May 2013 

NASA/TP–2013-217994 

Simplified Aircraft-Based Paired Approach: 

Concept Definition and Initial Analysis 

Sally C. Johnson, Gary W. Lohr, and Burnell T. McKissick 

Langley Research Center, Hampton, Virginia 

Terence S. Abbott 

Booz Allen Hamilton, McLean, Virginia 

Nelson M. Guerreiro 

Analytical Mechanics Associates, Inc., Hampton, Virginia 

Paul Volk 

Adaptive Aerospace Group, Inc., Hampton, Virginia 
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May 2013 

NASA/TP–2013-217994 

Simplified Aircraft-Based Paired Approach: 

Concept Definition and Initial Analysis 

Sally C. Johnson, Gary W. Lohr, and Burnell T. McKissick 

Langley Research Center, Hampton, Virginia 

Terence S. Abbott 

Booz Allen Hamilton, McLean, Virginia 

Nelson M. Guerreiro 

Analytical Mechanics Associates, Inc., Hampton, Virginia 

Paul Volk 

Adaptive Aerospace Group, Inc., Hampton, Virginia 
Available from: 

NASA Center for AeroSpace Information 

7115 Standard Drive 

Hanover, MD 21076-1320

443-757-5802 

The use of trademarks or names of manufacturers in this report is for accurate reporting and does not 

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National Aeronautics and Space Administration. 
v

Contents 

Table of Acronyms and Abbreviations ........................................................................................................vii

Introduction .................................................................................................................................................. 2

Overview of FAA SAPA Concept .................................................................................................................... 2

Lateral Navigation Requirements ................................................................................................................. 3

Operational Assumptions ............................................................................................................................. 8

Abeam Positioning .................................................................................................................................... 8

Speed and Altitude Profiles....................................................................................................................... 9

Characterization of SAPA Pair Positioning ..................................................................................................11

Wake Encounter Experiment Scenarios and Conditions ........................................................................11

Simulation Profile and Conditions ..........................................................................................................12

Principal Scenario Parameters ................................................................................................................13

Experiment Scenarios .............................................................................................................................15

Experimental Results and Discussion ......................................................................................................21

Limitations of Simulation Study...........................................................................................................31

Summary of Monte Carlo Simulation Results .........................................................................................31

Conduct of SAPA Operations ......................................................................................................................35

Speed Management ................................................................................................................................35

Initial Vertical Separation ........................................................................................................................37

Missed Approach ....................................................................................................................................38

SAPA Data Requirements and SAPA Status Information ........................................................................38

Set Up and Initialization of SAPA Procedure ...............................................................................................40

Current Operations ................................................................................................................................. 40

SAPA Procedures .....................................................................................................................................41

Positioning Task ......................................................................................................................................41

Separation Requirements .......................................................................................................................45

Roles and Responsibilities .......................................................................................................................46

Airspace Requirements ...........................................................................................................................47

Traffic Flow Management Considerations ..............................................................................................47

Role of Automation ................................................................................................................................. 47
vi

Concluding Remarks....................................................................................................................................49

Acknowledgements .....................................................................................................................................51

References ..................................................................................................................................................52

Appendix A – Speed Management Using a Station Keeping Model ...........................................................54

Appendix B – Box Plot of Initial In-Trail Distance Data ...............................................................................58

Appendix C – Asymptotic Confidence Intervals for Minimum Initial In-Trail Distances .............................60
vii

Table of Acronyms and Abbreviations 

ADS-B Automatic Dependent Surveillance – Broadcast 

AGL Above Ground Level 

APA AVOSS Prediction Algorithm 

ARTCC Air Route Traffic Control Center 

ATC Air Traffic Control 

ATIS Automatic Terminal Information Service 

AVOSS Aircraft VOrtex Spacing System 

CAS Calibrated Air Speed 

FAA Federal Aviation Administration 

FAF Final Approach Fix 

FTE Flight Technical Error 

GBAS Ground-Based Augmentation System 

GLS GNSS Landing System 

GNSS Global Navigation Satellite System 

GPS Global Positioning System 

ICAO International Civil Aviation Organization 

IGE In Ground Effect 

ILS Instrument Landing System 

IMC Instrument Meteorological Conditions 

KIAS Knots Indicated Airspeed 

LAAS Local-Area Augmentation System 

LPV Localizer Approach Vertical Guidance 

MASPS Minimum Aviation System Performance Standards 

NASA National Aeronautics and Space Administration 

NextGen Next Generation Air Transportation System 

NGE Near Ground Effect 

NSE Navigation System Error 

NTZ No Transgression Zone 

OGE Out of Ground Effect 

PDE Path Definition Error 

PFAS Planned Final Approach Speed 

RNAV aRea NAVigation 

SAP Stabilized Approach Point 

SAPA Simplified Aircraft-based Paired Approach 

SAPA-CZ SAPA Conformance Zone 

SBAS Satellite-Based Augmentation System 

SOCMAP SAPA Out of Conformance Missed Approach Point 

SFO San Francisco International Airport 

TRACON Terminal Radar Approach CONtrol 

TSE Total System Error 

WAAS Wide-Area Augmentation System 

WVSATTM Wake Vortex Simulation and Analysis Tool 
Abstract

Simplified Aircraft-based Parallel Approach (SAPA) is an advanced 

concept proposed by the Federal Aviation Administration (FAA) to 

support dependent parallel approach operations to runways with 

lateral spacing closer than 2500 ft. At the request of the FAA, NASA 

performed an initial assessment of the potential performance and 

feasibility of the SAPA concept, including developing and assessing 

an operational implementation of the concept and conducting a 

Monte Carlo wake simulation study to examine the longitudinal 

spacing requirements. The SAPA concept was shown to have 

significant operational advantages in supporting the pairing of 

aircraft with dissimilar final approach speeds. The wake simulation 

study showed that support for dissimilar final approach speeds could 

be significantly enhanced through the use of a two-phased altitudebased longitudinal positioning requirement, with larger longitudinal 

positioning allowed for higher altitudes out of ground effect and 

tighter longitudinal positioning defined for altitudes near and in 

ground effect. While this assessment is preliminary and there are a 

number of operational issues still to be examined, it has shown the 

basic SAPA concept to be technically and operationally feasible. 

Introduction 

Capacity reduction from restricted operation in Instrument Meteorological Conditions (IMC) of 

Closely Spaced Parallel Runways, defined as runways laterally spaced closer than 4300 ft apart, 

is a major limiting factor in supporting future air traffic demand. There are a number of current 

and proposed future solutions for conducting efficient, simultaneous independent, parallel 

operations in Instrument Meteorological Conditions (IMC) for runways spaced between 4300 

and 2500 feet (ft), such as high-update radar and path offset [CSPOWG2009]. For runways with 

centerline spacing closer than 2500 ft, wake turbulence encounters between aircraft on parallel 

paths become problematic, and dependent operations are required. Simplified Aircraft-based 

Parallel Approach (SAPA) [DeCleene2008] is an advanced concept being considered for 

dependent, low-visibility operations for dual and triple runways spaced closer than 2500 ft. 

At the request of the Federal Aviation Administration (FAA), an initial assessment of the 

potential performance and feasibility of the SAPA concept was performed. This initial 

assessment included an initial system study of the SAPA concept to assess operational benefits 

and constraints to inform SAPA standards development plus a preliminary technical feasibility 

analysis to aid the FAA in the development and refinement of the SAPA concept. This effort 

was jointly funded by the FAA and the National Aeronautics and Space Administration (NASA) 

under its Airspace Systems Program. 
2

Overview of FAA SAPA Concept 

The ability to conduct dependent parallel approaches to closely spaced runways under IMC 

offers an important opportunity for a significant increase in the rate of flight operations and the 

potential to foster growth of operations at airports. The Simplified Aircraft-based Paired 

Approach (SAPA) concept is a proposed mid-term (2013-2018) concept for operation of closely 

spaced parallel runways in IMC. SAPA is designed to increase capacity at airports in the United 

States where lateral runway spacing for parallel runway operations under IMC currently require 

aircraft-to-aircraft longitudinal separation equivalent to single runway operations. The target 

application is simultaneous, dependent operations to two or three parallel runways spaced closer 

than 2500 feet (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. 

