December 2014 

NASA/TM–2014-218664 

Advanced Interval Management (IM) 

Concepts of Operations 

Bryan E. Barmore, Nash’at N. Ahmad, and Matthew C. Underwood 

Langley Research Center, Hampton, Virginia 
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December 2014 

NASA/TM–2014-218664 

Advanced Interval Management (IM) 

Concepts of Operations 

Bryan E. Barmore, Nash’at N. Ahmad, and Matthew C. Underwood 

Langley Research Center, Hampton, Virginia 
Available from: 

NASA STI Program / Mail Stop 148 

NASA Langley Research Center 

Hampton, VA 23681-2199 

Fax: 757-864-6500 
iii 

Contents 

Contents ....................................................................................................................................................... iii 

List of Acronyms, Abbreviations, and Symbols .......................................................................................... iv 

Introduction ................................................................................................................................................... 1 

NEW IM Capabilities ................................................................................................................................... 2 

Interval Management for Closely Spaced Parallel Operations ..................................................................... 2 

Current System ...................................................................................................................................... 2 

Simplified Aircraft-based Paired Approach (SAPA) ................................................................................ 2 

Operational Scenario ............................................................................................................................. 5 

Open Questions ..................................................................................................................................... 6 

Interval Management for Dependent Parallel Approaches ....................................................................... 6

Operational Scenario ............................................................................................................................. 8 

Interval Management with Wake Mitigation .............................................................................................. 10 

Current System .................................................................................................................................... 10 

Justification for Change ...................................................................................................................... 13 

New Concept ....................................................................................................................................... 13 

Operational Scenario ........................................................................................................................... 14 

Open Questions ................................................................................................................................... 15 

References ................................................................................................................................................... 16 
iv 

List of Acronyms, Abbreviations, and Symbols 

ADS-B Automatic Dependent Surveillance – Broadcast 

AGL Above Ground Level 

ALAS Adjacent Landing Alerting System 

ARC Aviation Rulemaking Committee 

ATC Air Traffic Control 

ATM Air Traffic Management 

ATO Air Traffic Organization 

b Wingspan 

CAS Calibrated Air Speed 

CSPO Closely Spaced Parallel Operations 

CSPR Closely Spaced Parallel Runways 

DI Defined Interval 

FAA Federal Aviation Administration 

FAF Final Approach Fix 

FIM Flight Interval Management 

GNSS Global Navigation Satellite System 

IFR Instrument Flight Rules 

ILS Instrument Landing System

IM Interval Management 

IMC Instrument Meteorological Conditions 

MLS Microwave Landing System 

MRS Minimum Radar Separation 

MTOW Maximum Takeoff Weight 

NASA National Aeronautics and Space Administration 

NTZ No Transgression Zone 

OPD Optimized Profile Descent 

PFAS Planned Final Approach Speed 

PRM Precision Runway Monitoring 

RECAT FAA’s Wake Turbulence Recategorization Program 

RNP Required Navigation Performance 

SAPA Simplified Aircraft-Based Paired Approach 

SAPA-CZ SAPA Conformance Zone 

TPA Terminal Proximity Alert 

VFR Visual Flight Rules 

VMC Visual Meteorological Conditions 

WTMD Wake Turbulence Mitigation for Departures 

Γ vortex circulation 

v Cl vortex-induced rolling moment coefficient 

Lα C three-dimensional lift curve slope of the follower 
1 

Introduction 

The ADS-B In Aviation Rulemaking Committee (ARC) has identified three Interval 

Management (IM)1 applications that they feel will provide significant user benefit in the future. 

These applications include: 

• IM for Closely Spaced Parallel Operations (CSPO); 

• IM with Defined Interval (DI); and 

• IM with Wake Mitigation. 

The advanced IM capabilities should provide two related benefits: 

• Increased capacity, and 

• Increased throughput. 

A runway's capacity is the upper limit on the number of operations (in this case, arrivals) that can 

occur in a period of time. Advanced IM will look to increase a runway's capacity by reducing the 

separation requirements for aircraft landing on that runway. A runway's throughput is the 

realized utilization and is less than the runway capacity. Advanced IM will increase a runway's 

throughput by precisely meeting the applicable separation standards and thereby reducing lost 

space due to delivery uncertainty. Both will be measured based on analysis and simulation of 

relevant environments. 

