August 2001

NASA/TM-2001-211052

Enhanced Airport Capacity Through Safe,

Dynamic Reductions in Aircraft Separation:

NASA’s Aircraft VOrtex Spacing System

(AVOSS)

Cornelius J. O’Connor and David K. Rutishauser

Langley Research Center, Hampton, Virginia
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August 2001

NASA/TM-2001-211052

Enhanced Airport Capacity Through Safe,

Dynamic Reductions in Aircraft Separation:

NASA’s Aircraft VOrtex Spacing System

(AVOSS)

Cornelius J. O’Connor and David K. Rutishauser

Langley Research Center, Hampton, Virginia
Available from:

NASA Center for AeroSpace Information (CASI) National Technical Information Service (NTIS)

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Hanover, MD 21076-1320 Springfield, VA 22161-2171

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Acknowledgments

The authors would like to acknowledge the assistance of those individuals who aided in the

preparation of this document. In particular, we acknowledge Mr. David Hinton and the members

of the AVOSS team whose prior work laid the foundation for this paper. In addition, informal

conversations with members of the WakeVAS team as well as representatives of numerous

airport authorities contributed to the ideas presented herein.
1 

Abstract 

An aspect of airport terminal operations that holds potential for efficiency improvements is the 

separation criteria applied to aircraft for wake vortex avoidance. These criteria evolved to 

represent safe spacing under weather conditions conducive to the longest wake hazards, and are 

consequently overly conservative during a significant portion of operations. Under many ambient 

conditions, such as moderate crosswinds or turbulence, wake hazard durations are substantially 

reduced. To realize this reduction NASA has developed a proof-of-concept Aircraft VOrtex 

Spacing System (AVOSS). Successfully operated in a real-time field demonstration during July 

2000 at the Dallas Ft. Worth International Airport, AVOSS is a novel integration of weather 

sensors, wake sensors, and analytical wake prediction algorithms. Gains in airport throughput 

using AVOSS spacing as compared to the current criteria averaged 6%, with peak values 

approaching the theoretical maximum of 16%. The average throughput gain translates to 15-40% 

reductions in delay when applied to realistic capacity ratios at major airports. 

Introduction 

Since the late 1990s the national airspace system has been recognized as approaching a capacity 

crisis. In light of this condition, industry, government, user organizations, and educational 

institutions have been working on procedural and technological solutions to the problem. One 

aspect of system operations that holds potential for improvement is the separation criteria applied 

to aircraft for wake vortex avoidance. Wake vortices are generated during all phases of flight, as 

a by-product of a wing that is generating lift. Wake vortex encounters have caused injuries as 

well as loss-of-control accidents, particularly for smaller aircraft following larger aircraft, and 

must be avoided. The problem is most critical in terminal areas due to a combination of low 

altitude and high-density operations. Decades of research on wingtip vortices have shown that 

although the initial position and strength of a wake is dependent on physical parameters of the 

generating aircraft (weight, speed, wingspan, etc.), the wake position, movement, and strength 

persistence over time depend on local meteorological parameters (winds, turbulence, and 

temperature). 

The Federal Aviation Administration (FAA) aircraft separation criteria for wake vortex 

avoidance are summarized in Table 1. These criteria were empirically developed and provide 

adequate spacing in worst-case weather conditions. There is no provision to reduce spacing when 

the weather conditions are conducive to early wake dissipation or advection from the approach 

corridor. To realize this potential spacing reduction, the AVOSS provides dynamic spacing 

criteria based on the current weather conditions to maximize runway acceptance rates. AVOSS 

was developed for the problem of aircraft on approach to a single runway, but the concept 

extends to other terminal area scenarios. Wake prediction models provide estimates of the wake 

position and strength based on terminal area weather conditions. These estimates are used to 

optimize the spacing between pairs of aircraft on the approach. As a safety check, field wake 

sensors confirm the predicted behavior. An AVOSS proof-of-concept was successfully 

demonstrated in July of 2000 at Dallas Fort-Worth International Airport (DFW). Data collected 

during the demonstration showed that the reductions in aircraft spacing enabled by AVOSS 

would result in significant delay reductions at capacity constrained airports. This paper describes 

the AVOSS architecture and theory of operation, as well as the latest performance results. A 
2 

description of the field deployments is provided, and plans for system evolution from its current 

state as a proof-of-concept to one suitable for operational use are discussed. 