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

dependent surveillance to minimize parallel-approach spacing requirements. With the use of an 

augmented Global Navigation Satellite System (GNSS) navigation source for precise, reliable 

guidance, plus a modern autopilot to provide very reliable and accurate path tracking throughout 

the approach, safe IMC operations are possible at very close lateral runway spacing. 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. The pair of aircraft must remain in approximately

abeam positioning to avoid any wake vortex encounters. The approach path is based on 

constant-width navigation performance. 

Aircraft are initially established on final approach with a minimum of 1000 ft of vertical 

separation, and Air Traffic Control (ATC) is responsible for initially pairing the aircraft with 

appropriate relative longitudinal positioning. During the SAPA operation, the aircraft employ 

onboard flight-guidance speed cues to maintain longitudinal alignment within the required 

tolerance. 

During the SAPA procedure, a breakout maneuver must be initiated in the following situations: 

 lateral position error beyond tolerance, 

 longitudinal position error beyond tolerance, and 

 ADS-B status indication of loss of autopilot coupling or augmented GNSS navigation 

accuracy. 

The breakout maneuver is a climbing turn that is executed via actuation of a preprogrammed 

path. 
3

Lateral Navigation Requirements 

Similar to the Airborne Information for Lateral Spacing (AILS) concept [Abbott2001], the 

primary means for conducting safe SAPA operations is that the possibility of interference 

between the two aircraft on approach must be remote. In SAPA, this requirement is met by 

defining a high Required Navigational Performance standard for the performing aircraft, which 

could be achieved by flying coupled autopilot with high precision navigation. The SAPA pairing 

only occurs once the aircraft are established on the final approach course, conducting a straightin approach guided by highly accurate navigation and flight guidance. There are two key 

components that affect the go-around rate: the likelihood that an aircraft blunders (i.e., crosses 

unacceptably close to the paired aircraft’s approach path), and the likelihood that an aircraft’s 

alerting will falsely perceive a blunder when none has occurred. 

The current study does not include any assessment of the feasibility of designing alerting 

algorithms that are sensitive enough to detect blunders quickly yet yield acceptably low false 

alarm rates. This may be quite difficult to achieve because the available time to avoid a collision 

or strong wake encounter will be quite small given the target lateral runway spacing of 750 ft or 

lower. The current study also does not address the likelihood that a minor deviation from the 

center of the flight path might trigger a false alarm. Future studies aimed at designing the 

alerting algorithms will need to address issues such as whether deviations within these lateral 

bounds would trigger false alarms, necessitating even tighter lateral navigation requirements. 

Based on the results of several studies currently underway, standards for implementing parallel 

runway operations are likely to change before SAPA could be operational. For example, based 

on an assessment of historical operational data, the assumed likelihood used in safety case 

calculations of a 30oblunder along the final approach course may change significantly. 

What is presented in this section is essentially a discussion of the feasibility of a single aircraft 

conducting an approach to one of a pair of runways spaced as close as 750 ft laterally without 

crossing the mid-line between the two runways. This is a first cut at examining the navigation 

systems on the existing fleet, because there is a significant question whether existing navigation 

systems have sufficient precision to support the targeted 750 ft lateral runway spacing. The 

purpose of this section is not to offer recommended SAPA lateral navigation standards, but 

rather to examine key issues in order to make an initial assessment of the feasibility of 

operational implementation of SAPA based on the likely equipage of the relevant aircraft fleet. 

Navigation Accuracy of Existing Systems 

In order to characterize the navigation precision required for SAPA operations to a specific 

lateral runway spacing, a number of assumptions must be made. The SAPA approach path is 

based on constant-width navigation performance. 

Current and near-term future procedures for runways spaced closer than 2400 feet, such as 

Simultaneous Offset Instrument Approach (SOIA), RNP Parallel Approach Transition (RPAT), 
4

and RNP Parallel Approach (RPA) can and do require the use of a No Transgression Zone (NTZ) 

of up to 2000 feet during an IMC segment, followed by a visual segment supporting closer 

lateral runway spacings. Clearly, the requirement of a 2000 ft NTZ would preclude the use of 

SAPA for runways spaced closer than 2000 ft. In the current analysis, no additional safety buffer 

is assumed, and the maximum allowable Total System Error (TSE) is defined by the point at 

which the wingtip crosses the midline between the two runways, as shown in Figure 1. Clearly, 

this does not make any accommodation for blunder reaction time.

Figure 1. Maximum allowable TSE 

The components of TSE are Path Definition Error (PDE), Navigation System Error (NSE) and 

Flight Technical Error (FTE). Because the SAPA operation occurs on the straight final approach 

course to a heavily used runway, the contribution of PDE is assumed to be negligible compared 

to the other components. 

The NSE is a function of the navigation system in use at the given airport. In this study, the NSE 

for Ground-Based Augmentation System (GBAS) and a Localizer Approach Vertical Guidance 

(LPV) approach are examined. Many high density airports with very closely spaced parallel 

runways will have implemented GBAS ground stations in the timeframe that SAPA will be 

operationally deployed. However, it is envisioned that SAPA operations could also be 

implemented at airports without GBAS that have runways with larger spacings. 

The lateral accuracy of GBAS, as specified by the DO-245A Minimum Aviation System 

Performance Standards (MASPS), is specified as a function of distance from the touchdown 

point, and can vary from 5 meters (m) to 27.7 m. The aircraft systems using GBAS information 

for navigation, including displays and autopilot, are designed to mimic the angular accuracy of 

Instrument Landing System (ILS) equipment, with more sensitivity closer to the runway. Thus, 

direct comparison to NSE required for the constant-width SAPA approach is problematic. 

However, the FAA has performed flight tests to determine the accuracy of GBAS systems and 

verified a 95% lateral accuracy of 2 m for GBAS-equipped aircraft.1

 It is assumed that this 

 

1

 Information obtained from Jason Burns, Engineer, FAA Navigation Services, GBAS Program Office, valid as of 

12/08/2009. 
5

accuracy can be provided with a lateral integrity limit (99.999%) of 40 m, which is consistent 

with Satellite-Based Augmentation System (SBAS) requirements for LPV approaches. This 

lateral accuracy and integrity limit is independent of the FTE component of TSE (described 

below). 

GBAS operations are valid within a specified range of the ground station, as represented by the 

broadcast parameter Dmax. The currently deployed Honeywell ground stations have this 

parameter set to 23 nautical miles (nm). For SAPA operations, the aircraft will initially be 

established on the final approach course with 1000 ft altitude separation. It is assumed that the 

aircraft would need to be cleared to begin the SAPA procedure before the higher altitude aircraft 

intercepts the 3o glide slope, which would be more than 5 nm from the runway threshold, but 

well within the 23 nm limit. 

For airports without GBAS ground stations, the lateral accuracy (95%) of SBAS is characterized 

by the FAA to be on the order of 2-3 m. LPV approaches are designed to look like ILS 

approaches from an overlay standpoint, but the lateral accuracy of the approach is not angular 

(like ILS) and does not vary as a function of the distance from the runway. Thus, for all practical 

purposes, the use of SBAS for a SAPA procedure should give roughly the same NSE as GBAS. 

So, the NSE requirement for SAPA operations should not be difficult to obtain. The FTE 

requirement is the toughest component of TSE to obtain. 

The FAA SAPA concept paper [DeCleene2008] quotes an FTE range of 40 m for newer aircraft 

to 80 m for existing older aircraft. This seems reasonable and is confirmed by some public 

domain information from Boeing, which measured TSE from 695 GPS (not GBAS) coupled 

approaches. The 95% TSE figure was 59 m. In the current study, there were no statistically 

significant sources found for GBAS- or SBAS-coupled FTE data, and FTE data is closely held 

proprietary information. Additionally, the FTE allocation must support the worst case weather 

condition for which the SAPA operation is expected to be approved. The FTE requirement will 

be examined further in the following subsection. 