This document provides a high-level description of several advanced IM operations that NASA 

is considering for future research and development. It covers two versions of IM-CSPO and IM 

with Wake Mitigation. These are preliminary descriptions to support an initial benefits analysis. 

 

 

1

 The current FAA terminology is to call all of these operations Interval Management. NASA has continued to use 

the older terminology of Flight Deck Interval Management. The ADS-B In ARC used Interval Management, Flight 

Deck Interval Management and Ground-based Interval Management interchangeably. For consistency, this 

document will refer to all operations as Interval Management. 
2 

NEW IM Capabilities 

Interval Management for Closely Spaced Parallel Operations 

Current System 

Parallel runways with centerline spacing of less than 2500 ft are defined as closely spaced 

parallel runways (CSPR). Simultaneous operations are authorized on these runways only under 

Visual Meteorological Conditions (VMC) which results in the loss of throughput when visual 

separations cannot be maintained (Eftekari et al. 2011). The Federal Aviation Administration has 

provided guidance on how to conduct CSPR operations (FAA Order JO 7110.308) in Instrument 

Meteorological Conditions (IMC). Under this order, select airports are authorized to conduct 

1.5 nmi diagonally-spaced dependent ILS approaches to closely-spaced parallel runways (Figure 

1). Prior to both aircraft being established on their final approach courses, standard radar 

separation, 1000 ft vertical or 3 nmi lateral, is required. This allows for partial recovery of VMC 

arrival rates in low-visibility conditions. However, for many airports with CSPR but not covered 

by 7110.308, the loss of visual separation results in a major reduction in capacity. To overcome 

the loss of throughput under IMC, several procedures have been proposed in the past. These 

procedures include the Unified Paired Approach (Mundra et al. 2008) and the Simplified 

Aircraft-based Paired Approach (Johnson et al. 2013). A modification of the Unified Paired 

Approach concept is being included in the Advanced Interval Management concept being 

developed by RTCA SC-186. 

Figure 1: Top down view of parallel dependent ILS/MLS approaches (FAA Order JO 7110.308). 

Simplified Aircraft-based Paired Approach (SAPA) 

The Simplified Aircraft-based Paired Approach (SAPA) concept is a proposed mid-term concept 

for operation of closely spaced parallel runways in IMC. SAPA is designed to increase 

throughput capacity at airports in the United States where lateral runway spacing for parallel 
3 

runway operations under IMC currently require aircraft-to-aircraft longitudinal separation 

equivalent to single runway operations (e.g., San Francisco International Airport, where the 

lateral distance between runways is 750 ft). The target application is simultaneous, dependent 

operations to two or three parallel runways spaced closer than 2500 ft, possibly as close as 

700 ft. In the current analysis, SAPA operations to runways with lateral spacing as close as 750 

ft were examined (Johnson et al. 2013). 

The SAPA concept leverages advanced navigation and flight-guidance technology along with 

shared surveillance to minimize parallel-approach spacing requirements. By utilizing precise 

navigation, achieved through the use of augmented Global Navigation Satellite System (GNSS), 

and an advanced autopilot that can conform to RNP values of 0.02, safe IMC operations are 

possible at very close lateral runway spacing (Johnson et al. 2013; Torres-Pomales et al. 2014). 

The aircraft use Automatic Dependent Surveillance – Broadcast (ADS-B) to share precise 

position and velocity data, as well as other application-specific data that is not currently available 

in commercial systems such as autopilot status (Johnson et al. 2013). 

During the SAPA operation, the aircraft employ onboard speed cues to maintain longitudinal 

alignment within the required tolerance (Johnson et al. 2013). Air Traffic Control (ATC) will 

sequence the aircraft pair in the arrival flow such that the leading aircraft will have a planned 

final approach speed (PFAS) that is equal to or slower than the trailing aircraft. These aircraft are 

designated as the “slow aircraft” and the “fast aircraft,” respectively. 

The SAPA concept allows one aircraft to pass the other aircraft during the approach segment. 