Table 1. FAA Wake Avoidance Separation Criteria 

Notes: 

Small ≤ 41,000 Maximum Gross Takeoff Weight (MGW) 

41,001 lb < Large ≤ 255,000 lb MGW 

Heavy > 255,000 lb MGW 

2.5 nm separation increased to 3 nm when airport has >50 sec Runway occupancy time 

System Design 

The AVOSS architecture is shown in Figure 1. The weather subsystem was developed in 

cooperation with the Massachusetts Institute of Technology (MIT) Lincoln Laboratory, North 

Carolina State University, and the National Oceanographic and Atmospheric Administration 

(NOAA). The system consisted of two instrumented towers, Doppler radar and sodar profilers 

for measuring winds aloft, and a radio acoustic sounding system (RASS) to measure 

temperatures aloft. At 30-minute intervals, data from these sensors as well as two Terminal 

Doppler weather radars were integrated into vertical profiles of winds, temperature, and 

turbulence using a fusing algorithm developed at MIT Lincoln Labs [1]. This data is used as a 

short-term forecast of the weather that is input to a state-of-the-art wake-prediction model [2]. 

This model provides estimates of wake transport (lateral and vertical) and strength. The 30-

minute interval was considered to be an acceptable amount of time for a persistence-based 

forecast of the weather while providing realistic lead time for anticipated enroute controller 

planning. Northwest Research Associates developed the prediction subsystem, with participation 

from NASA and the Naval Postgraduate School. 

Following

Aircraft, (nm)

Small

Large

Heavy

Small

2.5

2.5

2.5

Large

4

2.5

2.5

B757

5

4

4

Heavy

6

5

4

Lead Aircraft Following

Aircraft, (nm)

Small

Large

Heavy

Small

2.5

2.5

2.5

Large

4

2.5

2.5

B757

5

4

4

Heavy

6

5

4

Lead Aircraft Following

Aircraft, (nm)

Small

Large

Heavy

Small

2.5

2.5

2.5

Large

4

2.5

2.5

B757

5

4

4

Heavy

6

5

4

Lead Aircraft
3 

The subsystem integration logic applies the estimates of wake behavior to a corridor of airspace 

about the nominal flight path (the center of the localizer and glide slope). Wakes can cease to be 

a hazard by drifting or sinking out of the corridor or by decaying to a circulation strength 

comparable to background turbulence. The dimensions of the corridor are based on a 3-sigma 

buffer applied to observed aircraft position dispersion data from radar tracking data [3]. 

To determine the AVOSS recommended spacing, the wake hazard times are computed for each 

aircraft type (e.g., B-747) present in the traffic mix for a given airport. The computations are 

performed at various points along the approach corridor to capture the changes in wake behavior 

with altitude. The wake factor (position or strength) that first clears the corridor at those points 

sets the wake existence time for that aircraft at that point. The worst-case spacing for each 

aircraft type is then taken as the required spacing for that aircraft’s category (e.g., heavy). Using 

the average approach speeds for each aircraft type and the predicted headwinds, the wake hazard 

times are converted to minimum spacing values (in nm) for each leader/follower pair. This 

results in the most conservative spacing being applied. The spacing is output as a categoryindexed table in nautical miles. 

The wake detection subsystem consists of various wake sensors that track the wakes from the 

approaching aircraft and provide time histories of wake position and strength as well as observed 

wake hazard times. These are used as a safety check and to validate the predictions. Figure 2 

conceptually shows the safety corridor and a sensor placement. The subsystem used in the DFW 

Prediction 

Subsystem

Weather 

Subsystem

Subsystem 

Integration

Wake 

Detection 

Subsystem

ATC 

Interface

Figure 1. AVOSS architecture 
4 

deployment consisted of a continuous-wave (CW) lidar system operated by Lincoln Labs, a 

pulsed lidar operated by NASA, and a ground wind vortex-sensing system (GWVSS), or 

windline, operated by Volpe. The CW lidar has the best range resolution but is limited to about 

300 meters in range, so it was used close to the runway threshold in the field deployments. The 

pulsed lidar can measure wakes out to several kilometers, but has poorer range resolution (~30 

meters). The windline is a row of pole-mounted anemometers, which has limitations as to where 

it can be installed in an airport environment, and wakes must sink into the sensor before they can 

be measured. The advantage of the wind line is its modest cost, unattended operation, and low 

maintenance. 

