26TH INTERNATIONAL CONGRESS OF THE AERONAUTICAL SCIENCES WAKE TURBULENCE MITIGATION FOR ARRIVALS (WTMA) Daniel Williams, Gary Lohr NASA Langley Research Center, Hampton, Virginia, USA Keywords: Wake, Vortex, Turbulence, CSPR, Arrivals Abstract Approach Procedures (IAPs). While instrument approaches are often used in visual conditions, The preliminary Wake Turbulence Mitigation an airport’s acceptance rate is degraded when for Arrivals (WTMA) concept of operations is the weather forces instrument-only conditions described in this paper. The WTMA concept for aircraft navigation and traffic separation, and provides further detail to work initiated by the ATC must control aircraft according to radar Wake Vortex Avoidance System Concept and wake separation standards. Evaluation Team and is an evolution of the This paper includes a background of the Wake Turbulence Mitigation for Departure current or state-of-the-art of operational concept. Anticipated benefits about reducing procedures including applicable research, and wake turbulence separation standards in cross- then provides a description of the WTMA wind conditions, and candidate WTMA system concept and system architecture considerations considerations are discussed. to improve those procedures for NextGen traffic projections. Finally future research efforts and 1 Introduction/Background recommendations are described. The authors are passionate about supporting operators with The current air traffic system is not prepared for appropriate technology and procedures, so this the two- to three-fold increase in traffic paper includes that perspective. projected for the 2025 time-frame [1]. Current system limitations, procedures, and the absence of automation-based tools define a highly 1.1 CSPR Description constrained environment. To cope with future Closely Spaced Parallel Runways (CSPRs) are traffic demands, fundamental changes are defined as runways whose centerlines are required to effectively manage traffic and separated by less than 2500’ [3]. Many airports maximize the utility of airports. In the U.S., the throughout the U.S. have CSPRs, and NextGen Next Generation Air Transportation System includes a vision for better utilizing them in (NextGen) is being developed to meet this IMC. When CSPRs are used for arrival projected traffic growth [2]. operations, the capacity can vary significantly An important area for supporting the based on whether visual approaches or potential air traffic growth is to improve the instrument approaches are in use. When visual capacities of airports when weather deteriorates approaches are used, simultaneous arrival from Visual Meteorological Conditions (VMC) operations can be conducted to both CSPRs, and to Instrument Meteorological Conditions (IMC). visual separation has to be applied between When ceiling and visibility are reduced such traffic on the parallel runways when standard that aircrew cannot visually navigate, or reliably separation does not exist. In the case of see and avoid other traffic, then Instrument instrument approaches, simultaneous operations Flight Rules (IFR) must be used. Air Traffic are not permitted, so the arrival rate is Control (ATC) shifts from allowing aircrew to significantly smaller than when visual fly visual approach procedures to Instrument approaches are in use. Concepts for increasing 1 DANIEL WILLIAMS, GARY LOHR the CSPR arrival rate could increase the utility 1.3 Wake Vortex Avoidance System of these airports to assist meeting increased Summary of Concepts for CSPR Approaches traffic projections. In 2005, a multi-organizational Wake Vortex Avoidance System (WakeVAS) Concept 1.2 Wake Separation Standard for In-Trail Evaluation Team (CET) documented five Approaches generic CSPR instrument approach geometries Terminal Radar Approach Control (TRACON) and procedures [5]. For these concepts, and Airport Traffic Control Tower (ATCT) approach controllers position aircraft as a controllers apply radar separation (R) of 2.5 NM “dependent-stagger” operation with a minimum or 3 NM [3] between aircraft on approach. In 1.5 NM diagonal stagger at the runway addition, if specific pairings by weight category threshold during appropriate cross-wind occur, wake separation standards are applied conditions. The concepts require cross-wind (Figure 1). These minima are applied for surveillance within an alternate separation zone single-runway Instrument Landing System (ASZ), a 3-D volume encompassing the IAPs of (ILS) approach procedures and straight-in concern, so that wake separation standards may approaches including approaches to CSPRs be reduced according to a “red-light/green- where the lateral separation between approach light” condition (Figure 2). The “green-light” paths is less than 2500 ft. Although