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With the change, which is being made mainly to save airlines billions in fuel costs, U.S. safety officials offer passengers some advice: Relax. The 1,000-foot standard has been in place elsewhere in the world for years without incident. In addition, safety equipment aboard planes has been upgraded.

"For the flying public, this change will be largely invisible," says Federal Aviation Administration spokesman William Shumann.

Nonetheless, the notion of planes routinely passing within half the vertical distance that air traffic controllers now allow rattles some high-mileage travelers and an air safety advocate.

John Foor, an engineer from Houston who flew about 125 times last year, says it increases the risk of a midair collision. "It cuts into the safety margin, regardless of the history of no incidents."

Though the FAA says the change probably won't be noticeable, air traffic controller Steve Entis says passengers looking out the windows of a jet flying at cruise altitude might see more planes than in the past, or at least see them closer.

"Don't worry," says Entis, a member of the National Air Traffic Controllers Association union who is working with the FAA on its plan. "You are safe, and the planes are separated properly."

Airlines have pushed for the change worldwide for decades. It creates more flight paths, allowing a plane to get more quickly into high-altitude airspace. At high altitudes, planes can take advantage of more favorable winds, and their engines burn fuel more efficiently.

The change will do little to reduce flight delays, the FAA and other aviation experts say. Most delays, they say, are caused by a large volume of flights taking off and landing on a limited number of airport runways.

No reason to worry

At 4:01 a.m. ET next Thursday, the required vertical separation between planes flying in a range from 29,000 to 41,000 feet will decrease to 1,000 feet from the current 2,000. The required horizontal separation will remain unchanged at 5¾ miles.

The new standard, known in the industry as RVSM, for reduced vertical separation minimum, is scheduled for the same date and time in the airspace above southern Canada, the Caribbean, Mexico and South America. It's already in effect in Europe, Australia, parts of Asia and across the Atlantic and Pacific oceans. Implementation is planned in Japan and South Korea later in 2005. Africa, Russia and parts of China will implement the new standard later.

Airliners and corporate jets routinely fly at altitudes of 29,000 feet and above. They operate at lower altitudes on short-distance flights and when there's heavy congestion above.

The FAA says there's no reason to worry about 1,000 feet of separation at high altitudes, because that amount of separation has been used below an altitude of 29,000 feet for decades. And abroad, the FAA adds, there have been no safety-related incidents caused by reduced vertical separation at high altitudes since it was introduced over the Atlantic in 1997 and Europe in 2002.

John Hansman, a professor of aeronautics and astronautics at the Massachusetts Institute of Technology, calls the switch to reduced vertical separation "a technical no-brainer." It's an "absolutely safe" move "if done correctly," he says. Hansman, the chairman of an FAA research advisory committee, worked with the agency on the program years ago but is no longer involved.

But safety advocate Paul Hudson, the executive director of the non-profit Aviation Consumer Action Project, says the move "reduces the margin for error." Equipment can still malfunction, and human error can occur, says Hudson, a member of the FAA's safety advisory executive committee.

The non-profit Flight Safety Foundation, which works with the aviation industry to improve safety, says planes that deviate from an assigned altitude present "greater risks" of a midair collision with 1,000 feet, instead of 2,000 feet, of separation. Yet, the group maintains that flying in airspace with reduced separation will be safe, because planes are required to be equipped with special equipment, and because air traffic controller training has increased.

In the 1950s, the government first established vertical separation of 2,000 feet for planes flying above an altitude of 29,000 feet. The FAA set the standard after technological advances gave an increasing number of planes the ability to fly at high altitudes that had previously been the domain of a small number of military aircraft.

For the airline's dominant trade group, the Air Transport Association, next week's change is the culmination of an effort begun more than 30 years ago.

Citing rising fuel costs and traffic volume, ATA unsuccessfully petitioned the FAA in 1973 to reduce the separation to 1,000 feet. The FAA said then that reduced vertical separation would adversely affect safety. Aircraft altimeters, which measure altitude, were not precise enough, and altitude correction systems weren't available in all planes.

But in the 1980s, the FAA began working with domestic and international aviation organizations to develop policies and standards aimed at implementing reduced separation. Technological advances also improved altimeters and related safety equipment.

The International Civil Aviation Organization, which represents governments' aviation authorities, concluded in 1987 that reduced separation was "technically feasible." Four years later, countries' air traffic control organizations, the world's airlines and an international pilots union agreed to move toward implementation.

