March 2012 

NASA/TM–2012-217346 

A Concept for Airborne Precision Spacing for 

Dependent Parallel Approaches 

Bryan E. Barmore and Brian T. Baxley 

Langley Research Center, Hampton, Virginia 

Terence S. Abbott 

Booz Allen Hamilton, Hampton, Virginia 

William R. Capron and Colin L. Smith 

Lockheed Martin Corporation, Hampton, Virginia 

Richard F. Shay and Clay Hubbs 

National Institute of Aerospace, Hampton, Virginia 
NASA STI Program . . . in Profile

Since its founding, NASA has been dedicated to 

the advancement of aeronautics and space science. 

The NASA scientific and technical information (STI) 

program plays a key part in helping NASA maintain 

this important role. 

 The NASA STI program operates under the 

auspices of the Agency Chief Information Officer. It 

collects, organizes, provides for archiving, and 

disseminates NASA’s STI. The NASA STI program 

provides access to the NASA Aeronautics and Space 

Database and its public interface, the NASA Technical 

Report Server, thus providing one of the largest 

collections of aeronautical and space science STI in 

the world. Results are published in both non-NASA 

channels and by NASA in the NASA STI Report 

Series, which includes the following report types: 

TECHNICAL PUBLICATION. Reports of 

completed research or a major significant phase 

of research that present the results of NASA 

programs and include extensive data or 

theoretical analysis. Includes compilations of 

significant scientific and technical data and 

information deemed to be of continuing 

reference value. NASA counterpart of peerreviewed formal professional papers, but having 

less stringent limitations on manuscript length 

and extent of graphic presentations. 

TECHNICAL MEMORANDUM. Scientific 

and technical findings that are preliminary or of 

specialized interest, e.g., quick release reports, 

working papers, and bibliographies that contain 

minimal annotation. Does not contain extensive 

analysis. 

CONTRACTOR REPORT. Scientific and 

technical findings by NASA-sponsored 

contractors and grantees. 

CONFERENCE PUBLICATION. Collected 

papers from scientific and technical 

conferences, symposia, seminars, or other 

meetings sponsored or co-sponsored by NASA. 

SPECIAL PUBLICATION. Scientific, 

technical, or historical information from NASA 

programs, projects, and missions, often 

concerned with subjects having substantial 

public interest. 

TECHNICAL TRANSLATION. Englishlanguage translations of foreign scientific and 

technical material pertinent to NASA’s mission. 

 Specialized services also include creating custom 

thesauri, building customized databases, and 

organizing and publishing research results. 

 For more information about the NASA STI 

program, see the following: 

Access the NASA STI program home page at 

http://www.sti.nasa.gov

E-mail your question via the Internet to 

help@sti.nasa.gov

Fax your question to the NASA STI Help Desk 

at 443-757-5803 

Phone the NASA STI Help Desk at 

443-757-5802 

Write to: 

 NASA STI Help Desk 

 NASA Center for AeroSpace Information 

 7115 Standard Drive 

 Hanover, MD 21076-1320
National Aeronautics and 

Space Administration 

Langley Research Center 

Hampton, Virginia 23681-2199 

 

March 2012 

NASA/TM–2012-217346 

A Concept for Airborne Precision Spacing for 

Dependent Parallel Approaches 

Bryan E. Barmore and Brian T. Baxley 

Langley Research Center, Hampton, Virginia 

Terence S. Abbott 

Booz Allen Hamilton, Hampton, Virginia 

William R. Capron and Colin L. Smith 

Lockheed Martin Corporation, Hampton, Virginia 

Richard F. Shay and Clay Hubbs 

National Institute of Aerospace, Hampton, Virginia 
Available from: 

NASA Center for AeroSpace Information 

7115 Standard Drive 

Hanover, MD 21076-1320 

443-757-5802 
1

Concept for Airborne Precision Spacing for 

Dependent Parallel Approaches 

Table of Contents

Introduction 2

Definitions 3

Concept Overview 4

APS Assumptions and Expectations 7

Assumptions 7

Expected Behaviors 8

PDS Guidance Modes 8

Operational Sequence of Events 10

Spacing Initiation Criteria 12

Procedures 12

Nominal Pair-Wise Spacing 13

Nominal Profile or RTA Operations 14

Off-Nominal Events/Procedures 15

No Valid IM Speed 15

Spacing Goal No Longer Feasible 16

Unacceptable IM speeds 16

PDS Failure 17

Information Requirements 17

Data Element 17

Users 17

Source 17

Example Scenario Descriptions 19

Scenario 1 20

Scenario 2 21

Scenario 3 22

Variations on Scenarios 23

Variation on Scenario 2 23

Variation on Scenario 3 23

Proposed ADS-B Message 23

Summary 25

Acronyms 25

References 26
2

Abstract

The Airborne Precision Spacing concept of operations has 

been previously developed to support the precise delivery of 

aircraft landing successively on the same runway. The highprecision and consistent delivery of inter-aircraft spacing allows 

for increased runway throughput and the use of energy-efficient 

arrivals routes such as Continuous Descent Arrivals and 

Optimized Profile Descents. This paper describes an extension to 

the Airborne Precision Spacing concept to enable dependent 

parallel approach operations where the spacing aircraft must 

manage their in-trail spacing from a leading aircraft on 

approach to the same runway and spacing from an aircraft on 

approach to a parallel runway. Functionality for supporting 

automation is discussed as well as procedures for pilots and 

controllers. An anaylsis is performed to identify the required 

information and a new ADS-B report is proposed to support 

these information needs. Finally, several scenarios are described 

in detail. 

Introduction

One of the major keys to airport landing rate efficiency is the ability to manage the longitudinal 

aircraft-to-aircraft spacing during the arrival and approach. While several new concepts and 

technologies are currently being developed and evaluated to further enhance Air Traffic Control’s 

(ATC) ability to accurately deliver arriving aircraft to the runway threshold, including NASA's 

Airborne Precision Spacing (APS) concept (refs. 1-3), these concepts have primarily focused on 

aircraft landing on the same runway. There are numerous airports, however, that routinely 

conduct parallel approach operations. Depending upon the airport layout, surveillance 

infrastructure and controller staffing, different types of operations can be performed on the 

parallel approaches. Under some situations, described in more detail in the Concept Overview 

section, arrivals to one parallel runway are dependent upon arrivals to the other. The ability to 

support ATC in the precision spacing of aircraft that are performing dependent parallel operations 

is the focus of NASA's next generation adaptation of its APS concept. 