Applicability of Required Navigation Performance Standards 

The SAPA concept is a potential mid-term solution under the NextGen (Next Generation Air 

Transportation System) concept of operations. Since one of the key transformations of NextGen 

is the principle of performance-based operations, it is appropriate to attempt to define the lateral 

navigation requirements in terms of Required Navigation Performance (RNP)2

. 

Lateral navigation accuracy in RNP RNAV (aRea NAVigation) is defined by a normal 

performance and a containment limit. For RNP-1, the aircraft is guaranteed to stay within the 

 

2

The ICAO (International Civil Aviation Organization) definition of RNP does not quantify the containment of 

navigation accuracy, leaving this to the “appropriate technical bodies.” The containment values for RNP used in this 

paper are from the RNP MASPS (Minimum Aviation System Performance Standards). Throughout this paper, the 

term RNP will be used to denote RNP RNAV, as defined in the RNP MASPS. 
6

RNP value, or 1 nm, of the desired path with 95% probability. The aircraft is guaranteed to stay 

within 2 x the RNP value, or 2 nm, of the desired path with 99.999% probability. To be 

operationally acceptable, a parallel operations concept must ensure that the go-around rate due to 

real or perceived blunders is acceptably low. For this study, a go-around rate on the order of 10-4 

per landing was used. To achieve a 10-4 rate for unacceptable navigation blunders, the 

containment limit is the relevant value. RNP containment is defined based on the desired path of 

the center of mass of the aircraft and does not consider the wingspan, so half the wingspan must 

be added for each aircraft. Thus, assuming there is no additional buffer to be added, the runway 

lateral spacing supported can be calculated for a given RNP value by: 

 Runway lateral spacing = left aircraft RNP x 2 + 1⁄2 wingspan 

 + right aircraft RNP x 2 + 1⁄2 wingspan 

For example, if we assume a worst case of two Airbus A380 aircraft paired on approach, with a 

wingspan of approximately 260 ft, then for RNP = 0.1, the minimum lateral runway spacing 

would be 2660 ft (4 x 600 + 260). Table 1 shows the RNP levels required for various runway 

lateral spacings. 

Table 1: Selected Minimum Lateral Runway Spacings Using 

RNP Containment Plus Wingspan Buffer of 260 ft 

RNP Level Lateral Runway Spacing (ft)

RNP-0.088 2400

RNP-0.047 1400

RNP-0.03 1000

RNP-0.02 750

It should be noted that all of the RNP values in Table 1 are below the minimum RNP RNAV 

applicability range of RNP-0.1 RNAV specified in the current MASPS for RNP. The MASPS 

state that future revisions of the MASPS may consider RNP values below this range to 

accommodate future potential applications. Despite this limitation, the following analysis is 

performed to determine the FTE requirement to satisfy SAPA operations at various lateral 

runway spacings. 

The TSE is computed statistically by adding the PDE, NSE and FTE in the following equation: 

TSE = 

2

PDE + 

2

NSE+ 

2

FTE

As discussed above, the PDE should be negligible compared to the NSE and FTE, and can be 

assumed to be zero for the purposes of this calculation. Using the formula shown above to 

derive the 95% requirement for TSE, substituting the RNP numbers derived above in place of 

TSE, and backing out expected NSE of 2m, yields the FTE requirement for various runway 

lateral spacings shown in Table 2. 
7

Table 2. FTE for Various Runway Lateral Spacings 

Runway Lateral Spacing RNP Value FTE

2400ft RNP-0.088 163 m (535 ft)

1400 ft RNP-0.047 102 m (334 ft)

1100 ft RNP-0.03 64 m (210 ft)

750 ft RNP-0.02 37 m (122 ft)

While these FTE levels are aggressive, there are certainly demonstrated auto-pilot applications 

that are well within these requirements, such as auto-land systems. For operations with RNP 

values below 0.1, a much higher integrity requirement may be considered appropriate. This 

could also significantly affect the system hardware fault detection/monitor design and software 

level. Additionally, it is not clear how SAPA operations would be impacted by weather 

conditions, such as crosswinds, atmospheric turbulence, wind gusts, and wind shear, and there 

would be limits on the weather conditions in which SAPA operations would be conducted. 

There are no operational systems currently certified to the RNP levels that would be required for 

SAPA operations to 750 ft lateral runway spacing, and the required navigation and FTE accuracy 

are aggressive; however, this navigation performance is probably within the realm of existing 

state of the art avionics. Again, these assessments were conducted assuming that the limitation 

on lateral deviations allowed the wingtip to graze the midline between the two runways, which 

may not be realistic. Additionally, there was no consideration of the lateral deviations triggering 

false alarms for the blunder alerting algorithms.
8

Operational Assumptions

In order to assess the technical and operational feasibility of the SAPA concept, a detailed 

description of how the concept might be implemented was developed and assessed. The SAPA 

concept uses procedures, precise navigation, ADS-B in and cockpit-based guidance to conduct 

simultaneous dependent instrument approaches to two or even three runways spaced from 2500 

down to 750 ft centerline spacing. The feasibility of applying the SAPA concept for runway 

lateral spacings less than 750 ft was not analyzed in this study. 

Abeam Positioning 

In a dependent operation, the pair of aircraft must maintain longitudinal spacing within a 

specified window for safe operation. Many of the proposed solutions for dependent parallel 

operations use an echelon pairing position, where one aircraft (the trailer) maintains a fixed, 

relative position slightly behind the other aircraft (the leader) for the entire final approach 

segment from before the Final Approach Fix (FAF) to landing [Stone1996, Hammer1999]. As 

shown on the left in Figure 2, the trailer maintains an echelon position behind the leader aircraft 

within a safe zone window defined by a rear boundary to keep the trailer ahead of any possible 

encounters with a wake generated by the leader and a front boundary defined such that if the 

leader blunders, it will pass ahead of the trailer with a low probability of a collision occurring 

[Landry2000]. 

Figure 2. Depiction of echelon safe zone versus SAPA conformance zone 
9

Unlike the echelon positioning, the SAPA concept allows one aircraft to pass the other aircraft 

during the approach segment. While the echelon longitudinal spacing window is often referred 

to as a safe zone, in this paper the term SAPA Conformance Zone (SAPA-CZ) will be used 

because the longitudinal maneuvering must conform to this zone, but staying within the zone 

does not provide blunder protection. As shown on the right 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. Since SAPA allows one 

aircraft to pass the other and the SAPA-CZ allows for maneuvering an equal distance ahead of or 

behind the paired aircraft, this longitudinal positioning is referred to herein as abeam positioning. 

However, it should be noted that the two aircraft will only be abeam as one passes the other, and 

for most of the operation the two aircraft will be separated by a significant longitudinal distance. 

Because the echelon safe zone window between blunder collision risk in front and wake 

encounter risk behind is fairly limited in size, echelon positioning requires that aircraft pairs are 

flying nearly identical approach speeds. For a given landing situation of aircraft landing weight, 

atmospheric conditions, headwind, etc., a typical civil air transport aircraft should land within 

approximately 5 knots of its nominal final approach speed. Any slower speed increases risk of a 

stall, while any higher speed may lead to potential runway overrun problems. In a realistic 

operational environment, there are significant inefficiencies associated with attempting to 

arrange for pairs of aircraft with similar approach speeds to arrive with appropriate timing for 

pairing. The SAPA concept allows for pairing of aircraft with more dissimilar approach speeds, 

but sacrifices the blunder collision protection. To mitigate the need for blunder collision 

protection, SAPA relies heavily on advanced navigation to reduce the probability of an aircraft 

blundering. 

Speed and Altitude Profiles 

This subsection describes the speed and altitude profiles assumed in this paper for the approach. 

As shown in Figure 3, the approach follows a 3o glide slope, which terminates 1000 ft beyond 

the runway threshold. The FAF is assumed to be 5 nm from the runway threshold, and the 

Stabilized Approach Point (SAP) is at 1000 ft Above Ground Level (AGL), which is slightly 

more than 3 nm from the runway threshold. 
10

Figure 3. Approach altitude profile 

Figure 4 shows the speed profile for the approach. The approach begins with a constant speed 

segment prior to the FAF, which could be different for a given airport or runway pair. From the 

FAF, the two aircraft slow to their respective final approach speeds and proceed to land open 

loop. 