Reduced vertical separation was introduced first in less-congested airspace: some routes over the Atlantic Ocean in 1997 and 1998 and over the Pacific Ocean in 2000.

Frequent fliers divided

Kathy Brousseau of Lexington, S.C., is another high-mileage business traveler with some safety concerns.

"There needs to be additional air traffic controllers to handle smaller separations, and the air traffic control system needs technology upgrades," says Brousseau, a warehouse consultant who took more than 230 flights last year.

Some other frequent fliers, however, express confidence.

Pat Reynolds, an auto industry consultant and a private pilot who flew on more than 200 airline flights last year, says he's unconcerned. "I'm glad it is finally happening."

Nancy Ilk, who flew on about 170 flights last year for a Georgia-based health care company, says she trusts "the expertise of the FAA and air traffic controllers."

The Air Line Pilots Association, which represents 64,000 commercial pilots, supports the reduced separation. "We consider the program successfully run by the FAA," says Larry Newman, chairman of ALPA's air traffic services group. "We have no outstanding issues."

To fly in airspace with reduced separation, the FAA requires planes to be equipped with two digital altimeters and other related avionics equipment. The equipment is essential above 29,000 feet, aviation experts say, because older planes' altimeters are less accurate at high altitudes.

About 95% of airline jets in the USA — and more than 62% of business jets — will have the required equipment in place on Jan. 20, Shumann says.

The FAA estimates it will cost $869 million to upgrade and modify planes that will operate in airspace with reduced separation. There also will be "minimal" recurrent maintenance costs, the agency says.

Modification costs from less than $100 for some aircraft to $235,000 for a small number of older planes, the FAA says.

According to the Aircraft Electronics Association, the cost of upgrading a single business jet ranges from $15,000 to $180,000, depending on an aircraft's age and the type of equipment installed.

Modification takes up to two weeks, and FAA certification can take three to six months, says Paula Derks, a spokeswoman for the AEA, which represents repair stations and avionics equipment manufacturers. Some business jet operators say it's too expensive to modify their planes.

The costs to upgrade altimeters and related equipment are expected to be more than offset by fuel savings. The FAA estimates that aircraft operators will save more than $5 billion in fuel costs through 2016, vs. the estimated $869 million cost of the required plane upgrades.

Jets without the required equipment can still fly but must do so below 29,000 feet, where altimeters perform more accurately. Some planes designed to fly above 41,000 feet are also allowed to ascend directly through airspace with reduced separation, but only when an air traffic controller's workload permits.

The FAA also will allow into that airspace some types of planes without the required equipment: military planes that can't be upgraded, jets carrying foreign government leaders, aircraft used for life-saving or humanitarian missions and new jets on manufacturers' certification flights.

Newman wonders if foreign airlines' planes — particularly those from Central and South America — may try to enter U.S. airspace without the required modifications. Some foreign governments' oversight over such maintenance work is weaker than that of the FAA, he says.

Violators can be fined, suspended

Aviation authorities of 14 countries with airlines that fly into the USA do not comply with ICAO standards. Those countries, which include Argentina, Ecuador and Greece, may lack laws, technical expertise, resources or organization to license or oversee airlines, the FAA says.

Russell Chew, the chief operating officer of the FAA's Air Traffic Organization, says the agency will rely on monitoring stations that receive radar data from a plane's transponder to detect whether it is properly equipped for flying in tighter quarters above 29,000 feet. Violators can be fined or suspended from operating future flights, Shumann says.

The narrower separation increases to 13 from seven the available flight corridors above 29,000 feet. That, says Entis of the National Air Traffic Controllers Association, gives controllers more flexibility in routing planes. It also reduces the likelihood of a controller denying a pilot entry to a requested altitude because of traffic congestion.

"We'll have more choices to direct traffic in bad weather or thunderstorms, which planes can't fly through," he says.

In September, the FAA began training its controllers in new procedures. Each controller received one to two hours of computer instruction and then completed a one-day training course. Two-hour refresher courses are now underway, Entis says. Pilots received no additional training.

Hudson, the consumer safety advocate, says the switch should be closely monitored by the FAA. If there's an increase in the number of "close calls" — planes flying too closely — the agency should re-institute the former margins, he says.

The FAA's Shumann says the agency will continually monitor the change. "There would have to be a major increase in operational errors to have us revert back," he says.