Beyond the requirement to track two Target Aircraft instead of just one, there are several 

challenges in developing an airborne system that accommodates dependent parallel approach 

operations. First, the airborne spacing tool must be able to compensate for runway threshold 

offsets. If the runway thresholds are not aligned, then the desired spacing interval relative to the 

landing runway must be adjusted to obtain the correct aircraft-to-aircraft longitudinal spacing to 

account for the difference in the longitudinal runway threshold distances. Second, the standard 

ATC longitudinal separation minimums change for parallel approach aircraft once both aircraft 

are on their respective final approach paths. In the U.S., the 3 nmi separation between aircraft 

within terminal airspace is reduced to either 1.5 or 2 nmi separation between aircraft on 

dependent parallel approaches, with the specific diagonal distance dictated by the distance 

between the two parallel runways. Assuming that spacing is desired against both common-
3

runway and parallel-runway Target Aircraft, then the airborne spacing system must determine 

which aircraft it is actually spacing against. In all spacing conditions noted to date, this 

determination is based on which of the two spacing requirements results in the greater 

longitudinal distance since it is operationally better to be too far from than too close to another 

aircraft. Being too far behind results in reducing the actual throughput from the theoretical 

maximum while being too close could result in separation violations and a reduction in safety. 

Airborne Precision Spacing is a specific operational application of the class of spacing 

applications called Interval Management (IM). In addition to the work on APS, there is much 

work being done to develop IM operational applications and define standards and requirements. 

Where appropriate, this document uses the language captured in the Safety, Performance and 

Interoperability Requirements Document for Airborne Spacing – Flight Deck Interval 

Management (ref. 4). 

Definitions

Active Target Aircraft – the Target Aircraft that is driving the currently presented IM speed. 

When spacing guidance is being provided relative to two Target Aircraft, PDS will determine 

which Target Aircraft provides the most stringent requirements and use that for the IM speed 

command. 

Airborne Precision Spacing (APS) – the operational concept where a properly equipped aircraft 

is instructed by the air traffic controller to achieve a spacing interval relative to one or two other 

aircraft at the runway threshold. There is no guarantee that the interval will be met prior to the 

threshold; however, the crew should conform to the spacing instruction or notify the controller of 

their inability to conform to the instruction. 

Assigned Spacing Goal – the ATC-assigned interval between aircraft to be achieved at the 

runway threshold. Can be given in time or distance. 

Automatic Dependent Surveillance – Broadcast (ADS-B) – a technology that automatically 

and periodically broadcasts the aircraft’s current position, velocity and other data to receivers on 

the ground or other aircraft. 

IM Aircraft – an aircraft that is performing the airborne spacing (interval management) 

operation. 

IM Speed – the speed presented by the PDS system to the flight crew. 

Ownship – the aircraft being flown by the flight crew. In this document, it is the aircraft 

receiving the spacing clearance from ATC. 

Pair Dependent Speed (PDS) – an onboard system, including automation and crew interfaces, 

that supports the crew in conforming to a spacing instruction. The PDS system calculates the 

airspeed required to achieve the assigned spacing goal using the required time of arrival, 

forecasted and sensed winds, and planned routing and final approach speed of the Target Aircraft 

and of the ownship. 

Required Time of Arrival (RTA) – the time at which an aircraft is assigned to arrive at the 
4

runway threshold. The RTA is used by PDS as an arrival goal time when there are no Target 

Aircraft within ADS-B range. 

Target Aircraft – any aircraft that the spacing operation is performed against. 

Concept Overview 

Airborne Precision Spacing 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. 

The main goal of APS is to enable high throughput operations when both single runway and 

dependent parallel runway operations are in use. The main benefits of APS are expected to be 

seen when there is high demand for the airport resources. By precisely controlling the spacing 

between landing aircraft, APS can be used to maximize runway utilization. The use of APS can 

also enable energy-efficient arrivals such as OPD, Continuous Descent Arrivals (CDA), and 

Green Arrivals by providing small speed changes that keep the aircraft close to the preferred 

descent profile (speed and vertical path) while actively managing the spacing between the 

aircraft. Another goal of APS for Dependent Parallel Approaches operations is to manage, prior 

to both aircraft being on final approach, the along track spacing so that vertical separation 

techniques between arriving aircraft are not needed as these tend to decrease the efficiency gains 

of the energy-efficient arrivals. 

The focus of the current work is on dependent parallel runway operations. However, it is 

expected that this concept of operations could be expanded to work for other types of dependent 

runway operations. 

Parallel runway approaches where the runway lateral separation is less than 9000 ft but greater 

than or equal to 4300 ft can be treated as independent if the following conditions are met: 

a 4.8 second radar update is available; 

a designated ATC monitoring position is in use; and 

a 2000 ft No Transgression Zone is displayed between the centerlines on the controller 

radar display. 

Parallel runway approaches with lateral separation between 4300 ft and 3400 ft, may be treated as 

independent if the above conditions are met plus: 

a Precision Runway Monitor (PRM) system is in use. 

Independent operations may be used down to 3000 ft runway lateral separation with the addition 

of:
5

a 1 second update PRM radar with display; and 

localizers offset by at least 2.5°. 

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

greater throughput. Dependent operations 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. The left side of Figure 

1 shows the case where the in-trail wake separation requirement is less than consecutive 

separation distances for the parallel approaches. The right side shows the case where the in-trail 

wake separation is greater than the consecutive separation distances for the parallel approaches. 

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. 

Since APS is a spacing concept where the controller retains responsibility for detecting and 

correcting possible separation violations, a 0.2 nmi buffer was added to the separation 

requirement to allow the controller time to react. The exact value of this buffer has not been 

determined and would depend upon controller experience, the reliability of the APS equipment, 

and any monitoring and alerting system available to the controller. The value of this buffer has no 

material effect on the concept but would affect the available benefits of performing APS 

operations. 