Figure 4. Approach speed profile 
11

Characterization of SAPA Pair Positioning 

A Monte Carlo wake encounter experiment was conducted to characterize the front and rear 

“wake-safe” boundaries for a SAPA-CZ. Prior to the current study, McKissick et al conducted a 

Monte Carlo experiment to examine wake encounters between pairs of aircraft conducting 

dependent approaches to closely spaced parallel runways, which provided some initial results 

applicable to the SAPA concept [McKissick2009]. The prior experiment examined within-pair 

wake encounters for runway centerline spacing ranging from 500 ft to 1500 ft. The experiment 

showed that, for each lateral runway spacing, the vast majority of the wake encounters occurred 

at low altitudes, less than 225 ft AGL, and that these low-altitude encounters occurred at much 

closer longitudinal distances between the aircraft than the encounters seen at higher altitudes. 

For example, for the 500 ft runway centerline spacing case with 15 kt crosswinds, below 225 

AGL there were 32,611 encounters (relative frequency of encounters 36%) with the closest 

encounter occurring 1382 ft behind the leader. Above 225 AGL there were 1984 encounters 

(relative frequency of encounters 2%) with the closest encounter occurring 3159 ft behind the 

leader. This result seems logical from an analytical viewpoint, since it is known that wakes that 

are in ground effect move laterally much faster than they do out of ground effect, and wakes 

descend more slowly and/or experience a bouncing behavior in ground effect. 

Based on the results from this previous experiment, the study team developed the idea of using 

more than one conformance zone for SAPA operations. Initial longitudinal positioning between 

a pair of aircraft would be governed by an Out-of-Ground-Effect (OGE) conformance zone. A 

second In-Ground-Effect (IGE) conformance zone might govern the longitudinal positioning for 

all altitudes in which the wake behavior might be subject to ground effects, from a 

conservatively defined ceiling for Near-Ground-Effect (NGE) down to landing. Alternatively, if 

the impact on performance was quite significant, having separate conformance zones for NearGround-Effect and In-Ground-Effect altitudes might be desirable. The boundary altitudes for 

Near-Ground-Effect and In-Ground-Effect wake behavior are a function of the wingspan of the 

aircraft generating the wake. To implement SAPA operations at a given airport, the boundary 

altitude between the two conformance zones could be conservatively defined based on the largest 

wingspan of aircraft expected to be operating at that airport. 

Wake Encounter Experiment Scenarios and Conditions 

A Monte Carlo simulation experiment to investigate wake encounters at various altitudes was 

conducted using the Wake Vortex Simulation and Analysis Tool (WVSATTM). WVSATTM,

developed by Air Traffic Simulation, Inc. [ATSI2010], uses the FAA’s Aviation System 

Standard database and procedure templates to simulate aircraft performance with a high level of 

accuracy and incorporates the Aircraft Vortex Spacing System (AVOSS) Prediction Algorithm 

(APA) version 3.2 wake model developed by NASA Langley [Proctor2009b, Proctor1998, 

Sarpkaya2000]. The purpose of the experiment was to provide an initial estimate of the extent of 

OGE and IGE conformance zones for SAPA operations. The process of establishing appropriate 

conformance zones for operational implementation of SAPA is beyond the scope of this study. 
12

The size and initial position of the wake that is generated depends on the weight, wingspan, 

speed, power and control surface configurations of the generating aircraft. If there is a 

significant crosswind, the wake may move laterally into the path of the aircraft on the parallel 

approach, but this lateral movement is heavily influenced by crosswind strength and direction 

and other atmospheric conditions. For a given runway lateral separation and crosswind, the safe 

distance for the trailing aircraft to stay ahead of the wake of the lead aircraft depends on the 

relative speed of the trailing aircraft. The relative distances between the two aircraft varies as 

they slow from the nominal terminal area maneuvering speed to the final approach speed. 

Simulation Profile and Conditions 

The simulation flight profile was designed to accurately reflect SAPA operations, but the 

conditions were chosen to reflect a near-worst case environment for generation and lateral 

transport of wakes. The simulation profile parameters are shown in Table 3. To maximize the 

size and strength of the generated wakes, the two simulated aircraft are comparable to the 

characteristics of Boeing 747-8F size aircraft. A low eddy dissipation rate for ambient 

turbulence in the atmosphere was chosen to reduce the rate of decay of the wakes. To achieve 

the desired speed differential, the fast aircraft simulates a heavily loaded aircraft with a nominal 

approach speed of 159 kt Indicated Air Speed (KIAS), while the slow aircraft is at the empty 

aircraft weight with a nominal approach speed of 149 KIAS. The simulated aircraft flew an 

approach with a 3oconstant glide slope with high navigation precision. At the request of the 

FAA, the aircraft flight-path accuracy for the study was chosen based on standards for WideArea Augmentation System (WAAS) approaches supplied by the FAA3

. The aircraft flew a 

nominal constant speed of 180 kt starting at the FAF, and each aircraft slowed to its nominal 

approach speed before reaching the SAP. In all cases, the crosswinds were constant at 15 kt, and 

crosswinds were always coming from an adverse direction; i.e., moving from the direction of the 

currently leading aircraft towards the currently trailing aircraft. 

 

3

 Information obtained from Martin Heller, FAA Aeronautical Information Systems (AJR-321), from 2009 Sun N 

Fun presentation. 
13

Table 3. Simulation Parameters4

Simulated aircraft: Wingspan 224.5 ft; Tail height 64.2 ft

 Fast aircraft: heavily loaded Landing weight: 749,000 lb

Nominal approach speed: 159 KIAS

Nominal landing speed: Normal (140 kt, 1.2 kt)

 Slow aircraft: empty Landing weight 425,000 lbs

Nominal approach speed: 149 KIAS

Nominal landing speed: Normal (130 kt, 1.2 kt)

Lateral navigation deviation Uniform (15 m, -15 m)

Vertical navigation deviation Uniform (4 m, -4 m)

Crosswinds Constant 15 kt; always adverse direction

Air temperature 15o C 0 ft AGL, 5o C 3000 ft AGL

Eddy dissipation rate 0.0001 m2/s3; Constant 0-3000 ft AGL

Principal Scenario Parameters 

There were three scenario parameters, shown in Table 4, in addition to the ones listed in Table 3, 

that were used in the experiment to examine various aspects of SAPA operations. The aim was 

to understand the basic SAPA dynamics as they relate to the safe following distance as several 

important parameters were varied. The parameters in Table 4 were expected to be the dominant 

parameters affecting the safe following distance. 

Table 4. Scenario Parameters 

Runway Lateral Spacing 750 ft

1000 ft

1400 ft

Wake Detection Surface Circle (673.5 ft diameter); 

Rectangle (673.5 ft lateral, 2000 ft vertical)

Runway Threshold Offset 0 ft

1500 ft

Runway Lateral Spacing 

The first scenario parameter was lateral spacing between runway centerlines. SAPA operations 

at parallel runways with lateral spacings of 750 ft, 1000 ft, and 1400 ft were examined. 

Although the use of SAPA operations at runways spaced as close as 700 ft has been suggested, a 

minimum spacing of 750 ft was deemed to be more likely. Wake experts suggest that for the 

size of aircraft used for this study at 700 ft lateral spacing, the wakes of abeam aircraft could 

 

4

 Throughout the paper, probability distributions will be formatted as: Name of distribution (Parameter1, 

Parameter2, ...) 
14

interact with each other in ways that are not yet well understood [Proctor2009a]. The 

WVSATTM tool does not attempt to model such wake interaction between such closely spaced 

abeam aircraft. Lateral spacings greater than 1400 ft were not modeled and would need to be 

validated in future studies. 

Wake Detection Surface 

The second scenario parameter in the study was wake detection surface. The WVSATTM tool 

models the wake generated behind each aircraft wing as a single point representing the center of 

the vortex and predicts the movement in space and decay over time. Detection of a potential 

wake encounter by another aircraft is accomplished by defining a wake detection surface 

surrounding the trailing aircraft. During a simulation run, a wake encounter is logged if the 

center of the wake vortex touches the detection surface. 