The AVOSS demonstration did not include an ATC interface, although a model that accounts for 

the performance impact of interfacing to ATC was included to add utility to the results. The 

model includes rounding of spacing values to 1⁄2 nautical mile increments and a spacing buffer to 

simulate variances in aircraft delivery to the top of approach. Performance statistics were 

collected for continued system evaluation and development. 

Figure 2. Safety corridor and sensor placement concept 
5 

AVOSS Performance 

Analysis of the field data from the 1999 and 2000 DFW deployments reveals the maximum 

Instrument Flight Rules (IFR) throughput gain averaged 6%, while ranging from 0% to 16%. The 

gain is computed by comparing the throughput using the AVOSS spacing recommendation to 

that achieved with the default FAA spacing. The 0% gain indicates that on some days the 

AVOSS did not recommend reducing the default spacing. The 16% gain in throughput is 

approximately equal to the maximum gain possible when comparing the default spacing to the 

minimum runway occupancy time (ROT) limited spacing. Figure 3 illustrates the importance of 

these results [4]. The plot shows the relationship between increases in throughput and delay, as a 

function of capacity. As the demand for a runway approaches maximum capacity (a/t=1), delays 

increase exponentially, providing the potential for large decreases in delays with relatively small 

increases in throughput. Many capacity-constrained airports operate near 100% capacity. As the 

chart shows, for a runway at 90% capacity, a 6% increase in throughput can offer a nearly 40% 

reduction in delay. 

Field data from the DFW deployments was also used to validate the AVOSS predictor 

algorithms. Of 2301 wake measurements that were compared with the predictions, 99% indicated 

AVOSS reduced separation could be applied based solely on the predicted behavior (i.e. 

observed wake hazard times did not exceed the predictions). In almost 2/3 of the cases, AVOSS 

Figure 3. Impact of throughput increase and delay as a function of capacity 
6 

recommended the minimum separation possible (ROT limited) with no sensor measurements 

contradicting the recommendation. The 1% of cases where observed wake hazard times 

exceeded the predicted times were all exceedances of less than 20 seconds, with half under 5 

seconds. These cases are not necessarily an indication that an inadvertent wake encounter would 

have occurred, since the wake hazard time is taken when the wake is observed to be clear of the 

safety corridor or indistinguishable from the background turbulence. An aircraft would have to 

be flying with a significant deviation from the localizer or glide slope course to encounter the 

wake, which is unlikely given FMS-coupled approaches and typical pilot performance. 

Future plans/roadmap to operation 

Potential applications 

A future AVOSS development notionally referred to as a Wake Vortex Advisory System 

(WakeVAS) may embrace one of the following concepts: single in-trail or closely-spaced 

parallel approaches, departures, or intersecting runway operations, depicted in figure 4. Each of 

these concepts would also require customization to a particular airport. Subsystems for weather 

and wake sensing, as well as initial procedure development will be site-specific, while weather 

forecasting and pilot and controller interfaces will be more generic. 

Arrivals

2 minute wait to land due 

to vortex 

of departure aircraft

Intersecting

Departures

Closely 

Spaced 

Figure 4. Potential WakeVAS terminal configurations 
7 

Single in-trail approaches 

The earliest operational deployment of a WakeVAS will likely be for single in-trail approaches, 

building directly on the results of the AVOSS development, as described earlier. Operational 

deployment of single in-trail WakeVAS systems would benefit nearly every capacity-constrained 

airport in the United States. The potential for delay reductions is significant: the impact of the 

increased spacing for aircraft following the 757 instituted in 1996 resulted in an immediate 12% 

increase in delays at Los Angeles International Airport [5]. On average, the AVOSS 

recommended spacing reductions of nearly one mile for large category aircraft following 757s. 