The term SAPA Conformance Zone (SAPA-CZ) will be used to describe the area where the 

faster aircraft may transgress. It should be noted, however, that staying within the zone does not 

provide blunder protection (Johnson et al. 2013). As shown in Figure 2, the faster aircraft 

maintains a position relative to the slower aircraft within a SAPA-CZ defined by a rear boundary 

to keep the faster aircraft ahead of any possible encounters with a wake generated by the slower 

aircraft and a front boundary defined to keep the faster aircraft from moving so far ahead that the 

slower aircraft will encounter a wake generated by the faster aircraft (Johnson et al. 2013). 
4 

Figure 2: Simplified Aircraft-based Paired Approach (SAPA) Conformance Zone (Johnson et al. 2013). 

Once it is known which two aircraft will be paired together and which will be the slow aircraft 

and the fast aircraft, the controller can issue the IM clearance. The IM Clearance is given to the 

fast aircraft to achieve a spacing prior to transitioning to SAPA operations. This operation makes 

use of the non-coincident route capability expected in the second generation FIM equipment 

(DO-328). For the SAPA operation, it does not matter if IM is initiated prior to top-of-descent or 

much closer to where the SAPA operation will begin. It is expected that the IM operation must 

start at least 10 nmi prior to transitioning to SAPA so that the IM aircraft can achieve the correct 

spacing. 

The IM Achieve-by Point is on the final approach course. The aircraft should be separated by 

1000 ft vertically and the assigned spacing goal within the SAPA window. The IM Operation 

terminates at the final approach fix; however, the SAPA guidance continues until both aircraft 

have touched down on the runway. 

After the SAPA portion of the operation has started, the slow aircraft will maintain the last 

assigned speed until it reaches the FAF or the position designated in the procedure where the 

deceleration to the PFAS begins. At this point, the slow aircraft will decelerate to its PFAS with 

an assumed linear deceleration schedule such that it will reach its PFAS at a distance from the 

runway touchdown point corresponding to 1000 ft AGL on the glide slope (Figure 3). It will then 

maintain that speed until landing. If the slow aircraft is also conducting an IM Operation, it 

should be terminated prior to the start of this SAPA operation. 
5 

Figure 3: Generic Speed Profile for “Slow Aircraft” (Johnson et al. 2013). 

The fast aircraft in the SAPA pair is the active spacing aircraft. As stated previously, this is the 

aircraft that ATC initially places in the trailing position in the pair. After the SAPA clearance is 

issued, the fast aircraft will adjust its speed, which is given by the SAPA algorithm, to obtain the 

correct position within the SAPA-CZ. This correct position is dependent on the PFAS of both 

aircraft in the SAPA pair (Johnson et al. 2013). 

Fast-time and high-fidelity simulations have shown that using the Adjacent Landing Alerting 

System (ALAS) algorithm, which is an alerting algorithm designed to detect intrusions on 

closely spaced parallel runways (Torres-Pomales et al. 2014), SAPA procedures may be carried 

out at 1050 ft separations with next-generation avionics and at 1400 ft separations with current 

ADS-B compliant avionics with an aggressive escape maneuver (Torres-Pomales et al. 2014). 

This concept is closely related to the Paired Approach concept in the FAA’s Advanced Interval 

Management Concept of Operations (FAA 2014). The main difference is that the SAPA 

capabilities allow for the initially trailing aircraft to pass the slow aircraft due to the advanced 

surveillance and navigation requirements. This increases the opportunity for a pair of aircraft to 

perform the operation as there are lesser requirements on the difference between the aircraft’s 

respective PFAS. 

Operational Scenario 

Below is one example of how IM operations can be combined with SAPA operations. 
6 

Figure 4: Schematic for FIM-SAPA Operation 

When the two aircraft that will participate in the SAPA operation are about 25 nmi flight 

distance from the airport, the controllers are notified that they should be paired and are provided 

with the information to know which aircraft will be the fast aircraft. The controller for the fast 

aircraft issues that aircraft an IM Clearance to achieve a spacing interval relative to the slow, aka 

target, aircraft. The assigned spacing goal is 10 seconds (approximately 0.5 nmi). The IM 

Aircraft is attempting to cross the Achieve-by Point 10 seconds after the Target Aircraft crosses 

the Target Reference Point. The IM Aircraft then manages their speed to cross the Achieve-by 

Point at the proper spacing. At this point the controller is able to clear both aircraft for the SAPA 

procedure. Although the IM Operation is terminated at this point, the IM Aircraft continues to 

have speeds presented to them as part of the SAPA operation to ensure that they stay within the 

safe window. 