A heavy aircraft can generate a vortex as large as half a wingspan in diameter. Also, a vortex 

tends to bobble and bounce as it moves, which is not captured in an engineering model of wake 

behavior, such as APA. Thus, there is uncertainty in the position of the vortex. Observations of 

wake vortices generated by a B747 aircraft in flight show that the full position uncertainty plus 

vortex size are bounded within one wingspan. To be conservative for this study, a wake 

detection surface in the shape of a circle with a diameter three times the wingspan (673.5 ft) was 

used. The flight profile used in the experiment, which maintains a constant 3o

slope, always has 

the trailing aircraft approximately co-altitude or above the altitude of the leading aircraft when

both aircraft are approaching the same longitudinal runway threshold position. The detection 

circle was sized to detect wake encounters assuming both aircraft are approximately following 

the glide slope. 

However, the initial longitudinal pairing of the aircraft occurs before the FAF with the aircraft 

separated vertically by 1000 ft, and then the higher aircraft descends until it is approximately coaltitude with the other aircraft. During this transition maneuver, if the higher aircraft is the 

leading aircraft, then wakes generated from the higher aircraft could descend and encounter the 

lower aircraft. To explore design alternatives for implementing the initial transition into the 

SAPA operational procedure, it was desired to also investigate a wake-safe boundary without 

this relative-altitude restriction for OGE altitudes. To accomplish this, a tall rectangular wake 

detection surface was also used in a second set of simulations for the OGE scenario cases. The 

rectangle was the same size laterally as the detection circle, but extended 1000 ft above and 

below the trailing aircraft. Thus, any wake that passed within 1000 ft above or below the trailing 

aircraft would be detected as an encounter. 

Runway Threshold Offset 

The third scenario parameter in the study, runway threshold offset, was not used to assess the 

benefits of runway threshold offsets, but rather was used to examine the worst case scenario of a 

runway threshold offset in the presence of adverse crosswinds. Runway threshold offsets of 0 ft 

and 1500 ft were simulated. 
15

When two aircraft are in abeam positioning approaching a pair of parallel runways where one 

runway has a threshold that is longitudinally offset relative to the other runway threshold, the 

aircraft that is approaching the nearer runway will be at a slightly lower altitude than the other 

aircraft. After a wake is generated by an aircraft, it will slowly descend or perhaps persist at a 

constant altitude, and unless unusual atmospheric conditions are present, it can generally be 

assumed that a wake will not rise significantly. It is probable that in some cases this altitude 

difference could result favorably in relaxed longitudinal wake spacing requirements for a 

dependent paired operation, as long as the operational restriction is in place that the leading 

aircraft is always at a lower altitude than the trailing aircraft. However, in SAPA operations, this 

operational restriction is not feasible, because SAPA allows the fast aircraft to overtake and pass 

the slow aircraft. 

Experiment Scenarios 

In order to investigate wake encounters at various altitudes, Monte Carlo simulations were 

conducted using a sequence of three scenarios, as shown in Figure 5. These scenarios were used 

to characterize wake behavior in OGE, NGE, and IGE, respectively. As shown in Figure 5, the 

ground effect boundaries for wakes generated by the simulated aircraft are as follows 

[Proctor1999]: 

IGE lower boundary = 0 ft AGL 

IGE upper boundary = b0 = 

 

 

= 176 ft AGL 

NGE upper boundary = 3 x bo = 530 ft AGL 

where b0 is the initial vortex span. 
16

Figure 5. Altitude and speed profiles for three experiment scenarios: OGE, NGE, and IGE 

OGE Scenario: Out of Ground Effect 

The OGE Scenario was designed to explore encounters of wakes out of ground effect (above 

530 ft AGL). The altitude and speed profiles for this scenario are shown in Figure 6. 
17

Figure 6. Altitude and speed profiles for OGE scenario 

This scenario begins with the slow aircraft at the FAF and the fast aircraft on the 3o

 glide slope 

some longitudinal distance behind. Each aircraft is assigned an initial speed based on the 

distribution Uniform (178 kt, 182 kt), and in the first nautical mile the relative longitudinal 

spacing between the aircraft may be slightly increasing or decreasing because they are both 

traveling at the same speed ± 2 kt. At a distance Uniform (3.8 nm, 4.2 nm) before the runway 

threshold, the slow leading aircraft begins to decelerate to reach Uniform (147 kt, 151 kt) at 

some point just prior to the SAP. Once the slow aircraft begins the deceleration, the fast aircraft 

will be moving closer to the slow aircraft. The slow aircraft continues at this speed until it 

reaches 250 ft AGL, at which point it finishes its simulation run and ceases to generate a wake. 

The fast aircraft also begins to slow when it reaches a distance Uniform (3.8 nm, 4.2 nm) before 

the runway threshold, decelerating to reach Uniform (157 kt, 161 kt) before the SAP. The 

simulation of the fast aircraft was continued below the lower boundary to ensure continuity 

between scenarios, but only wakes encountered by the fast aircraft at or above 530 ft AGL were 

recorded for this scenario. 

For the OGE scenario, 2500 runs were executed for each of the three runway lateral spacings 

using the wake detection circle. An additional 2500 runs were executed for each of the runway 

lateral spacings in the OGE scenario using the tall detection rectangle. The in-trail distance 

between the slow and fast aircraft at the beginning of each run was varied uniformly within the 

windows shown in Table 5. 

Begin slowing 

3.8 – 4.2 nm

from Runway Threshold
18

Table 5. Initial In-Trail Distance Windows for OGE Scenario 

Runway Lateral Spacing Initial In-Trail Distance Window

750 ft 0.3 nm – 1.3 nm

1000 ft 0.8 nm – 1.8 nm

1400 ft 1.6 nm – 2.6 nm

During each run, the two aircraft were released at a given in-trail distance and the run was 

monitored to determine whether or not a wake encounter occurred. The initial in-trail distance 

windows were chosen for each lateral spacing to ensure that the boundary of the conformance 

zone would be exercised, i.e., so some runs would result in encounters and other runs would be 

clear of encounters. 

NGE Scenario: Near Ground Effect 

The NGE scenario was designed to explore encounters of wakes at altitudes that transition from 

OGE into IGE (i.e., from 530 ft AGL to 176 ft AGL). This scenario, shown in Figure 7, begins 

 

Figure 7. Altitude and speed profiles for NGE scenario 

with the slow aircraft on the 3o glide slope at 530 ft AGL, at approximately 1.5 nm from the 

runway threshold. The fast aircraft is initiated on the glide slope some longitudinal distance 

behind. 

Both aircraft maintain their constant approach speeds during this scenario, with the slow aircraft 

traveling at Uniform (147 kt, 151 kt), and the fast aircraft closing from behind at Uniform (157 
19

kt, 161 kt). The slow aircraft continues until it reaches 150 ft AGL, at which point it finishes its 

simulation run and ceases to generate a wake. Only wake encounters occurring at or above 176 

ft AGL, however, were recorded for this scenario. 

There were 2500 runs executed for each of the three runway lateral spacings using the wake 

detection circle. The in-trail distance between the slow and fast aircraft at the beginning of each 

run was varied uniformly within the windows shown in Table 6. 

 Table 6. Target Initial In-Trail Distance Windows for NGE Scenario 

Runway Lateral Spacing Initial In-Trail Distance Window

750 ft 608 ft – 6684 ft

1000 ft 3646 ft – 9722 ft

1400 ft 8507 ft – 14,583 ft

These in-trail distances, however, are positions relative to a fixed location on the approach path 

in both NGE and IGE scenarios. Because the starting altitude of the leading aircraft is specified, 

and because each aircraft has some level of vertical navigational error, the leading aircraft will 

not always start at the same position on the approach path for every simulation run. Because the 

in-trail distance is in the earth reference system and not along the glide slope, this navigational 

error introduces more randomness to the in-trail distribution. This also changes the actual in-trail 

windows simulated by increasing their size by approximately 500 ft, and thus the effective intrail windows simulated in the NGE scenario are as shown in Table 7. 