Although the goal of this concept is to allow simultaneous OPDs to dependent parallel 

approaches, the separation requirements on final approach are not the only constraint. Prior to 

both aircraft being established on their final approach courses, the normal 3 nmi lateral separation 

or 1000 ft vertical separation must be maintained. Requiring vertical separation at the turn onto 

final approach course would require one of the two aircraft to intercept the final approach course 

at least 1000 ft below the OPD altitude. This would require a low-altitude, level flight segment 

which is counter to the efficiency objectives of an OPD. Such level-flight segments use much 

more fuel and create more noise than the OPD segment. Therefore, vertically separating the 

aircraft is undesirable. 

Figure1:Placementofaircraftbasedonparallelapproachandintrailseparationrequirements.
6 

Figure 2 shows the expected lateral distance between aircraft, blue circles and green triangle, as 

the trailing aircraft turns onto the final approach course. The respective goal at the runway 

threshold is shown by the blue and green lines. The calculations are for the aircraft flying 200 kt 

at the turn onto final and a final approach speed of 135 kt. For runways separated by more than 

3200 ft, the trailing aircraft would have to violate the 3 nmi separation standard prior to turning 

on to final in order to achieve the desired spacing of 1.7 nmi or 2.2 nmi. Figure 3 shows the 

achievable spacing relative to the aircraft on the parallel approach when the separation at the turn 

onto final approach course, 3.2 nmi, is used as the delivery constraint. For runways separated by 

between 2500 ft and 4300 ft, the achievable spacing is very close to what the expected spacing 

goal would be (2 nmi separation requirement plus a 0.2 nmi buffer). For runways separated by 

more than 4300 ft, the achievable spacing is at least 0.5 nmi greater than desired (1.5 nmi 

separation requirement plus the 0.2 nmi buffer). Therefore, for runways separated by greater than 

4300 ft, either OPD benefit must be reduced to allow for vertical separation prior to both aircraft 

being established on their final approach courses, or potential throughput is lost to ensure lateral 

separation prior to both aircraft being established on their final approach courses. Which choice is 

more beneficial requires specific knowledge of the operating environment and the OPD design. 

Figure 2: Distance between aircraft when the trailing aircraft turns onto its

final approach course when achieving the minimum spacing for the parallel

approach at the runway threshold.
7

To support the use of APS, a new onboard guidance system has been developed called PairDependent Speed (PDS). PDS allows for the selection of two Target Aircraft and associated 

spacing goals and provides IM speeds to achieve the tightest spacing possible that is consistent 

with the spacing goals. 

APS Assumptions and Expectations 

This concept of operations is based upon the following fundamental assumptions and expected 

behavior. Changes to these assumptions and expectations would likely lead to changes to the 

human-machine interfaces, data exchange, or procedures. Assumptions capture the assumed 

infrastructure and capabilities available to support APS. The expected behaviors section captures 

the behaviors and actions of the controller and flight crew from which the procedures are 

developed. 

Assumptions 

An IM Aircraft can have only one active spacing clearance. The identification of a 

second Target Aircraft is an amendment to the spacing clearance and not a separate 

clearance. 

The assigned spacing goal behind an aircraft proceeding to a parallel runway is given in 

distance, and is based upon the distance between runway centerlines. 

The assigned spacing goal behind an aircraft to the same runway is given in time, and is 

based upon the weight category of both aircraft and the forecasted winds. 

Figure3:Achievedspacingattherunwaythresholdwhenlateralseparationis

appliedwhenthetrailingaircraftturnsontoitsfinalapproachcourse.
8

Information to build a four-dimensional (4D) trajectory for the Target Aircraft, including 

route and planned final approach speed, is available via ATC voice radio communication, 

an on-condition ADS-B report, or Controller Pilot Data Link Communications (CPDLC). 

APS status information is broadcast via a new on-condition ADS-B report and is 

available to the controller for monitoring. [Note: A proposal for this report is made in the 

section “New ADS-B Message.”] 

ATC uses a scheduling and sequencing tool to provide a feasible arrival sequence for the 

pair of runways, and sufficient spacing between aircraft to meet all separation 

requirements (wake vortex, dependent parallel operations, etc). 

At some point prior to starting spacing operations, the schedule and sequence are frozen. 

Once the spacing operations have started, further modifications to the sequence are 

considered undesirable. 

The scheduling and sequencing tool provides the controller with the information that 

must be passed to the flight crew as part of the spacing clearance including Target 

Aircraft identification, assigned spacing goals and a Required Time of Arrival (RTA) at 

the runway threshold. 

The PDS system will provide IM speeds to achieve the RTA until sufficient surveillance 

information is available for at least one Target Aircraft. At that point, the PDS system 

will provide IM speeds to achieve the assigned spacing goal. 

Information is made available to the flight crews through the Automatic Terminal 

Information Service (ATIS) or other methods that parallel approach operations and 

spacing operations are in effect at the airport. 

Expected Behaviors 

The controller issues the initial spacing clearance prior to the aircraft’s top of descent. If 

needed, amendments may occur later in the operation. 

The controller includes at least the Target Aircraft identification, the assigned spacing 

goal, and an RTA. 

The controller includes a second Target Aircraft only if it is operationally relevant. 

The controller issues navigation clearances that are complete paths to their respective 

runway thresholds for all participating aircraft prior to initiating the spacing operations. 

The flight crew will follow the IM speed in a timely manner. 

PDS Guidance Modes 

The PDS system has five internal modes that determine how the IM speed is calculated. 

Depending on the circumstance, some modes may transition to others. A state diagram is included 

in Figure 4. 
9

PROFILE 

is active only when explicitly selected 

provides IM speeds that exactly match the published speeds on the intended route 

does not deviate from the published speeds to achieve an RTA or spacing interval

RTA

is active when RTA-only is selected 

is active at initiation of a spacing operation if the Target Aircraft data are not valid 

provides IM speeds to achieve the RTA at the runway threshold 

PAIRED

is active when a Target Aircraft is selected and that aircraft’s surveillance data are 

received and are valid 

provides an IM speed to achieve the assigned spacing goal relative to the Target Aircraft 

at the runway threshold 

FINAL 

is active when the aircraft must start its final deceleration to ensure a stabilized approach 

within the specified constraints 

provides IM speed that matches the planned final approach speed 

SPEED REVERSION 

is a fail-safe mode if PDS is unable to provide a valid IM speed 

provides the lesser of the published speed and last IM speed if the target data become 

invalid

provides the last IM speed if the ownship data becomes invalid. 
10 

Figure4:StatelogicdiagramforPDS.

