ATM Modernization, Business & GA, Commercial

Product Focus: Taking the Pulse of Domestic RVSM

By Charlotte Adams | April 1, 2006
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Introduced by FAA in January 2005, domestic reduced vertical separation minimum (DRVSM) added six flight levels (FLs) to the high-altitude structure of the National Airspace System (NAS): FL 300, 320, 340, 360, 380 and 400. Aircraft flying in the upper airspace above the 48 contiguous states, Alaska, southern Canada and part of the Gulf of Mexico now maintain 1,000-foot vertical separation, half the distance required before DRVSM rules went into effect.

With this change, the United States caught up with Europe, Southeast Asia, Australia and northern Canada, as well as Pacific and Atlantic Ocean regions, where RVSM rules already were in effect. (Because their airspaces are necessarily linked, southern Canada had to wait until the United States adopted RVSM before it could do likewise.)

U.S.-registered carriers operating in international RVSM airspace had equipped well before the 2005 DRVSM deadline. According to the Air Transport Association of America (ATA), the trade organization of the major U.S. airlines, most of its members' 4,500 airplanes are equipped for RVSM, with the exception of older DC-9s.

Rush to Equip

But there was a rush among unequipped operators to obtain the necessary approvals. ARINC, which provides RVSM monitoring services, experienced a record year in 2004, handling 838 aircraft. Approximately 90 percent of those monitoring flights were for business or general aviation (GA) operators, according to ARINC's RVSM services manager, Timothy Fritz.

Equipping for RVSM is not a trivial undertaking. Among the avionics systems required for RVSM operations are: dual, independent, cross-coupled digital altimeters; Mode C or Mode S transponder; automatic altitude hold system; altitude alerter system; dual air data computers; and an autopilot able to maintain tight altitude control.

Now that the flood has peaked, users are finding out what DRVSM means. The economic news is good--better than expected, says FAA. The additional flight levels between 29,000 and 41,000 feet have saved customers an estimated $800 million in 2005 vs. the expected $393 million, according to Ardyth Williams, operations performance support manager with FAA's Air Traffic Organization (ATO). Although some of the increase reflects skyrocketing fuel prices, the change has worked well in the airspace system, she says.

Besides the reduction in fuel consumption, DRVSM may have increased safety margins. Operational errors in DRVSM altitudes, resulting in violations of required separation standards, declined by an average of 22.4 percent in the first 12 months, Williams says. The most severe errors, categories A and B, decreased by 27 percent during that period.

Capacity Increase?

As to capacity increases, an argument made in the first paragraph of FAA's final rule, the results are less clear. DRVSM has "provided the potential for enhanced en-route airspace capacity," Williams says. But adding six new altitudes in a sector doesn't automatically mean that six more aircraft can be added, she explains. "It just means we are reducing the conflict points."

Weather Effects

FAA emphasizes the flexibility that DRVSM gives controllers, particularly when dealing with convective weather systems. "If airplanes run into turbulence, and they ask for a different altitude, they are more likely to get it," Williams says. And there is less frequency congestion on air traffic control voice channels because there is less vectoring, she adds. "Airplanes are more vertically separated than laterally separated."

An area with severe weather is basically unusable airspace. Before DRVSM, for example, if there was a line of storms from Cleveland to Kansas City, controllers might have taken the planes south. Some of the airplanes would have operated at FL 370 and above, but most would have been at FL 350 to 290. Others would have been pushed down into the "lower 20s," Williams says. DRVSM simplifies airspace complexity. "You can add more airplanes in the same corridor," she says. Weather still wreaks havoc when airplanes get near an airport, but DRVSM makes weather situations more manageable in en-route airspace.

Initial evidence points to capacity benefits, but the end result is unclear. A DRVSM issues paper posted by the National Business Aviation Association at states: "NBAA does not agree that DRVSM alone will provide any net capacity gain for the ... NAS. DRVSM is one of several necessary steps for the NAS to absorb increased operational demand over the next decade. In the short term, DRVSM will provide controllers with a much larger set of options to maintain VFR [visual flight rules] -like traffic flows during severe weather conditions at en-route altitudes. ... No capacity is gained, but `no thunderstorm' capacity might be maintained."