Table 7. Actual Initial In-Trail Distance Windows for NGE Scenario 

Runway Lateral Spacing Initial In-Trail Distance Window

750 ft 350 ft – 6934 ft

1000 ft 3396 ft – 9972 ft

1400 ft 8256 ft – 14,833 ft

With respect to these simulated in-trail distances, the OGE scenarios do not need these 

corrections. In the OGE scenarios, the initial positions of each aircraft were referenced to the 

runway thresholds (and not specific altitudes) and thus, for each simulation run, the exact initial 

in-trail distance was specified from the uniform distributions tested. 

IGE Scenario: In Ground Effect 

The IGE scenario was designed to explore encounters of wakes at altitudes considered to be in 

ground effect (from 176 ft AGL to landing). The altitude and speed profiles for this scenario are 

shown in Figure 8. This scenario examined the front wake-safe boundary of the SAPA-CZ under 

worst case conditions where the fast aircraft has passed the slow aircraft and moved out in front. 

The scenario begins with the fast aircraft ahead on the 3o

 glide slope at 176 ft AGL, which is 

approximately 0.396 nm from the runway threshold. The slow aircraft is initiated on the glide 

slope some longitudinal distance behind. 
20

Figure 8. Altitude and speed profiles for IGE scenario 

The aircraft begin the scenario at their respective approach speeds, with the fast aircraft moving 

out ahead at Uniform (157 kt, 161 kt) and the slow aircraft lagging behind at Uniform (147 kt, 

151 kt). Upon crossing the runway threshold, each aircraft decelerates and flares at AGL 

altitude Normal (45 ft, 2 ft, with bounds 40-50 ft), with the fast aircraft landing at Normal (140 

kt, 1.2 kt) and the slow aircraft landing at Normal (130 kt, 1.2 kt). The 3o glide slope intercepts 

the runway 1000 ft beyond the threshold, but after flaring, the aircraft touchdown is at a constant 

distance of 1800 ft beyond the threshold. 

There were 2500 runs executed for each of the three runway lateral spacings using the wake 

detection circle. The in-trail distance between the slow and fast aircraft at the beginning of each 

run was varied uniformly within the windows shown in Table 8. 

 Table 8. Targeted Initial In-Trail Distance Windows for IGE Scenario 

Runway Lateral Spacing Initial In-Trail Distance Window

750 ft 800 ft - 2,800 ft

1000 ft 1,500 ft - 3,500 ft

1400 ft 3,500 ft - 5,500 ft

As in the NGE scenario, the simulated RNP navigational errors and the requirement for a 

specific leading-aircraft start altitude imply that the effective in-trail windows with respect to the 

actual leader's starting position on the approach path are as shown in Table 9.
21

 Table 9. Actual Initial In-trail Distance Windows for IGE Scenario 

Runway Lateral Spacing Initial In-Trail Distance Window

750 ft 550 ft – 3050 ft

1000 ft 1250 ft – 3750 ft

1400 ft 3250 ft – 5750 ft

Experimental Results and Discussion 

This subsection presents the results of the Monte Carlo simulation experiment. 

Out-of-Ground-Effect Wake Encounters 

The OGE scenario was designed to investigate encounters of wakes out of ground effect. There 

were 2500 runs conducted for the OGE scenario with 750 ft runway lateral spacing, with the 

slow aircraft starting at the FAF and the fast aircraft starting with a distance behind the FAF 

varied uniformly from 1800 ft to 7800 ft. During each run, the wake was monitored to determine 

whether at any time during the run the trailing aircraft encountered a vortex generated by the 

leading aircraft. The OGE scenario results for runway lateral spacing of 750 ft are shown in 

Figure 9. 

Figure 9. OGE scenario, 750 ft lateral runway spacing

The histogram on the left in Figure 9 shows the number of runs in which wake encounters 

occurred for a given initial in-trail distance, with each bar representing a span of in-trail distance 

of 50 ft. Out of 2500 runs, wake encounters occurred in 1627 runs. The smallest initial in-trail 

distance for which a wake was encountered was 3044 ft. The initial in-trail distances for the 

OGE runs had an average of approximately 20 runs per bin for the OGE case, so the bins above 

5000-ft initial in-trail distance are saturated with 100% of runs generating an encounter. 
22

The graph on the right in Figure 9 shows the wake encounters recorded for the entire scenario 

simulation. Each recorded wake encounter – from the 1Hz sampling – is shown at the altitude of 

encounter and versus the initial in-trail distance of the respective simulation run. Thus, each 

vertical strip of data represents the same simulation run and all recorded wake encounters each 

second for that run. Note that at the right side of the right graph, the trailing aircraft was in a 

continuous wake encounter from the FAF (approximately 1644 ft AGL) down to the OGE lower 

boundary (530 ft AGL). For the 3000 ft initial in-trail distance, the trailing aircraft started with a 

wake encounter at the FAF but pulled ahead of the leading aircraft’s wake after it had descended 

below an altitude of 870 ft AGL. 

As can be seen in the encounter altitude graph, when the two aircraft were released with an initial 

in-trail distance of approximately 4000 ft or more, the trailing aircraft was encountering the wake 

of the leading aircraft throughout the entire run. When the two aircraft were released with initial 

in-trail distance of approximately 3500 ft or less, the trailing aircraft moved ahead of the wake 

generated by the leading aircraft towards the end of the scenario. At the beginning of the OGE 

scenario, the two aircraft are traveling at approximately the same speed during the constant speed 

segment and thus maintaining a steady in-trail distance, but once the leading aircraft slows just 

before the SAP, the distance between the two aircraft decreases, and the trailing aircraft may 

move ahead of the wake generated by the leading aircraft. 

There were a small number of runs in which wake encounters did not occur early in the scenario 

at higher altitudes but did occur at lower altitudes ranging from approximately 1000 to 1300 ft 

AGL. In these simulation runs, the speeds assigned to the two aircraft were such that the trailing 

“fast” aircraft was in fact slower than the leading “slow” aircraft during this constant-speed 

segment as a result of small speed variations. Thus, instead of maintaining a constant in-trail 

distance during this segment, the trailing aircraft lagged further behind the leading aircraft until it 

began to encounter the wake generated by the leading aircraft. These only occurred when the 

two aircraft were released within an initial in-trail distance range of approximately 3000 to 3300 

ft, so the initial positioning was such that the trailing aircraft was initially just ahead of the wake 

generated by the leading aircraft. In operational practice, if the trailing aircraft was utilizing 

onboard speed guidance to stay within the SAPA-CZ, then these cases would presumably not 

have resulted in wake encounters. 

For the case of 1000 ft lateral runway spacing, the results for the OGE scenario are shown in 

Figure 10. As expected, with the larger lateral runway spacing, the time required for the wake to 

move across to the other runway increased significantly. The smallest initial in-trail distance 

leading to wake encounters was 6026 ft. 
23

Figure 10. OGE scenario, 1000 ft lateral runway spacing 

For the case of 1400 ft lateral runway spacing, the results for the OGE scenario are shown in 

Figure 11. Again, with the larger lateral runway spacing, the time required for the wake to move 

across to the other runway increases significantly. The smallest initial in-trail distance leading to 

wake encounters was 11,171 ft. 

Figure 11. OGE scenario, 1400 ft lateral runway spacing 
24

Near-Ground-Effect Wake Encounter 

The NGE scenario was designed to investigate encounters of wakes near ground effect, at 

altitudes ranging from 176 ft to 530 ft. The scenario began with the fast aircraft at a given initial 

in-trail distance behind the slow aircraft, with the two aircraft traveling at their respective final 

approach speeds, so the fast aircraft was closing from behind on the slow leading aircraft. 

Figure 12. NGE scenario, 750 ft runway lateral spacing 

For the NGE scenario case of 750 ft lateral runway spacing, the results are shown in Figure 12. 

As shown in the histogram on the left, the initial in-trail distances resulting in wake encounters 

were slightly smaller for the NGE scenario than for the OGE scenario. This was expected, 

because, as vortices begin to experience ground effect, they move laterally more quickly and thus 

arrive at the parallel runway faster than out of ground effect. Out of 2500 runs, wake encounters 

occurred in 1326 runs. The smallest initial in-trail distance for which a wake was encountered 

was 2621 ft. 