Open Questions 

1. Where should the Achieve-by Point be relative to initiating the SAPA operation? Does 

there need to be a maintain phase (on non-coincident routes) so that the two aircraft are at 

a common speed as well as properly spaced? 

2. What is the relationship between the SAPA window and the assigned spacing goal for 

IM? 

3. How much of the routing and special points can be inferred from the operation versus 

must be explicitly communicated? Does IM transitioning to SAPA require a new IM 

clearance type? 

Interval Management for Dependent Parallel Approaches 

Interval Management for Dependent Parallel Approaches enables an air traffic controller to 

instruct a properly equipped aircraft to achieve precise spacing relative to two other aircraft in 

order to increase both runway throughput and utilization of Optimized Profile Descents (OPD) 

when parallel dependent approaches are in use. The controller uses advanced scheduling and 

sequencing tools to determine the desired landing runway and sequence. This tool also provides 

the information that must be passed to the aircraft. The flight crew uses onboard automation that 

provides speed guidance to meet the controller’s desired inter-aircraft spacing (Barmore et al.
7 

2012). This section describes two closely related operations labeled Dependent Staggered 

Approaches with One Target (DSA1) and Dependent Staggered Approaches with Two Targets 

(DSA2) in the FAA’s Advanced Interval Management Concept of Operations (FAA 2014). 

For independent parallel operations to be conducted where the lateral separation between the 

runways is less than 9000 ft but greater than or equal to 4300 ft, a 4.8 second radar update rate is 

required, a designated ATC monitoring position must be in use, and a No Transgression Zone 

(NTZ) of 2000 ft must be displayed between the centerlines of the parallel runways on the 

controller radar display. For independent parallel operations occurring on runways with 

centerline separation between 4300 ft and 3400 ft, the aforementioned conditions must be met, 

with the addition that a precision runway monitoring (PRM) system must be in use. Finally, 

independent operations occurring on runways with down to 3000 ft centerline separation may be 

achieved with the aforementioned conditions, with the addition of PRM radar display with a 1 

second update rate and localizers that are offset by at least 2.5 degrees (Barmore et al. 2012). 

Independent operations on parallel approaches are often preferred as they provide generally 

greater throughput. Dependent operations, however, have fewer infrastructure requirements, but 

specific separation between the aircraft proceeding to different runways is now required. For 

dependent operations to runways with more than 4300 ft lateral runway separation, a minimum 

distance of 2 nmi must be maintained between aircraft established on parallel approaches. This is 

in addition to the in-trail wake separation requirements for common runway operations. For 

runways with lateral spacing between 2500 ft and 4300 ft, a distance of 1.5 nmi must be 

maintained. Again, the in-trail wake separation requirements may require greater separation 

(Barmore et al. 2012). 

The upper diagram in Figure 5 shows the typical case of high-demand operations for dependent 

parallel runways. For a given aircraft, there is an aircraft at approximately the separation 

minimum directly in front of them going to the same runway and another aircraft on the parallel 

approach that is “in-between” the first two. In this case, the two-target operation should be used. 

The spacing aircraft will space to achieve the larger spacing between the two requirements (intrail and stagger). If the same-runway leading aircraft is far enough in front of the aircraft of 

consideration that separation will be easily met, then a one-target stagger operation can be used 

(lower left diagram in Figure 5). If the aircraft on the parallel approach is further ahead than the 

same-runway leading aircraft (lower right diagram in Figure 5) so that the stagger separation 

requirement does not apply, then the typical in-trail spacing operation can be used. 
8 

Figure 5: One-Target vs. Two-Target Dependent Parallel Airborne Spacing Operations (Smith 2012). 

In two-target operations, a distinction is made between a primary target and a secondary target. 

When determining this, the spacing aircraft calculates a resultant spacing position for each 

spacing aircraft. The aircraft whose spacing interval results in a spacing position further back is 

defined as the primary target, while the secondary target’s spacing interval results in a closer 

spacing position. The spacing aircraft precisely meets the spacing interval of the primary aircraft, 

while exceeding or meeting the spacing interval of the secondary aircraft. This is illustrated 

below in Figure 6 (Smith 2012). 