Operational Sequence of Events 

A typical spacing operation consists of the following sequence of events: 

1. As aircraft approach the destination airport, an arrival sequence of aircraft to the airport is 

generated by the controller’s automation. This includes runway and arrival route 

assignments and scheduled times of arrival to the runways. An aircraft is added to the 

sequence approximately 60-90 minutes prior to landing. 

2. The controller’s automation fixes the arrival sequence and runway assignment of a 

particular aircraft approximately 20 minutes prior to the aircraft’s top of descent. 

3. ATC issues an arrival clearance. 

4. ATC decides to use airborne spacing operations for an aircraft. 

5. ATC issues clearance(s) for airborne spacing. There are three types of airborne spacing 

clearance that can be given: 

Standard Profile – Aircraft uses PDS to fly the published profile. No additional 
11 

information is needed1.

Pair-wise Spacing – Aircraft uses PDS to achieve a pair-wise interval at the runway 

threshold. This is the expected operational mode of PDS. The controller must include 

the Target Aircraft identification (ID), the assigned spacing goal and type, and an 

RTA in the clearance. 

Required Time of Arrival – Aircraft uses PDS to achieve a required time of arrival at 

the runway threshold2

. ATC includes an RTA in the clearance. 

6. Crew enters the spacing clearance data into PDS. 

7. PDS calculates the IM speed. 

8. PDS provides feasibility feedback to the flight crew. 

9. Crew accepts or rejects the IM clearance from ATC. 

10. If accepted, the crew implements IM speed. If rejected, the crew continue flying the 

current speed. 

11. The crew actions for IM operations are: 

fly the arrival procedure and monitor for conformance; 

fly the IM speed; 

notify ATC once PDS couples to each of the Target Aircraft; and 

monitor PDS for notifications or alerts. 

12. ATC actions during IM operations are: 

provide separation assurance from all traffic; 

terminate IM if required; and 

identify additional Target Aircraft, if needed (repeat steps 5-8, excluding the RTA). 

13. ATC issues descent clearances as able. 

14. When necessary, the controller hands off aircraft to the next sector controller along with 

information on the clearance and spacing status. This information could include the IM 

 

1

 This would be very similar to the flight crew following published procedure as in current-day operations. The added 

benefit of using PDS is that the flight crew would receive guidance to achieve a stabilized approach. 

2

 PDS uses the same trajectory calculation and speed logic to meet an RTA as to do pair-wise spacing. This capability 

allows RTAs to the runway threshold regardless of Flight Management System capability. 
12 

Aircraft ID, the Target Aircraft ID, the assigned spacing goal and type, and other 

operationally relevant information. 

15. Crew and ATC manage off-nominal events. 

16. Controller issues approach clearance. 

17. Controller issues landing clearance. 

18. PDS transitions to FINAL mode and ends active spacing guidance. 

19. Procedure terminates at the runway threshold. 

Spacing Initiation Criteria 

In order to initiate APS, the criteria listed in Table 1 must be met. These criteria would be applied 

independently to both Target Aircraft if two Target Aircraft are to be used. 

Table1:APSInitiationCriteria.

Criteria Justification

IM Aircraft and Target 

Aircraft runway assignments 

are fixed. 

Needed to support assignment of arrival and approach 

procedures to the runway. 

IM Aircraft and Target 

Aircraft are cleared for 

published arrivals that extend 

to the assigned runway. 

Need to have aircraft on known arrival routes that extend to 

the runway so that reasonably accurate trajectory calculations 

can be done by both the ground scheduling tool and by PDS. 

Predicted sequence is stable. Need to know the expected arrival sequence for both runways 

to assign appropriate Target Aircraft and type of spacing. 

Changes to the sequence during spacing operations are 

possible, but could be disruptive and are, hence, discouraged. 

Desired spacing between the 

IM Aircraft and Target 

Aircraft is known. 

Need to consider all influences on the spacing goal such as 

wake turbulence, separation requirements, runway utilization, 

and arrival times. 

RTA for the IM Aircraft is 

known. 

Needed as part of the spacing clearance to enable stable 

operations when a Target Aircraft is beyond ADS-B range of 

the IM Aircraft. 

Procedures

The following are proposed procedures to support APS. It should be noted that there may be 
13 

multiple ways to achieve the same goal, but only one method is described in these procedures. 

Nominal Pair-Wise Spacing 

1. The controller determines to issue a spacing clearance. 

2. The controller ensures that the initiation criteria are met. 

3. The controller issues a spacing clearance to the IM Aircraft. The clearance includes the 

Target Aircraft ID, the assigned spacing goal, and the RTA at the runway threshold. 

4. The flight crew enters the information from the spacing clearance into the PDS. 

5. The flight crew enters the ownship’s planned final approach speed and verifies it in PDS. 

6. The flight crew accepts or rejects clearance. 

7. The flight crew follows the IM speed. 

8. If the data for the Target Aircraft (e.g., ADS-B position and trajectory information) are 

valid, the procedure goes to step 11. 

9. PDS presents IM speed to achieve the RTA. 

10. The flight crew follows IM speed. 

11. As PDS receives valid data for each Target Aircraft and couples to them, the crew 

notifies ATC that they have coupled to that target. 

12. If the controller expects the crew to have begun spacing, based on observed range 

between aircraft and ADS-B performance, and the crew has not reported beginning 

spacing, the controller queries the crew regarding their spacing status. The controller may 

cancel the spacing clearance. 

13. When the Target Aircraft data are valid, PDS presents IM speed. The RTA part of the 

clearance is no longer valid. 

14. The flight crew follows IM speed based on paired aircraft. 

15. The flight crew follows normal arrival procedures as cleared, with the exception of using 

IM speed instead of following the published arrival procedure speeds. 