The graph on the right in Figure 12 shows that all of the wake encounters occurred near the 

beginning of the scenario. This was as expected, since the in-trail distance between the two 

aircraft was decreasing throughout the scenario. This resulted in either a simulation run that 

started and stayed in a continuous encounter, or a simulation run that started in an encounter and 

exited the encounter before the end of the run. There were no runs that started free of encounters 

and then entered an encounter. 

For the case of 1000 ft lateral runway spacing, the results for the NGE scenario are shown in 

Figure 13. As expected, with the larger lateral runway spacing, the NGE wake-encounter 

distances for 1000 ft lateral spacing were much larger than for 750 ft lateral spacing, and the 

smallest in-trail distance leading to a wake encounter was 5374 ft. Again, the NGE wakeencounter distances for the 1000 ft lateral case were slightly smaller than the OGE 1000 ft wakeencounter distances. 
25

Figure 13. NGE scenario, 1000 ft lateral runway spacing 

Results for the NGE scenario for the case of 1400 ft lateral runway spacing are shown in Figure 

14. The NGE wake-encounter distances for 1400 ft lateral spacing were again much larger than 

for 1000 ft lateral spacing, and the smallest in-trail distance leading to a wake encounter was 

9802 ft. Again, the NGE wake-encounter distances for the 1400 ft lateral case were slightly 

smaller than the OGE 1400 ft wake-encounter distances. 

Figure 14. NGE scenario, 1400 ft lateral runway spacing 

In-Ground-Effect Wake Encounters 

The IGE scenario was designed to investigate encounters of wakes in ground effect, below 176 ft 

AGL. Unlike the two other scenarios, this scenario began with the fast aircraft ahead and the 

slow aircraft at a given initial in-trail distance behind. During the scenario, the distance between 

the two aircraft was continually increasing, with the fast aircraft moving farther ahead of the 

slow aircraft. 
26

For the IGE scenario case of 750 ft lateral runway spacing, the results are shown in Figure 15. 

As shown in the histogram on the left, the initial in-trail distances resulting in wake encounters 

were significantly smaller for the IGE scenario than for the NGE and OGE scenarios. Out of 

2500 runs, wake encounters occurred in 1886 runs. The smallest initial in-trail distance for 

which a wake was encountered was 1008 ft, which was less than half the 2621 ft encounter 

distance for the NGE 750 ft case. It was expected that shorter in-trail distances would be seen 

for the IGE case, and the influence of ground effect on vortex lateral movement would be very 

pronounced. 

Figure 15. IGE scenario, 750 ft runway lateral spacing 

The graph on the right in Figure 15 shows the wake-encounter altitudes. The encounter patterns 

in this graph are quite different from the previous two scenarios. Since the faster aircraft is now 

leading, once it has pulled far enough ahead to cause a wake encounter for the slow trailing 

aircraft, the trailing aircraft was in a continuous wake encounter for the remainder of the 

simulation. Note that the altitude of a wake encounter was measured at the center of gravity of 

the trailing aircraft, which is 21.2 ft when the simulated aircraft is on its landing gear on the 

ground; therefore, there were no encounters detected below 21.2 ft. The graph shows that the 

wake encounters occurred at all altitudes in the scenario, but the smallest encounter in-trail 

distances occurred at very low altitudes, with many occurring at less than 50 ft AGL. There 

appear to be three reasons for the prevalence of wake encounters just before touchdown. First, 

the in-trail distance between the two aircraft was increasing throughout the scenario, so the two 

aircraft were the farthest apart just before landing. Second, the influence of ground effect on 

vortex lateral movement became more pronounced with closer proximity to the ground. Third, 

just prior to landing, the aircraft flared and slowed considerably for touchdown. The flareslowing maneuver created vortices with stronger circulation, which then translated into faster 

lateral movement when the vortices neared the ground. Examination of sample cases using the 

WVSATTM wake-visualization feature showed a pronounced change in wake lateral movement 

at this point in the simulation. 

For the case of 1000 ft lateral runway spacing, the results for the IGE scenario are shown in 

Figure 16. In this case, out of the 2500 runs, there were 1292 encounters. As expected, with the 
27

larger lateral runway spacing, the IGE wake-encounter distances for 1000 ft lateral spacing were 

much larger than for 750 ft lateral spacing, and the smallest in-trail distance leading to a wake 

encounter was 2141 ft. Again, the IGE wake-encounter distances for the 1000 ft lateral runway 

spacing case were significantly smaller than the 5374 ft minimum for the NGE 1000 ft lateral 

runway spacing case. 

Figure 16. IGE scenario, 1000 ft lateral runway spacing 

Results for the IGE scenario for the case of 1400 ft lateral runway spacing are shown in Figure 

17. In this case, out of the 2500 runs, there were 1037 encounters. With the 1400 ft lateral 

runway spacing, the IGE wake-encounter distances were much larger than for the 750 ft or 1000 

ft lateral runway spacing cases, and the smallest in-trail distance leading to a wake encounter was 

4319 ft. Again, the IGE wake-encounter distances for the 1400 ft lateral spacing case were 

significantly smaller than the 9802 ft for the NGE 1400 ft case. 

Figure 17. IGE scenario, 1400 ft lateral runway spacing 
28

Out-of-Ground-Effect Wake Encounters with Tall Detection Rectangle

To investigate the effect of relative altitudes of the aircraft on wake-encounter distances, 2500 

runs with a tall, rectangular wake-detection surface were conducted for each of the three lateral 

runway spacings for the OGE scenario. The detection rectangle was the same size laterally as 

the detection circle, but extended 1000 ft above and below the trailing aircraft center of gravity 

so that any wake that passed within 1000 ft above or below the trailing aircraft would be detected 

as an encounter. For the simulation scenarios, this was essentially positive and negative infinity 

in the vertical direction. The results for the OGE case of 750 ft lateral runway spacing for the 

rectangular wake detection surface are shown in Figure 18. 

Figure 18. OGE scenario, 750 ft runway lateral spacing 

with 2000 ft vertical detection rectangle 

Compared to the detection circle, the use of the vertical detection rectangle resulted in slightly 

smaller initial in-trail distances leading to wake encounters. For the 750 ft case, the smallest 

observed in-trail distance leading to a wake encounter was 2886 ft, compared to 3044 ft with the 

detection circle. The histogram on the right in Figure 18 shows that, again, the smallest initial 

in-trail distance resulting in wake encounters occur in runs where the in-trail distance increased 

during the start of the scenario (i.e., the “fast” aircraft was traveling slower than the “slow” 

aircraft during the initial constant speed segment). Onboard speed management could perhaps 

preclude these cases. 

Similarly, the results for 1000 ft and 1400 ft lateral runway spacing also showed only slight 

decreases for initial in-trail distance that led to a wake encounter. For the 1000 ft case, the 

closest initial in-trail distance that led to an encounter was 5800 ft, which is slightly smaller than 

the 6026 ft distance observed with the detection circle. For the 1400 ft case, the closest initial intrail distance resulting in an encounter was 10,550 ft, which was slightly less than the 11,171 ft 

distance observed with the detection circle. 
29

Runway Threshold Offset 

To investigate the worst case effect of an offset runway threshold on wake-encounter distances, a 

set of 2500 runs was conducted for each of the three scenarios (OGE, NGE, and IGE) with each 

of the three lateral runway spacings. The circular detection surface was used for all of the 

runway threshold offset simulations. In each scenario, the initially trailing aircraft is 

approaching the nearer runway to maximize the likelihood of wake encounters. The results for 

the 1500 ft runway threshold offset for 750 ft lateral runway spacing for the OGE, NGE, and 

IGE scenarios are shown in Figures 19, 20, and 21, respectively. 

