Range, or what is known as a "stacked" downwind to the west, where arriving flights are essentially put on top of each other. Or by pulling the southeast corner of the four-corner post farther south, so it would not conflict with the ski and wilderness areas.

"There's no question that, based upon what [the FAA] has told us and what our consultants have said, that the current procedures are not as efficient as they could be," said Campbell. "There is a need for some sort of modified four-corner post. We've said we would support that, and it would probably eliminate the need for an environmental study."

The consultants, HNTB, based in Arlington, Va., and HMMH, in Burlington, Mass., declined to be interviewed for this story. They are expected to release their recommendations early next year, about the same time that the FAA's draft of the environmental study is to be completed.

Desing, the tower manager, says he welcomes the input.

"If they've got a better idea, we want to hear it," he says.

Even within his own ranks, support of redesign is not unanimous. During a reporter's recent visit to the Salt Lake tower, one controller pasted a can't-miss "Save Our Canyons" logo above his work station. As a member of the controllers' union, he is prohibited from publicly opposing its support of the plan.

Still, controllers such as Drew Crampton say some sort of solution to the cramped airspace situation is badly needed within 60 miles of Salt Lake City International, which includes Hill Air Force Base, the airports at Ogden, Provo, Tooele and Salt Lake No. 2 in West Jordan.

Crampton, who returned to Utah after a stint at Phoenix's Sky Harbor International Airport, calls working the airspace here more challenging than Phoenix, even though Sky Harbor handles nearly 150,000 more annual flights than Salt Lake.

"The volume there is higher, but it's much less complex," he said. "Volume is easy; you just move the planes from point to point. The difficulty presented by the mountains and the weather here make it vastly more complicated. And at peak periods, there is a huge demand placed on the system."

In other words, that evening-bank adrenaline rush is exhilarating - but only to a point. At the end of the day, Salt Lake City International's air traffic controllers would like just a little more room to breathe when things get busy and the wind and snow start blowing.

"I know it's complicated; a lot of this goes over people's heads," says Allred. "But what we're essentially doing now is putting I-15 traffic on Redwood Road. And it's a real hindrance."