Figure 6: Primary vs. Secondary Targets while Conducting Dependent Parallel Airborne Spacing Operations 

(Smith 2012). 

Smith 2012 found that a secondary target was a necessity in order to maintain safe separation 

between both the primary target aircraft and the secondary target aircraft. These violations 

occurred specifically because the spacing between an aircraft and its secondary target was not 

being monitored. As well, it was observed that the addition of a secondary target did not appear 

to change the stability of the operations. 

Operational Scenario 

As previously mentioned, Smith concludes that, for safety reasons, if dependent parallel 

operations are conducted while using Interval Management, the aircraft performing the spacing 

must monitor both a primary and secondary target. In the example scenario, see Figure 7, the 
9 

scheduling and sequencing automation tools determines that aircraft J must actively monitor and 

perform dependent spacing operations on aircraft I and aircraft H. In this scenario, the controller 

provides a clearance to aircraft J to achieve 130 second spacing behind aircraft H and to achieve 

2.2 nmi spacing from aircraft I (Barmore e al. 2012). Although the clearance is given in terms of 

distance, this clearance could have also been given in terms of time. 

The crew would acknowledge the receipt of the clearance and enter this information into the 

flight deck automation equipment. An IM speed is presented to the crew, who will follow the 

speed guidance. The speeds given to the crew will satisfy the greater separation distance between 

the two targets (i.e., the speeds given will satisfy achieving spacing behind the primary target). In 

this example, the primary target for aircraft J is aircraft H (Barmore et al. 2012). 

If aircraft J only required spacing of 90 seconds behind aircraft H and 2.2 nmi behind aircraft I, 

then aircraft I would be the primary target, and speeds would be given to the flight crew that 

satisfy the 2.2 nmi spacing from aircraft I (Barmore et al. 2012). 

Figure 7: Schematic of two-target dependent parallel airborne spacing operation. The spacing clearance is 

issued at time t1 (blue aircraft). Time t2 (green aircraft) shows the relative positions when the first target 

(aircraft H) reaches the runway threshold (Barmore et al. 2012). 
10 

Interval Management with Wake Mitigation 

Current System 

A brief description of the current wake separation standards and newly introduced wake vortex 

separation rules is given in this section. 

Under the current FAA regulations, the Air Traffic Control imposes wake spacing standards for 

IFR aircraft based on maximum certified takeoff weight (FAA AC 90-23G). The separation 

standards and weight categories are listed in Tables 1-2. Although B757 is in the large weight 

category, a special wake separation standard has been set for it. 

Table 1: FAA Wake Separation Standards (At the Threshold) Leader 

Follower (Nautical Miles) 

Super Heavy B757 Large Small 

Super MRS 6 7 7 8 

Heavy MRS 4 5 5 6 

B757 MRS 4 4 4 5 

Large MRS MRS MRS MRS 4 

Small MRS MRS MRS MRS MRS 

MRS = Minimum Radar Separation 

Table 2: Aircraft Weight Classes 

Category Weight

Heavy MTOW ≥ 300,000lbs 

Large 41,000lbs < MTOW < 300,000 

Small MRS ≤ 41,000lbs 

The “super” category has been approved on an interim basis and at present consists of two 

aircraft – Airbus A380 and Antonov AN225. 

To alleviate the capacity constraints imposed by the current wake separation standards, the FAA 

has suggested a phased approach to re-categorize the standards. FAA’s Wake Recategorization 

Program consists of three implementation phases: 
11 

• RECAT-I Static Six Category Separation 

• RECAT-II Static Pair-wise Separation 

• RECAT-III Dynamic Pair-wise Separation 

New wake separation standards under RECAT-I (Tables 3-4) were recently introduced and are 

described in FAA Order N JO 7110.608. These standards are based on aircraft dynamics 

parameters (wingspan, final approach speed, and roll moment capability) in addition to the 

maximum takeoff weight (MTOW). The effort also took into account the traffic mix at five US 

airports (Atlanta, Newark, John F. Kennedy, Chicago O’Hare, and San Francisco) and four 

European airports (Heathrow, Amsterdam, Frankfurt, and Charles de Gaulle) to further optimize 

the re-categorization. Enhancements to Terminal Proximity Alert (TPA) software are also 

underway to aid controllers. 

