Editor's Note

PARR: Resolving Problems Strategically

By Win Heagy and Dan Kirk | June 1, 2003
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A tool for Free Flight, URET is being enhanced to automatically provide controllers with resolutions for conflicts with aircraft, airspace and even weather. The results are to achieve safer, more efficient and more flexible air traffic management.

The Federal Aviation Administration’s User Request Evaluation Tool (URET), assisting en route sector controllers at six air route traffic control centers (ARTCCs), appears to be proving its worth, as it supports the shift away from tactical operations based on radar data and toward strategic air traffic control (ATC) planning, based on flight plans and associated trajectories. Studies indicate that airlines are saving roughly $18 million a year due to the removal of selected altitude restrictions and the more direct routing enabled by URET at just two ARTCCs–Indianapolis and Memphis.

Enhancements Being Made

However, since the initial URET prototype deployment by the Mitre Corp.’s Center for Advanced Aviation System Development (CAASD) to Indianapolis in 1996 and Memphis in 1997, URET has been continuously enhanced through controller feedback. Specific areas of enhancement include the development of strategic resolutions for aircraft-to-aircraft and aircraft-to-airspace conflicts, problems involving severe weather, and resolutions that assist in the efficient implementation of constraints for congestion management. FAA and Mitre CAASD jointly are developing these capabilities, collectively termed Problem Analysis, Resolution and Ranking, or PARR.

As with previous URET development, PARR capabilities are being expanded through a series of builds. FAA and Mitre CAASD are developing initial PARR capabilities as part of Free Flight Phase 2 (FFP2). FFP2, which follows Free Flight Phase 1, includes URET implementation. FAA and National Airspace System (NAS) users established the two-phase Free Flight initiative to meet user demands and accommodate air traffic growth.

Initial PARR capabilities are designed to help support the resolution of aircraft-to-aircraft and aircraft-to-airspace conflicts. Subsequent PARR enhancements–for example, those that deal with severe weather problems–are termed "advanced PARR."

Initial Capabilities

Initial PARR capabilities include two URET planning enhancements. The first is designed to simplify actions such as going direct to a downstream fix by conflict-checking a range of options (for example, multiple downstream fixes) with a single button click. The second feature is the provision, on controller request, of strategic lateral, vertical and speed resolutions to conflict situations detected by URET. Both features present options for the controller’s consideration .

With automatic conflict-checking for assigned altitude, speed and direct-to-fix options, a controller using URET can open menus with alternatives for a new assigned altitude, a new assigned speed, or a downstream fix to bypass a route bend. The controller may select one of these entries and graphically view any conflicts that were found.

The conflict-check results are used to color-code the corresponding menu entries. For example, severe conflicts (predicted aircraft separation of less than 5 nautical miles) are coded red. In this way, the controller can see the conflict status of each alternative at a glance. This is expected to be particularly useful when the pilot has requested an altitude or route change due to severe weather, turbulence or other factors.

FAA and Mitre CAASD have completed the research on the menu-coding feature of PARR. The capability is planned as a near term URET enhancement.

More Options

With strategic resolutions for aircraft-to-aircraft and aircraft-to-airspace conflicts, PARR can save the controller time by examining resolution options across a variety of dimensions, directions and types of maneuvers. These can include maneuvers not available on the URET menus, such as step climbs and vectors. The generated PARR resolutions are ranked according to estimated airspace user and controller preferences, to facilitate the resolution selection process. They are made available to the controller using the URET textual and graphical displays.

Only one aircraft is maneuvered for each resolution. (FAA and Mitre CAASD anticipate multiple aircraft maneuvers per resolution as a future enhancement.) PARR searches for conflict-free trajectories (within the next 20 minutes) to resolve all problems with the aircraft being maneuvered. And it does so in an operationally acceptable manner, without introducing new problems.

Aircraft Equipage Considered

PARR examines climbs and descents, left and right vectors, VOR reroutes, directs, restriction removals and speed change maneuvers. In merging situations, a composite maneuver may be created by combining a vector maneuver with a speed change that matches the speed of the preceding aircraft. Otherwise, each resolution contains only one type of maneuver.

The resolutions are considered closed maneuvers. In other words, the aircraft always is returned to its original route of flight. Phase of flight also is considered when generating resolutions. For example, descent maneuvers will be preferred in the arrival phase of flight. Lateral resolutions also are based on aircraft equipage so that, for example, only area navigation- (RNAV) equipped aircraft will be cleared to a non-VOR fix. PARR also provides a clearance description for each resolution, employing similar considerations of aircraft equipage to ensure navigability.

Advanced Capabilities

These initial PARR capabilities are undergoing controller evaluation and functional performance assessment. Their deployment as a URET enhancement is currently being planned.

The severe weather resolution capability of advanced PARR will benefit the cockpit by providing the controller with the ability to quickly, safely and accurately generate resolutions to weather problems.