16. The flight crew follows IM speed and makes adjustments to speed and aircraft 

configuration. 

17. The controller monitors the aircraft to ensure separation from all traffic and ensure 

efficient flow of traffic. 

18. If needed, the controller issues a spacing clearance amendment. The amended clearance 
14 

includes the Target Aircraft ID and the spacing interval of the second aircraft. No RTA is 

given with an amended clearance. 

19. The controller monitors the spacing data to ensure conformance and expected behavior. 

20. PDS commands final approach speed to enable a stable approach by 1000 ft above 

ground level (AGL). 

Sample phraseology: 

ATC: NASA 557, Denver Center, spacing clearance, advise when ready to copy. 

NASA557: Denver Center, NASA 557, ready to copy. 

ATC: NASA 557, cross runway seventeen right at 10:10:43. When able, cleared IM 

Spacing 120 seconds with UAL122. Report commencing IM spacing. 

NASA557: Cross runway seventeen right at 10:10:43, when able cleared for IM spacing

120 seconds with UAL122, NASA557. 

.

---- later time ---- 

.

NASA557: Denver Center, NASA557 has begun IM spacing with UAL222. 

ATC: NASA557, roger. 

Nominal Profile or RTA Operations 

1. The controller, using supporting automation, determines to issue a standard arrival profile 

or RTA clearance. 

2. The controller ensures that initiation criteria are met noting that criteria related to Target 

Aircraft do not apply. 

3. The controller clears the aircraft for a standard arrival profile or an RTA at the runway 

threshold. The message includes the RTA value, if applicable. 

4. The flight crew enters or verifies information in PDS. 

5. The flight crew enters or verifies their planned final approach speed in PDS. 

6. PDS presents IM speeds to meet the RTA or matches the standard profile. 
15 

7. The flight crew accepts the clearance. 

8. The flight crew follows normal arrival procedures with the exception of using the IM 

speed.

9. The flight crew follows speeds from the PDS system and makes adjustments to speed and 

aircraft configuration. 

10. The controller monitors aircraft to ensure separation from all traffic and ensure efficient 

flow of traffic. 

11. PDS commands final approach speed to enable a stable approach by 1000 ft AGL. 

Off-Nominal Events/Procedures 

Several situations could necessitate the use of off-nominal procedures. The expected resolutions 

are provided in the subsequent text with mitigation methods that are included in either PDS or the 

procedures.

No Valid IM Speed 

The lack of speed guidance occurs when PDS is unable to produce a valid speed for the crew. 

This condition can occur due to the following situations: 

no ADS-B data is received from a Target Aircraft; 

received data is of insufficient quality; 

no intent information available for Target Aircraft or ownship; 

PDS is unable to construct a 4D trajectory from the intent data for either Target Aircraft 

or ownship; 

Target Aircraft or ownship is out of conformance with the intent information; 

no spacing goal has been entered; 

there is a hardware failure; or 

no RTA has been entered and there is no ADS-B data from the Target Aircraft. 

In this case, the procedures are: 

PDS alerts the crew of loss of IM speeds. This alert would include the source of the 

problem, such as “ownship data invalid,” or target ID of invalid data. 

The flight crew flies current speed or published speed, whichever is less. 

The flight crew advises ATC of failure that includes the appropriate Target Aircraft ID if 
16 

provided by PDS. 

The controller determines if the canceled spacing clearance should be replaced by 

another. Particularly in the case of spacing relative to two Target Aircraft, it may still be 

useful to have the IM Aircraft space relative to the remaining Target Aircraft. However, 

the spacing clearance for two Target Aircraft is a single clearance, so a new clearance for 

the single Target Aircraft must be issued. 

The controller issues a new clearance to the crew or resumes non-spacing operations. 

Spacing Goal No Longer Feasible 

The predicted spacing at the runway is continually compared to the spacing goal. If the deviation 

is larger than the conformance bounds, PDS will alert the pilot that there is a high-probability of 

not being able to achieve the assigned goal. The conformance tolerance bounds decrease as the 

aircraft gets closer to the runway. A 30-second deviation might be acceptable at 70 nmi, it would 

not be acceptable at 20 nmi. The IM speed remains active. 

If this occurs at initiation of paired spacing: 

the crew verifies that the Target Aircraft ID, the spacing interval and the RTA match the 

clearance. 

If the spacing information matches the clearance, the crew notifies ATC they are unable 

to accept the spacing clearance. 

If the spacing information differs from the clearance, the flight crew corrects the data in 

PDS and reevaluates the IM speed. 

If crew has already started IM spacing: 

the crew continues following IM speed; and 

the crew advises ATC that they are outside of the PDS conformance range. 

The controller may cancel spacing on that Target Aircraft, cancel all spacing, or modify 

the spacing interval on the current Target Aircraft. 

Unacceptable IM speeds 

The crew may determine that the IM speed is unacceptable. This situation exists if the crew 

determines that the current IM speed is inconsistent with safety of flight. This may be due to 

changing atmospheric conditions, e.g., turbulence, aircraft configuration, or other concerns.

To limit the occurrence of unacceptable speeds, PDS applies the following speed limitations: 

not greater than the current maximum maneuver speed of the aircraft; 

not greater than 250 kt when below 10,000 ft; 
17 

not greater than 110% of the published speed for the current route leg; and 

not less than 90% of the published speed for the current route leg. 

If this occurs, the procedures are: 

The flight crew selects a safe speed. 

The flight crew notifies the controller that they are unable to continue the spacing 

clearance and provide the reason for this deviation. 

ATC determines whether to cancel spacing or to allow the crew to maintain their current 

speed and resume spacing when able. 

PDS Failure 

If the PDS system should fail for a reason not identified above, it will terminate all spacing and 

alert the crew. 

The flight crew flies current speed or published speed, whichever is less. 

The flight crew notifies ATC that they must terminate spacing. 

The controller issues a new clearance, or resumes non-spacing operations. 