RECAT-I has been implemented at four airports: Memphis International Airport and Louisville 

International Airport. Implementation at other airports (Cincinnati, Miami, and Charlotte) is 

planned and work is underway on RECAT-II which is expected to go into effect by 2016. 

Table 3: RECAT-I Wake Separation Standards Leader 

Follower (Nautical Mile) 

A B C D E F 

A MRS 5 6 7 7 8 

B MRS 3 4 5 5 7 

C MRS MRS MRS 3.5 3.5 6 

D MRS MRS MRS MRS MRS 5 

E MRS MRS MRS MRS MRS 4 

F MRS MRS MRS MRS MRS MRS 

MRS = Minimum Radar Separation 

Table 4: RECAT-I Weight Categories 

Category Weight Wingspan 

A MTOW ≥ 300,000lbs b > 245ft 

B MTOW ≥ 300,000lbs 175ft < b ≤ 245ft 

C MTOW ≥ 300,000lbs 125ft < b ≤ 175ft 

D 

MTOW < 300,000lbs 125ft < b ≤ 175ft 

or aircraft with: 90ft < b ≤ 125ft 
12 

E MTOW > 41,000lbs 65ft < b ≤ 90ft 

F 

MTOW < 41,000lbs b ≤ 125ft 

or aircraft capable of a MTOW less than 15,500lbs regardless of 

wingspan, or a powered sailplane 

Wake Turbulence Mitigation for Departures (WTMD) is another recently introduced rule that 

allows reduced separation behind Heavy/B757 for CSPR operations (FAA Order JO 7110.316). 

WTMD depends on current and forecasted weather conditions and allows runways to operate as 

wake independent from Heavy/B757 under favorable crosswind conditions (Figures 8-9). 

Figure 8: Transport of wake due to crosswinds (FAA AC 90-23G). 

Figure 9: Reduced separations under WTMD (FAA AC 90-23G). 
13 

Justification for Change 

Wake vortex based separation standards are a major constraint to the airspace capacity. A 

reduction in wake spacing standards while maintaining the current levels of operational safety 

will substantially improve the efficiency and capacity of the NAS. A wake-informed IM 

procedure can further increase the traffic throughput. 

New Concept 

As aircraft approach an airport, a scheduling tool in the ground automation predicts the expected 

wake separation distance needed and schedules the aircraft appropriately. Standard Interval 

Management operations are then used to allow the aircraft to space precisely at the predicted 

wake separation requirement. Supporting the ground scheduling tool is a real-time wake 

modeling tool. Dynamic wake separation standards can be set based on wake models (Proctor 

2009; Robins et al. 2001; Ahmad et al. 2014). The following steps lead to the scheduling of the 

aircraft: 

1. Weather forecast (approximately one hour in advance) for providing environmental 

initial conditions to the fast-time wake models. These initial conditions include 

crosswinds to predict the transport of the wakes and the level of ambient atmospheric 

turbulence and stratification to predict the decay of wake vortex circulation strength ( Γ ). 

2. Calculation of an aircraft response metric (e.g., v Cl , the vortex-induced rolling moment 

coefficient) based on wake circulation strength. 

3. Estimation of time/distance separation between aircraft pairs based on the model output. 

4. The scheduler uses the model-based time separation estimates to schedule aircraft in its 

queue. 

Figure 10 shows the flow of information needed by the scheduler. The scheduler requires 

aircraft data (route, weight, aircraft type, and planned final approach speed), and surveillance 

data to set up the queue and issue the FIM clearance constrained by the airport 

acceptance/departure rate. In the new concept, an additional static aircraft database is required 

that includes estimates of characteristics such as wingspan, and the three-dimensional lift curve 

slope ( Lα C ). 
14 

Figure 10: Modeling-based dynamic wake separation. The fast-time models prediction of wake transport and 

circulation decay based on weather forecast is used to set the wake separations. 