Closely-spaced parallel approaches 

A closely spaced parallel approach WakeVAS system may offer the greatest potential for delay 

reduction. Based on AVOSS and similar windline-derived data, wake transport from one runway 

to an adjacent runway appears to be a rare event. Utilization of appropriate procedures can 

prevent a problem in Visual Flight Rules (VFR) conditions. During reduced-visibility conditions, 

however, the inability to use these procedures results in as much as a 50% reduction in arrival 

capacity, given the loss of the second runway’s capacity. A WakeVAS combined with an offset 

ILS, RNAV, or Simultaneous Offset Instrument Approach (SOIA) arrival procedure may restore 

the usability of the second runway under most reduced-visibility conditions, since these 

procedures are designed to lower the operational ceilings at which pilots may accept VFR arrival 

clearances. Such a combined system could significantly reduce a parallel runway’s downtime. 

Departures 

Separation reductions for single runway departures represent another solution with high potential 

benefit. Wake vortex spacing requirements are included in both VFR and IFR departure 

clearances, so a wake-avoidance solution would provide departure capacity improvements during 

both VMC and IMC. Wake turbulence monitoring and prediction would have to cover both the 

near-ground area off the runway and any common departure path used by sequential departures. 

The complexity of a departure WakeVAS would be dependent on the airport and local airspace. 

Additional impacts, such as interaction with noise abatement and fan-out procedures, may arise. 

Intersecting runway operations 

An intersecting runway system may also offer an opportunity for early implementation. When 

arriving and departing aircraft cross each other’s paths while airborne, there is a potential for 

encountering a wake at nearly right angles. A good example is New York’s La Guardia (LGA) 

airport. At LGA, when aircraft are arriving on Runway 22 and departing on Runway 31, 

controllers frequently need to apply additional separation between aircraft. This situation 

requires controllers to apply 2 minutes of separation between subsequent Runway 22 arrivals and 

Runway 31 departures, almost double the separation if wake turbulence were not a factor. 

Although this airport configuration only occurs about 20% of the time, studies have shown that it 

accounts for roughly half the delays at LGA. An intersecting runway WakeVAS would likely 

require less procedural development than would other WakeVAS applications. The system 
8 

would set the time interval for consecutive operations, to be followed by the controllers, with no 

other procedural changes required. Since the critical area for potential wake encounters is largely 

within ground effect, current wake sensors are adequate for detection (no issues of sensor 

coverage). An intersecting runway system, however, has less nationwide applicability because 

the number of capacity-constrained airports with an intersecting runway problem is limited. 

Initial steps 

A NASA-led government/industry team is currently laying the groundwork for a WakeVAS. 

Participants include: the FAA, MIT/Lincoln Labs, MITRE, NASA, and the Volpe Center. The 

group is researching subsystem issues such as evaluating existing technologies (including 

sensors, wake vortex prediction, and other factors) with regard to maturity for deployment of 

wake-vortex-related systems and procedures. 

Work is also focused on identification of candidate airports for the development and deployment 

of a prototype WakeVAS. Airport selection criteria is based on several factors, including a cost 

and benefit analysis, the projected impact of the various WakeVAS concepts on airport delay and 

capacity, and the willingness and ability of the airport authority to participate in advocating and 

funding a WakeVAS. 

Specific definition of user needs at those airports deemed promising will follow. An initial 

system architecture concept along with an analysis of the technical and operational risks of each 

type of system will also be developed. An exploration of the proposed architecture will lead to 

the development of candidate procedures. 

The successful development of an ambitious system such as a WakeVAS would not be possible 

without the participation of all affected users. The formation of working groups is anticipated to 

focus the entire operational development. These groups would include, at a minimum, 

professional controller and pilot organizations, airport authorities, airlines, industry, and 

government agencies. This article represents one element of the initial informational phase. 

Phased implementation approach 

It is envisioned that the initial deployment of a WakeVAS would be a limited system, likely 

based on subsystem confidence levels. For example, wake behavior must initially be measured 

from the runway to the glide-slope intercept height to verify that the wake predictions are 

reliable. To date, only low altitude (below 600 feet) wake behavior has been measured. Phased 

testing can be done with ceilings above the glide slope height to establish the zone over which an 

operational WakeVAS would need to sense wake behavior. Research to date indicates that wake 

behavior at altitudes above ground effect (approximately twice the wingspan of the generating 

aircraft) is more accurately predicted and may not require sensor verification in an operational 

system. If this were determined to be the case, then ceilings would only need to be above 300 to 

600 feet to use lidars for wake sensing. Historical data (see figure 5) shows that with a ceiling 

limit of 300 feet (approximately the final mile of a standard approach), and a wake sensor limited 
9 

by ceiling (i.e. lidar) a WakeVAS would still have about 90% availability in instrument approach 

conditions [6]. All-weather wake sensors are therefore not required for a usable WakeVAS. 