Information Requirements 

Table 2 lists the data elements, users, source, and whether it is required (‘R’), expected (‘E’) or 

desired (‘D’). Required data are necessary for the operation to occur as described here, including 

safely reacting to non-normal events. Expected data are required to achieve the expected 

outcome; however, if these data are unavailable a small percentage of the time, the operation can 

continue with less than optimal results. Desired data are not required to achieve the desired 

results, but are useful to maintain operational efficiency during non-normal events. 

Table2:DataElements

Data Element Users Source 

Ground

Automation

Controller Flight

Crew 

PDS

Flight plan including 

runway assignment for 

all relevant traffic 

E R Existing ground 

automation

Aircraft position for all 

relevant traffic 

R R E Radar or ADS-B 
18 

Forecast of winds and 

temperatures aloft 

E D E Ground-based 

system 

Airport configuration E R Existing ground 

automation

Runway allocation and 

other requirements (e.g., 

scheduled departures, 

configuration changes, 

closures) 

E E Tower scheduler, 

if available 

Inter-aircraft spacing 

requirements 

E R Standard

procedures or 

controller

automation

Planned arrival sequence, 

and schedule 

E R Ground 

automation, if 

available 

Aircraft capabilities such 

as Required Navigation 

Performance level, ADSB, APS, etc. (all traffic 

aircraft) 

E R Part of filed flight 

plan or existing 

ground automation 

For IM Aircraft, first 

Target Aircraft and 

spacing interval 

E R R R Ground 

automation or 

controller

For IM Aircraft, RTA at 

runway

R R R R Ground

automation or 

controller

For IM Aircraft, second 

Target Aircraft and 

spacing interval 

E E E E Ground 

automation or 

controller

Ownship and Target 

Aircraft intent including 

planned final approach 

speed

D R ADS-B from 

Target Aircraft or 

data link from 

ground

List of eligible ADS-B 

aircraft

 D R ADS-B

Real-time winds and 

outside temperature aloft 

E Ownship sensors 
19 

from ownship sensors

Real-time winds and 

outside temperature aloft 

from ground source or 

other traffic 

 D ADS-B (for other 

aircraft) or data 

link from the 

ground 

Feasibility of spacing 

clearance 

E Calculated in PDS 

Aircraft speed limits E Ownship 

IM speed R PDS

PDS guidance mode R PDS 

Spacing deviation trend R PDS

Spacing status including 

Target Aircraft, spacing 

interval, current interval, 

other flags 

D D R PDS

The ownship and target-aircraft intent constitute the information needed to reasonably calculate 

an estimated time of arrival at the runway. This information is expected to include route 

information, modifications for different cruise or descent speeds and the final approach speed. 

For this concept, route information on the ADS-B is a named arrival route, including the 

approach and its transition. This translates into a path using an onboard database. 

Example Scenario Descriptions 

This section includes several scenarios that explore the proposed procedures. All of these are 

based on the following conditions: 

Airport KZYX has two parallel runways, 9R and 9L, which are 3400 ft apart. The runways are 

currently handling both arrivals and departures. Dependent operations are in use and require a 

minimum of 1.5 nmi distance between aircraft on the parallel approaches. Normal wake and radar 

separation is required for aircraft that are in-trail. Three nautical mile radar separation is required 

prior to both aircraft being established on the final approach course during parallel operations. All 

arrival routes are OPD so lateral separation is being provided prior to the aircraft being 

established on their final approach courses. The controller managing the arrival stream has access 

to a scheduling and sequencing tool that provides suggestions for pairing aircraft for airborne 

spacing operations. This tool also provides the necessary information for the spacing clearances. 

The controller could be presented with a timeline that would also provide the necessary 

information for initiating an IM operation. That information includes which aircraft to pair 

together, whether they will be conducting single or dependent runway spacing, the appropriate 

assigned spacing goals, and the RTA at the runway threshold. 
20 

Each scenario description is accompanied by a schematic diagram showing the situation when the 

spacing clearance is issued and when the first Target Aircraft arrives at the runway threshold. The 

spacing clearance is issued at time t1; the aircraft are shown in blue. The assigned spacing goal is 

given by the text accompanying the arrow pointing from the spacing aircraft to the Target 

Aircraft. The aircraft are repeated at time t2, this time colored green, when the Target Aircraft 

reaches the runway threshold. The schematic is not drawn to scale but is to illustrate the relative 

position and timing of the aircraft. 

Scenario 1 

Aircraft H and G are arriving along the same route to runway 9R (Figure 5). Aircraft H has 

reached the point prior to its top of descent where spacing clearances are normally issued. The 

controller checks their automation display and sees the suggestion to have aircraft H space 100 s 

relative to aircraft G. The 100 s spacing corresponds to at least 3.2 nmi at the closest point of 

approach. The controller clears aircraft H to space relative to aircraft G at 100 s along with an 

RTA of 12:05:50. The RTA of 12:05:50 correlates to the RTA of aircraft G plus 100 s. The crew 

acknowledges and enters the information into their PDS system. This entry includes verifying 

their planned final approach speed. PDS is able to calculate a spacing interval on aircraft G and 

determines that spacing is feasible. The crew accepts the clearance. The crew follows the IM 

speeds presented and implements new speeds as the IM speed changes. The crew configures the 

aircraft as necessary to maintain the IM speeds. Since the Target Aircraft was within ADS-B 

range at the time of the clearance, the RTA was never used by the PDS system. As the aircraft 

transition from sector to sector, each receiving controller is able to see that the IM and Target 

Aircraft are conducting spacing operations. The controllers can call up the active spacing 

Figure5:Schematicofintrailspacing(scenario1).Thespacingclearanceisissuedattimet1(blueaircraft).Timet2

(green)showstherelativepositionswhentheTargetAircraft(G)reachestherunwaythreshold.
21 

clearance and the status information from the aircraft for monitoring. Aircraft H eventually 

crosses the runway threshold at 12:06:03 which is 98 s after aircraft G crossed the threshold 

(12:04:25). This is 13 s after the scheduled time (RTA) but is within tolerances for the interaircraft spacing. 