The National Convective Weather Forecast (NCWF) product, provided by the National Center for Atmospheric Research (NCAR), is an example of the kind of information that will be needed to support this weather resolution feature. This product can provide URET/PARR with areas of weather that are forecast to have convective activity greater than a predefined severity (for example, level three), as well as turbulence, lightning and other hazardous activity. The NCWF product combines this information into well-defined areas of severe weather that are considered unacceptable for flight.

Because of attenuation and limitations in its effective range, airborne weather radar often is limited in its ability to provide pilots with a complete picture of the weather affecting an aircraft. The NCWF product, however, gives controllers an overview of the severe weather impacting a flight, so they can strategically and tactically assist the pilot in avoiding these areas.

Weather Avoidance

In addition, since URET will check aircraft trajectories against severe weather areas, PARR can generate resolutions around these areas that also avoid other aircraft and special-use airspace in an efficient and operationally acceptable manner. This safely minimizes off-route deviations due to severe weather and makes available additional strategic information for en-route weather decision making. (This feature in no way changes the pilot’s and controller’s responsibilities concerning severe weather avoidance.)

Additional weather products that will benefit pilots–such as areas of known and forecast icing and turbulence–are being considered for future implementation. These will be presented to controllers in much the same way as the previously discussed severe weather product. URET will check aircraft for conflicts with these areas, and PARR will be able to generate resolutions to avoid them.

Despite increased capacity due to improvements such as PARR, traffic flow rates still will be affected by severe weather and limited runway capacity. URET/PARR can assist the controller in the efficient implementation of these constraints.

For example, when additional spacing is required for aircraft that are en route to a flow-constrained airport, URET can help the controller quickly determine which aircraft are affected by the constraint and provide strategic notification where additional spacing is required. This is done using detailed aircraft performance and atmospheric data. URET will assist in the implementation of fuel-efficient maneuvers and help reduce pilot workload by avoiding tactical vectoring.

Controllers also will be able to use congestion management to assess the predicted spacing on the entry to a flow-constrained area without forcing the aircraft into a single stream. This avoids the unnecessary slowdown of faster aircraft. Strategic notification and coordination features will allow a reduction in the number of intermediate restrictions required upstream of the arrival airport. Finally, PAAR’s resolution ability can be used to help determine efficient maneuvers, for example, a direct-to-fix maneuver to use an existing spacing gap, and thus avoid an aircraft delay.

ADS-B and Data Link

As air/ground data link and automatic dependent surveillance-broadcast (ADS-B) capabilities become available, they will provide URET with highly accurate cockpit intent information such as speed, heading and climb/descent rates. This information, when coupled with improved system intent information (such as the resolutions for congestion management described above), will support increased trajectory and conflict detection accuracy. This, in turn, will lead to improved PARR resolutions.

Data link also will allow more accurate, detailed and efficient clearances to be sent. For example, turn points could easily be expressed as lat/longs instead of vector headings and VOR radial intercepts.

These advanced PARR features are undergoing initial evaluation and functional performance assessment.

Expected Cockpit Benefits

FAA and Mitre CAASD expect PARR will make significant contributions to safe, orderly and efficient air traffic flow. They believe it will accommodate pilots’ needs and increase controller productivity. These contributions include:

  • Enhanced safety–The expected greater use of airspace creates the potential for a higher number of conflict situations, and the relaxation of ATC restrictions can lead to more complex traffic patterns. PARR is expected to increase safety through improved situational understanding and the implementation of problem-free resolutions. Strategic resolutions will allow increased time for decision making and coordination.

  • Improved airspace capacity–By supporting an improved situational understanding and the implementation of strategic, problem-free resolutions, PARR is expected to enable the accommodation of greater numbers of aircraft operating in less structured airspace.

  • Improved ability to respond to pilot requests–Providing assistance in strategic planning and problem solving will allow controllers more time to address pilot requests and facilitate the granting of these requests.

  • Improved maneuver efficiency– PARR can generate efficient maneuvers using detailed aircraft performance and atmospheric data, and quickly examine a variety of dimensions and directions for increased resolution efficiency. Strategic resolutions can also reduce the number of large, tactical maneuvers.

Money Saved

A previous cost/benefit study estimated the benefits for a full problem resolution capability, such as with advanced PARR, to be $2.5 billion in constant 1988 dollars, for a 20-year life-cycle span.

A Free Flight concept has been formulated to provide users with as much flexibility of flight as possible, while maintaining or increasing NAS safety and predictability. Evaluations to date indicate that the concept’s capabilities can be implemented as an enhancement to URET and yield significant benefits to both the controller and the airspace user.

Winfield Heagy (wheagy@mitre.org) and Dan Kirk (dkirk@mitre.org) are task leads for the PARR effort at The Mitre Corp.’s Center for Advanced Aviation System Development (CAASD). Charles Large of Mitre-CAASD created the graphics. For more information about URET and PARR, see the Mitre-CAASD Web site at www.mitrecaasd.org.

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