Conclusion 

NASA has developed a proof-of-concept Aircraft VOrtex Spacing System (AVOSS). 

Successfully demonstrated in a real-time field demonstration during July 2000 at the Dallas Ft. 

Worth International Airport, AVOSS is a novel integration of state-of-the-art weather sensors, 

wake sensors, and analytical wake prediction algorithms. Gains in airport throughput if AVOSS 

spacing were used as compared to the current criteria averaged 6%, with peak values 

approaching the theoretical maximum of 16%. The average throughput gain translates to 15-40% 

reductions in delay when applied to realistic capacity ratios at major airports. A low percentage 

(1%) of validation measurements indicated variances in the system that need to be eliminated as 

the system is developed for operational use. 

A NASA-led government/ industry team is working to define the requirements for an operational 

system. High-level concepts for single and parallel approaches, departures, and intersecting 

runways are being studied. AVOSS subsystem maturity levels are being assessed and cost 

benefit analyses are being conducted to target potential WakeVAS locations and configurations 

offering the lowest risk/highest payoff for a phased introduction. Stakeholder support is being 

generated through team communication, leading to the eventual formation of working groups to 

guide development and implementation. 

Figure 5. Hourly ceiling and visibility data collected from 1961-1990 at various airports 
10 

References 

 [1]. Cole, R., Denneno, A., Matthews, M.; The AVOSS Winds Analysis System, Project Report 

NASA/L-5, Lincoln Laboratory, Massachusetts Institute of Technology, April, 2001. 

[2]. Robins, R. E., Delisi, D. P.; NWRA AVOSS Wake Vortex Prediction Algorithm Version 3.1.1, 

NWRA-CR-00-R229A, Northwest Research Associates, November 2000. 

[3]. Hinton, D.A.: An Aircraft Vortex Spacing System (AVOSS) For Dynamical Wake Vortex 

Spacing Criteria, AGARD 78th Fluid Dynamics Panel Meeting and Symposium on the 

Characterisation & Modification of Wakes from Lifting Vehicles in Fluids, Trondheim, Norway, 

20-23 May, 1996, AGARD CP-584, Paper 23. 

[4]. Credeur, L.; Basic Analysis of Terminal Operation Benefits Resulting from Reduced Vortex 

Separation Minima, NASA-TM78624, October, 1977. 

[5]. Simpson, Robert W.; A Critical Review of the FAA Safety Rulemaking on Wake Turbulence, 

Air Transport Association of America, Sept. 1997. 

[6]. Hinton, David A.; Charnock, James K.; Bagwell, Donald R.; Grigsby, Donner: NASA 

Aircraft Vortex Spacing System Development Status, AIAA 99-0753, 37th Aerospace Sciences 

Meeting & Exhibit, January 11-14, 1999, Reno, NV. 
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Enhanced Airport Capacity Through Safe, Dynamic Reductions in Aircraft

Separation: NASA’s Aircraft VOrtex Spacing System (AVOSS)

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6. AUTHOR(S)

Cornelius J. O’Connor and David K. Rutishauser 

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NASA Langley Research Center

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13. ABSTRACT (Maximum 200 words)

An aspect of airport terminal operations that holds potential for efficiency improvements is the separation

criteria applied to aircraft for wake vortex avoidance. These criteria evolved to represent safe spacing under

weather conditions conducive to the longest wake hazards, and are consequently overly conservative during a

significant portion of operations. Under many ambient conditions, such as moderate crosswinds or turbulence,

wake hazard durations are substantially reduced. To realize this reduction NASA has developed a proof-ofconcept Aircraft VOrtex Spacing System (AVOSS). Successfully operated in a real-time field demonstration

during July 2000 at the Dallas Ft. Worth International Airport, AVOSS is a novel integration of weather sensors,

wake sensors, and analytical wake prediction algorithms. Gains in airport throughput using AVOSS spacing as

compared to the current criteria averaged 6%, with peak values approaching the theoretical maximum of 16%.

The average throughput gain translates to 15-40% reductions in delay when applied to realistic capacity ratios at

major airports.

14. SUBJECT TERMS

Air Traffic Control; Airport capacity; Wake vortex

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15

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