Meteorological forecast data for the terminal area will be needed approximately one hour in 

advance to initialize the wake models. This data can be provided by the National Weather 

Service (NWS) model forecasts. Additional meteorological data can be obtained via datalink 

from preceding aircraft landing/departing from the airport. The wake models will take the 

aircraft information (route, weight, true airspeed) and the meteorological forecast to predict the 

circulation decay. The scheduler will estimate wake hazard based on an aircraft response model 

(e.g., vortex-induced rolling moment coefficient, Clv) from the wake model prediction and 

update the queue which optimally should result in a time compression. The queue is updated at 

the freeze horizon (30-45 min in advance) based on the wake information. A real-time wake 

warning sensor can be deployed initially to build confidence in the system. For example, pulsed 

lidars can provide estimates of crosswind, turbulence and wake circulation strength. 

Operational Scenario 

Figure 11 shows an operational scenario. Aircraft B follows Aircraft A in the scheduler queue. 

The separation distance for Aircraft B is 5 nmi. Based on the wake model forecasts it is 

predicted that this distance can be reduced to 4.5 nmi. Similar updates are made for all aircraft 

pairs in the queue resulting in a time compression while maintaining the constraints imposed by 

the airport/airspace design such as the acceptance/departure rates. The scheduler update is done 

30-45 min in advance at the freeze horizon. Information from the real-time wake warning 

system is used to update the queue again at 15 min to take into account observed differences 

from the forecast. 
15 

Figure 11: A weather forecast of 30-45min in advance will be required by the scheduler to queue aircraft 

based on dynamic wake separations. 

Open Questions 

Implementation would need to address the following questions: 

 Given the current accuracy/reliability of weather forecasts (time window is 

approximately 1hr), what is the feasibility of the proposed method? 

 What are the computational constraints, if any? A computationally feasible method might 

require building look-up tables based on models and measurements for characterizing 

wake decay and transport under varying atmospheric conditions instead of running the 

models. 

 Availability of aircraft dependent parameters, e.g., Lα C for the aircraft response module. 

Can these parameters be approximated with reasonable accuracy from data available in 

open literature? 

 Current state-of-the-art in wake sensing technology for deploying a real-time wake 

warning system. Pulsed lidars can provide estimates of crosswinds, eddy dissipation 

rates and wake vortex circulation strength. The processing of raw lidar data for 

estimating wake circulation strength is computationally intensive. 
16 

References 

Ahmad, N.N., R.L. VanValkenburg, M. Pruis, “NASA AVOSS Fast-Time Wake Prediction Models: 

User’s Guide,” National Aeronautics and Space Administration, NASA/TM-2014-218152. 

Barmore, B.E., B.T. Baxley, T.S. Abbott, W.R. Capron, C.L. Smith, R.F. Shay, C. Hubbs, “A Concept for 

Airborne Precision Spacing for Dependent Parallel Approaches”, National Aeronautics and Space 

Administration, NASA/TM-2012-217346. 

Eftekari, R.R., J.B. Hammer, D.A. Havens, A.D. Mundra, “Feasibility Analysis for Paired Approach 

Procedures for Closely Spaced Parallel Runways,” Integrated Communications Navigation and 

Surveillance (ICNS) Conference, May 10-12, 2011. 

Federal Aviation Administration, “1.5-Nautical Mile Dependent Approaches to Parallel Runways Spaced 

Less Than 2, 500 Feet Apart,” Order JO 7110.308. Effective Date: November 5, 2008. 

Federal Aviation Administration, “Aircraft Wake Turbulence,” Advisory Circular AC 90-23G. Effective 

Date: February 10, 2014. 

Federal Aviation Administration, “Guidance for the Implementation of Wake Turbulence 

Recategorization Separation Standards at Memphis International Airport,” FAA Order N JO 7110.608. 

Effective Date: November 1, 2012. 

Federal Aviation Administration, “Reduced Wake Turbulence Separation on Departure from Heavy/B757 

Aircraft Departing Parallel Runways, Spaced Less Than 2,500 Feet, Using Wake Turbulence 

Mitigation for Departures (WTMD),” Order JO 7110.316. Effective Date: January 28, 2013. 

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Version 1.0, uppublished. 

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218151. 
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Advanced Interval Management (IM) Initial Concepts of Operations 

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Barmore, Bryan E.; Ahmad, Nash'at N.; Underwood, Matthew C.

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This document provides a high-level description of several advanced IM operations that NASA is considering for future 

research and development. It covers two versions of IM-CSPO and IM with Wake Mitigation. These are preliminary 

descriptions to support an initial benefits analysis. 

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Interval Management; Mitigation; Operations; Runway; Wake

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