Scenario 2 

As aircraft I approaches its top of descent point (Figure 6), the controller sees the 

recommendation for aircraft I, which is going to runway 9L, to space 2.2 nmi relative to aircraft 

H. A spacing of 2.2 nmi at the runway threshold will give at least 3.2 nmi prior to each aircraft 

being established on final. The 3.2 nmi spacing requirement prior to final approach is now the 

limiting factor on how close aircraft I can get to aircraft H. Note that there is no arrival 

immediately in front of aircraft I going to runway 9L. The controller instructs aircraft I to space 

2.2 nmi relative to aircraft H with an RTA of 12:06:40. At the expected final approach speeds of 

aircraft H and I, the scheduling tool estimates the 2.2 nmi spacing will be equivalent to 50 s. 

Therefore, aircraft I’s RTA is aircraft H’s 12:05:50 plus 50 s. The crew acknowledges and enters 

the information into their PDS system. This entry includes verifying their planned final approach 

speed. Aircraft H is outside ADS-B range; therefore, the PDS system provides speeds to achieve 

the RTA and the crew notifies the controller they are managing to the RTA. A few minutes later 

aircraft H comes within ADS-B range and the PDS system on aircraft I is able to transition to 

paired spacing. PDS is now providing speeds to achieve the 2.2 nmi relative to aircraft H and is 

no longer providing guidance relative to the RTA. The crew is notified that PDS guidance is now 

based on aircraft H. The crew informs the controller that they have begun spacing relative to 

aircraft H. Aircraft I is 2.2 nmi from aircraft H as aircraft H reaches the runway threshold. 

Figure6:Schematicofspacingtoaparallelapproach(scenario2).Thespacingclearanceisissuedattimet1(blue

aircraft).Timet2(green)showstherelativepositionswhentheTargetAircraft(H)reachestherunwaythreshold.
22 

Scenario 3 

As Aircraft J approaches its top of descent (Figure 7), the controller’s automation tool determines 

that aircraft J should perform dependent spacing operations relative to both aircraft H and I. The 

controller therefore clears aircraft J to space relative to aircraft H to achieve 130 s spacing and 

relative to aircraft I to achieve 2.2 nmi spacing with an RTA of 12:08:00. The RTA time is based 

on 130 s after the RTA for Aircraft H which is predicted to be a greater distance than the two 

distances for the parallel approaches: H to I and I to J. The crew acknowledges and enters the 

information into their PDS system. This entry includes verifying their planned final approach 

speed. Aircraft H is a valid Target Aircraft and PDS determines spacing is feasible. IM speed is 

presented to the crew who implements that speed and notifies ATC they have begun spacing 

relative to aircraft H. The crew follows the IM speed and configures the aircraft as necessary. 

PDS provides speeds to achieve the greater of the 130 s relative to aircraft H and the 2.2 nmi 

relative to aircraft I. In this example aircraft J is 2.5 nmi from aircraft I when aircraft I crosses the 

threshold of 9L and crosses the threshold of 9R 134 s after aircraft H. 

Figure7:Schematicofcombinedintrailandparallelapproachspacing(scenario3).Thespacingclearanceisissued

attimet1(blueaircraft).Timet2(green)showstherelativepositionswhenthefirstTargetAircraft(H)reachesthe

runwaythreshold.
23 

Variations on Scenarios 

In this section, variations on the above scenarios will be discussed. 

Variation on Scenario 2 

When aircraft I receives its spacing clearance, the crew reports that they are conforming to the 

RTA. Not reporting that they are spacing relative to aircraft H implicitly informs the controller of 

this situation via the procedural reporting requirement. The controller monitors the spacing data 

coming across ADS-B and notices that the crew has selected the wrong Target Aircraft. The 

controller contacts the crew and clarifies the spacing instruction. The crew then selects the proper 

aircraft and is able to begin spacing. 

If the controller did not have the status information from the aircraft, the controller would need to 

wait until the spacing and Target Aircraft were within the expected ADS-B range and then query 

the crew. This additional monitoring would increase the controller’s workload relative to having 

the status information available. 

Variation on Scenario 3 

Aircraft J needs only 90 s behind aircraft H on runway 9R (Figure 7). In this case, the two 

intervals of 2.2 nmi are greater than the 90 s in-trail spacing, so aircraft J eventually achieves 2.3 

nmi spacing relative to aircraft I and 105 s relative to aircraft H. 

Proposed ADS-B Message 

Table 3 describes a new ADS-B message proposed to support APS operations. This message 

would include intent information needed by PDS and monitoring information for the controller. 

An update rate on the order of 30 s is sufficient for this application. 

Table3:ElementsofOnconditionADSBMessagetoSupportAPSOperations.

Parameter / Content Container Comments / Notes 

Arrival route name 40 character 

array

A unique code that identifies the arrival routing. 

Planned final approach 

speed

Unsigned

integer

< 255; given in knots 

Planned cruise speed Unsigned 

integer

The planned cruise speed in Indicated Air Speed or 

Mach. If less than 100, interpret as hundredth of 

Mach. Can be taken from the Flight Management 

Computer or the current speed when spacing is 

engaged.
24 

Parameter / Content Container Comments / Notes 

Planned cruise altitude Unsigned 

integer

The planned cruise altitude in flight levels if less 

than 100 and in feet if greater than 100. Can be 

taken from the Flight Management Computer or the 

current speed when spacing is engaged. 

Wake vortex category 2 bit flag 00 = Small 

01 = Large 

10 = B-757 

11 = Heavy 

State of Target 

Aircraft #1 

2 bit flag 00 = no target selected [default]

01 = target identified; not coupled

10 = target identified and coupled

11 = target not located (after 300 s)

Target Aircraft #1 ID 7 character 

array

Call sign of first Target Aircraft [ignored if Target 

Aircraft #1 state is 00]

Spacing goal for 

Target Aircraft #1 

Unsigned

integer

< 255; given in seconds [ignored if Target Aircraft 

#1 state is 00] [if assigned spacing goal is in 

distance this is the time conversion PDS is using.]