Actual capacity increases are difficult to measure. A report prepared last summer for ATO by the Collaborative Decision Making/DRVSM Work Group attempted to quantify en-route airspace capacity increases in an extremely preliminary way, based on monitor alert parameter (MAP) data collected during the first 28 days of DRVSM implementation. MAP is a concept that FAA traffic flow managers in the system command center in Washington, D.C., and other air traffic control centers around the country use to monitor sector loads in order to better handle traffic flows across the NAS. MAP values, which are variable, are thresholds at which managers are alerted about the number of aircraft currently in, or predicted to be in, a sector. In response to these alerts, managers adjust flows to avoid overloading an individual sector or controller.

In the first month of DRVSM implementation, MAP exceedances throughout the NAS were up more than 65 percent--from 28,629 to 47,350--the report states. It continues: "The number of MAP exceeds in DVRSM sectors has increased NAS-wide because these sectors were able to accommodate more aircraft without implementing TMIs [traffic management initiatives] to control sector volume. This indicates a direct correlation between DRVSM and increased capacity." TMIs are the actions that managers take, for example, to redirect some airplanes out of an overloaded sector or hold aircraft on the ground.


MAP exceedances in the early days of DRVSM suggest that en-route capacity is increasing. But more data needs to be gathered to verify and quantify the increase. If MAP values can be adjusted upward, this would create a foundation for increased en-route capacity that airspace managers, controllers, and users could incorporate into everyday planning and operations.

The element of predictability, based on a quantification of the benefits, is what's missing, according to industry sources. Operators are not yet able to plan for increased throughput. Industry officials who were involved in last year's work group have offered to support a second effort. FAA understands the issues and is expected to review MAP values. It will take considerable time to realign these values, however, as the data must be collected and analyzed. The issue also is important to air traffic controllers, who will watch very carefully to make sure sector throughput thresholds are not set too high.

According to a representative of Northwest Airlines, a fiscally conservative carrier which equipped more than 100 of its DC-9s for DRVSM at considerable expense, DRVSM does increase capacity but to an unknown level. "There's probably between a 3 and 5 percent increase that's easily obtainable, easily publishable, and therefore easily predictable," he says. "But we've got to get the data to support it. Three more airplanes through a sector in 15 minutes could be huge in terms of the overall picture of the day or the year."

Among its recommendations, the work group report calls for FAA to explore how system capacity can be measured more accurately. It recommends the "implementation of a national program to review and manage the monitor alert process within the FAA."

Fuel Savings

The report estimates that the average flight level increased by 380 feet--from FL 346.5 to FL 350.3--after DRVSM implementation, based on a preliminary survey of 12 city pairs. It goes on to project significant reductions in fuel consumption, assuming this data holds true over a longer time period and larger airspace volume. However, much more data is needed to firm up these predictions.

The level of savings derived from DRVSM is difficult to determine. Although FAA's initial estimate of $393 million in savings over the first year is a large sum of money, if that's divided by 20 or so airlines, it comes to about $20 million per airline, points out Basil Barimo, vice president of operations and safety for ATA. The jet fuel bill for the industry, by contrast, was around $32 billion in 2005, he says. Because DRVSM is just one of several FAA enhancements that have been put into place, it's difficult to correlate with a specific dollar benefit.

The work group report also cites anecdotal evidence of more efficient air traffic management during severe weather. One FAA facility directly attributed its clean handling of 29 aircraft in a "complex, high-altitude sector during a weather event" to the availability of new DRVSM altitudes. The traffic at the time involved a "mix of deviating flights and a normal arrival/departure rush." Other facilities reported "much less vectoring of aircraft," a phenomenon associated with deviations around weather. Additionally, more aircraft were able to use the new flight levels to surmount Hurricane Dennis storms in July 2005, the report states.

Today NAS users are more likely to be able to operate at more optimal altitudes than they were before DRVSM was initiated. And more aircraft are being equipped, including those of the U.S. military, whose unapproved aircraft are currently being accommodated in DRVSM altitudes. While the U.S. military has no plans to equip its fighter aircraft, Williams says, officials are trying to retrofit the nonfighter fleet when those aircraft come in for major maintenance.



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