not condition occurs when cross-winds reach a explicitly stated, these minima account for wake specified threshold value (a local constraint vortex descent and decay to acceptable levels of dependent upon RCL spacing and IAP safety for wake avoidance. geometry), that will inhibit the transport of lead aircraft wake vortices to the CSPR approach path of an “up-wind” trailing aircraft. Conceptually, this will allow ATC to reduce the lead-trail wake vortex separation within the ASZ for specific diagonal pairs. Fig. 1. ILS Approach In-Trail Wake Separation Minima at the Threshold [3] Reducing the wake separation minima to the radar separation minima is the target benefit Fig. 2. WakeVAS CET Dependent-Stagger for WTMA on a per aircraft-pair case. Procedure Determining benefits by airport was the focus of a 2005 MITRE report [4]. It highlighted nine of Another IAP-based concept and wake the busiest 35 U.S. airports as potentially avoidance procedure included assigning trailing benefitting by WTMA due to the CSPR runway aircraft to IAPs with steeper approach paths. configuration and cross-wind availability. The This ensured that trailing aircraft would avoid specific airports analyzed were Boston (BOS), the wake of a lead aircraft by procedural Cleveland (CLE), Detroit (DTW), Newark separation and flight path guidance [6]. (EWR), Los Angeles (LAX), Philadelphia Another feature of this concept took advantage (PHL), Seattle (SEA), San Francisco (SFO), and of CSPRs with displaced thresholds, using the St. Louis (STL). displacement in conjunction with the steeper approach for the trailing aircraft to fly above 2 WAKE TURBULENCE MITIGATION FOR ARRIVALS (WTMA) and land beyond the leading aircraft’s wake assessment of the information requirements vortices. needed to support approval of the procedure was also conducted. Supervisors felt that the 1.4 Wake Turbulence Mitigation for required information was currently available in Departures (WTMD) Summary the ATCT and that the decision to approve and monitor use of the procedure could be integrated A concept focusing on wind-dependent into their workload. A full reporting of the departure operations has been developed [7]. results can be found in [9]. The current version of this concept is called Wake Turbulence Mitigation for Departures (WTMD). This concept would be applied to 1.5 Current CSPR Approach Procedures and operations at airports with CSPRs, and takes Application of Wake Separation Standards advantage of the fact that cross-winds of 1.5.1 Visual Pairings sufficient velocity transport wakes generated by Air Traffic Control (ATC) positioning of two “heavy” and B757 category aircraft on the aircraft to approach a CSPR so they arrive as a downwind runway away from the upwind “visual pair” is currently conducted at several runway. This means that departures on the U.S. airports. These aircraft often fly a Charted upwind runway are not affected by wakes Visual Flight Procedure (CVFP) highlighting generated on the downwind runway; therefore key visual navigation features and ATC contact wake separation of upwind runway departure points. Aircraft may be paired by ATC under traffic from traffic on the downwind runway is visual approach conditions where aircrew not required. Wake standard separation would assume the responsibility for visual navigation still have to be applied between consecutive to the runway, traffic separation and wake departures from the same runway and for vortex avoidance from traffic they are following departures from the downwind runway to a CSPR. Within the pair, aircraft may be following departures from the upwind runway. separated laterally during the final approach by Human-in-the-loop simulations were conducted only the runway centerline spacing, (minus a that focused on the role of the “local” ATCT wingspan distance), since they may be adjacent controller, who would apply the requisite to each other “wingtip to wingtip.” Wake separation standards. The results clearly separation standards (Figure 1) are applied indicate that using the WTMD procedures was between pairs by approach controllers until relatively easy, with workload remaining within aircrew can assume separation responsibility. acceptable limits. Further, the prototype interface provided adequate information to 1.5.2 Simultaneous Offset Instrument Approach accomplish responsibilities with respect to the As a way of maximizing CSPR throughput, procedure. Finally, departure rate Simultaneous Offset Instrument Approach improvements were observed when WTMD (SOIA) procedures (Figure 3) have been operations were in effect [8]. Supervisory Air developed to enable dual arrival streams in Traffic Controllers would be responsible for conditions of limited ceiling and visibility. authorizing use of