State of Target 

Aircraft #2 

2 bit flag 00 = no target selected [default]

01 = target identified; not paired

10 = target identified and paired

11 = target not located (after 300 s)

Target Aircraft #2 ID 7 character 

array

Call sign of second Target Aircraft [ignored if 

Target Aircraft #2 state is 00]

Spacing goal for 

Target Aircraft #2 

Unsigned

integer

< 255; given in seconds [ignored if Target Aircraft 

#2 state is 00] [if assigned spacing goal is in 

distance this is the time conversion PDS is using.]
25 

Parameter / Content Container Comments / Notes 

Route Conformance 

Flag

1 bit flag 0 = aircraft has moved off path3

1 = aircraft is on-path

The two spacing goal elements may need to be enlarged to support intermixing arrival and 

departure operations on the same runway and to support distance-based spacing goals where 

values beyond 255 s are needed. 

Summary

This document describes a new operational concept to enable high throughput operations when 

dependent runway operations are in use. This is an extension of a previous Airborne Precision 

Spacing concept [1]. Dependent runway operations are enabled by allowing for the selection of 

two Target Aircraft, one going to the same runway as the IM Aircraft and the other going to the 

dependent, parallel runway. The onboard spacing tool will ensure that the IM speed provided to 

the flight crew meets both spacing criteria. 

Acronyms

4D: Four dimensional 

ADS-B: Automatic Dependent Surveillance - Broadcast 

AGL: above ground level 

APS: Airborne Precision Spacing 

ATC: Air Traffic Control 

ATIS: Automatic Terminal Information Service 

CDA: Continuous Descent Arrival 

CPDLC: Controller Pilot Data Link Communications 

ID: identification 

IM: Interval Management 

NASA: National Aeronautics and Space Administration 

 

3

 The proposed criteria for route conformance are within 2.5 nmi laterally of the path, 6000 ft vertically of the expected 

path, and a track within 90° of current leg. 
26 

OPD: Optimized Profile Descent 

PDS: Pair Dependent Speed 

PRM: Precision Runway Monitor 

RTA: Required Time of Arrival 

t1: first time in sequence 

t2: second time in sequence 

References 

[1] Barmore, B., Abbott, T., and Krishnamurthy, K., “Airborne-Managed Spacing in 

Multiple Arrival Streams,” Proceedings of the 24th Congress of the International Council of 

Aeronautical Sciences, Stockholm, Sweden, ICAS Secretariat, 2004.

[2] Barmore, B., “Airborne Precision Spacing: A Trajectory-Based Approach to Improve 

Terminal Area Operations,” 25th Digital Avionics Systems Conference. Portland, OR, Oct., 2006. 

[3] Barmore, B., Abbott, T., and Capron, W., Baxley, B., "Simulation Results for Airborne 

Precision Spacing along Continuous Descent Arrivals," AIAA-2008-8931. The 26th Congress of 

the International Council of Aeronautical Sciences and 8th AIAA Aviation Technology, 

Integration, and Operations Conference. Anchorage, AK, Sept., 2008. 

[4] RTCA, “Safety and Performance and Interoperability Requirements Document for 

Airborne Spacing – Flight Deck Interval Management (ASPA-FIM)” DO-238. Washington, DC., 

June 2011. 
REPORT DOCUMENTATION PAGE Form Approved

OMB No. 0704-0188

2. REPORT TYPE 

Technical Memorandum

 4. TITLE AND SUBTITLE

A Concept for Airborne Precision Spacing for Dependent Parallel 

Approaches

5a. CONTRACT NUMBER

 6. AUTHOR(S)

Barmore, Bryan E.; Baxley, Brian T.; Abbott, Terence S.; Capron, William 

R.; Smith, Colin L.; Shay, Richard F.; Hubbs, Clay

 7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)

NASA Langley Research Center

Hampton, VA 23681-2199

 9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES)

National Aeronautics and Space Administration

Washington, DC 20546-0001

 8. PERFORMING ORGANIZATION

 REPORT NUMBER

L-20021

10. SPONSOR/MONITOR'S ACRONYM(S)

NASA

13. SUPPLEMENTARY NOTES

12. DISTRIBUTION/AVAILABILITY STATEMENT

Unclassified - Unlimited

Subject Category 03

Availability: NASA CASI (443) 757-5802

19a. NAME OF RESPONSIBLE PERSON

STI Help Desk (email: help@sti.nasa.gov)

14. ABSTRACT

The Airborne Precision Spacing concept of operations has been previously developed to support the precise delivery of 

aircraft landing successively on the same runway. The high-precision and consistent delivery of inter-aircraft spacing allows 

for increased runway throughput and the use of energy-efficient arrivals routes such as Continuous Descent Arrivals and 

Optimized Profile Descents. This paper describes an extension to the Airborne Precision Spacing concept to enable dependent 

parallel approach operations where the spacing aircraft must manage their in-trail spacing from a leading aircraft on approach 

to the same runway and spacing from an aircraft on approach to a parallel runway. Functionality for supporting automation is 

discussed as well as procedures for pilots and controllers. An anaylsis is performed to identify the required information and a

new ADS-B report is proposed to support these information needs. Finally, several scenarios are described in detail..

15. SUBJECT TERMS

aircraft approach spacing; descent; precision; runways

18. NUMBER

 OF 

 PAGES

31

19b. TELEPHONE NUMBER (Include area code)

(443) 757-5802

a. REPORT

U

c. THIS PAGE

U

b. ABSTRACT

U

17. LIMITATION OF 

 ABSTRACT

UU

Prescribed by ANSI Std. Z39.18

Standard Form 298 (Rev. 8-98)

3. DATES COVERED (From - To)

5b. GRANT NUMBER

5c. PROGRAM ELEMENT NUMBER

5d. PROJECT NUMBER

5e. TASK NUMBER

5f. WORK UNIT NUMBER

411931.02.61.07.01

11. SPONSOR/MONITOR'S REPORT

 NUMBER(S)

NASA/TM-2012-217346

16. SECURITY CLASSIFICATION OF:

The public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, 

gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this 

collection of information, including suggestions for reducing this burden, to Department of Defense, Washington Headquarters Services, Directorate for Information Operations and 

Reports (0704-0188), 1215 Jefferson Davis Highway, Suite 1204, Arlington, VA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person 

shall be subject to any penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number.

PLEASE DO NOT RETURN YOUR FORM TO THE ABOVE ADDRESS.

1. REPORT DATE (DD-MM-YYYY)

01- 03 - 2012