Figure 19. 1500 ft runway threshold offset, OGE scenario, 750 ft runway lateral spacing 

Figure 20. 1500 ft runway threshold offset, NGE scenario, 750 ft runway lateral spacing 
30

Figure 21. 1500 ft runway threshold offset, IGE scenario, 750 ft runway lateral spacing 

Interestingly, the comparison of the 1500 ft runway threshold offset to the results with no offset 

yielded mixed results. For the OGE scenario, the smallest observed in-trail distance leading to a 

wake encounter was 2477 ft, which was less than the 3044 ft with no threshold offset. For the 

NGE scenario, the smallest observed in-trail distance leading to a wake encounter was 2799 ft, 

which was larger than the 2621 ft with no threshold offset. For the IGE scenario, the smallest 

observed in-trail distance leading to a wake encounter was 1115 ft, which was slightly larger 

than the 1008 ft with no threshold offset. The OGE scenario histogram on the right in Figure 19 

shows that there were quite a number of cases where the smallest initial in-trail distances 

resulting in wake encounters occurred in runs where the in-trail distance increased during the 

scenario (i.e., the “fast” aircraft was traveling slower than the “slow” aircraft during the constant 

speed segment). Onboard speed management could perhaps preclude these cases. 

The same mixed results were found for the 1000 ft and 1400 ft lateral runway spacing cases, as 

shown in Table 10. 

Table 10. Smallest Observed Initial In-Trail Distance Leading to a Wake Encounter 

for 1500 Ft Runway Threshold Offset vs. No Offset 

for All Three Runway Lateral Spacings and All Three Scenarios 

750 ft Runway Spacing 1000 ft Runway Spacing 1400 ft Runway Spacing

Scenario No Offset 1500 ft Offset No Offset 1500 ft Offset No Offset 1500 ft Offset

OGE 3044 2477 6026 5824 11,171 10,718

NGE 2621 2799 5374 5385 9802 9383

IGE 1008 1115 2141 2257 4319 4360

For the OGE cases for 1000 ft and 1400 ft lateral runway spacings, as in the 750 ft case, the 

runway threshold offset resulted in smaller initial in-trail distances leading to wake encounters, 

and the smallest distances were in situations where the wake encounters did not occur near the 

beginning of the scenario. For the NGE and IGE cases for all of the three lateral runway 
31

spacings, the runway threshold offset counter-intuitively resulted in larger distances for the 

closest initial in-trail distances leading to wake encounters. 

Limitations of Simulation Study 

The Monte Carlo simulation study investigated the effects of parametric variations on wake 

encounter distances. The results are useful in furthering understanding of the basic SAPA 

dynamics as they relate to the safe following distance, examining implementation options as they 

relate to wake lateral movement, and investigating the overall feasibility of the SAPA concept. 

If a decision is made to implement SAPA in an operational environment, additional studies will 

be required to quantify the appropriate sizes of conformance zones for a given set of operational 

parameters and assumptions. 

In this study, a limited investigation of the initial in-trail distance leading to a wake encounter 

was examined for a specific set of cases. The quantified results of the study are only directly 

applicable to the specified aircraft pair with the assumed speed profiles, atmospheric conditions, 

crosswinds, and other assumptions. However, the test scenarios were chosen to reflect nearworst case conditions, and some general characteristics of the SAPA-CZ can be inferred from the 

analysis of these cases. For a detailed performance analysis of SAPA, additional wake encounter 

studies will be required to quantify a SAPA-CZ for any given aircraft pair. 

The fact that this is a simulation and not a real-world experiment, imposes some limitations on 

the results. A number of studies have been conducted to observe the behavior of wakes in a realworld environment out of ground effect. These studies typically entail ground-based sensors 

measuring wakes of over-flying aircraft or an instrumented aircraft flying through aircraft wakes. 

However, there is a significant lack of observational data on wake behavior in the presence of 

ground effect. Additionally, wake behavior in ground effect is more difficult to model accurately 

because the interaction with the ground is complex. Thus, state-of-the-art wake behavior models, 

including the APA algorithm implemented in WVSATTM, are more reliable predictors of wake 

transport and decay at higher altitudes than in ground effect [Proctor2000]. Our results have 

shown that wake behavior in ground effect is likely to be the main driver of pair positioning 

requirements, yet this IGE wake behavior is, in fact, the least understood. 

With these limitations in mind, the next subsection presents a summary of the results of the 

simulation study. 

Summary of Monte Carlo Simulation Results 

The Monte Carlo simulation experiment confirmed that the initial in-trail distances that result in 

wake encounters vary significantly between the three lateral runway spacings and between the 

OGE, NGE, and IGE scenarios, as shown in Table 11. 
32

Table 11. Comparison of Closest Initial In-Trail Distance Resulting 

in a Wake Encounter for Each Lateral Runway Spacing for Each Scenario 

with the Circular Detection Surface and No Runway Threshold Offset 

Scenario Lateral Runway Spacing

750 ft 1000 ft 1400 ft

OGE 3044 ft 6026 ft 11,171 ft

NGE 2621 ft 5374 ft 9802 ft

IGE 1008 ft 2141 ft 4319 ft

As stated in the limitations discussion above, these numerical results should not be interpreted as 

recommended final values for conformance zone boundaries, but are indicative of relative 

SAPA-CZ sizes. As expected, the requirements for pair positioning to avoid wake encounters 

are much tighter for very closely spaced parallel runways than for runways with larger lateral 

spacing. To support a more comprehensive comparison to minimum in-trail distances leading to 

wake encounters across all of the scenario parameters of the experiment (Table 4), see the box 

plot in Appendix B. Appendix C presents a statistical analysis of the data from the experiment, 

defining asymptotic confidence intervals for the minimum in-trail distances for each case. These 

statistical analyses are provided as an example of how extreme value theory might be used to 

define a SAPA-CZ, not as recommended values. 

More significantly, this study confirmed that proximity to the ground has a significant effect on 

the requirements for pair positioning. This suggests that an altitude-based approach to defining 

pair positioning requirements is prudent for SAPA operations. It is clear from the results above 

that any future implementation of a SAPA operational procedure will be driven by the 

requirement to be within the SAPA-CZ in ground effect. For every combination of parameters in 

this study, the SAPA-CZ would be much smaller in IGE than in OGE. Since the SAPA-CZ in 

NGE would be only slightly smaller than in OGE, the NGE wake boundary will likely not be a 

factor due to the tight IGE window. This suggests that a two-phased conformance zone may be 

most appropriate for operational implementation of SAPA. 

This altitude-based difference in wake behavior also means that in order to accommodate a 

maximum approach speed differential between the two aircraft, the optimal initial positioning is 

for the aircraft with the faster approach speed to be trailing the aircraft with the lower approach 

speed. The maximum distance between the two aircraft should occur while they are within 

larger OGE SAPA-CZ. 

For the purposes of example, let's consider an aircraft pairing on a 750 ft runway centerline 

spacing and no runway threshold offset, where two aircraft are at the nominal experimental 

profile speeds and with zero navigational errors of the speed and altitude profiles used in this 

experiment. Further, assume that the trailing aircraft does not overtake the slower aircraft on the 

approach, and place this aircraft at the back of the IGE SAPA-CZ (1000 ft in-trail) at the point 

where the leader reaches 176 ft altitude. Performing a backwards integration of the speed 

profiles for the two aircraft, the in-trail distance between the two aircraft at the beginning of the 

NGE altitude boundary is approximately 1453 ft, which is much smaller than the experimentally 

observed smallest in-trail distance leading to a wake encounter of 2621 ft. This shows that the 
33

NGE wake boundary is not a limiting factor due to the much smaller IGE wake boundary. 

Further, integrating the speed profiles of the two aircraft until the leader is at 5 nm from the 

runway threshold (the start of the OGE zone for this experiment) results in an in-trail distance 

between the two aircraft of 2765 ft, which is still well within the experimentally observed OGE 

smallest distance leading to a wake encounter of 3044 ft. Conversely, if a pair of aircraft are 

initially positioned such that the faster aircraft is at the rear of the OGE SAPA-CZ and the pair 

are to be positioned within the IGE conformance zone by the time they reach 176 ft altitude, then 

the speed differential between the two aircraft must be greater than the 10 kt used in this 

experiment. 

This simple example serves to illustrate that aircraft pairing on a proposed SAPA procedure will 

have to consider the planned final approach speeds of both aircraft and the size of the IGE 

SAPA-CZ to determine the appropriate initial in-trail distance for any SAPA pairing. It is not as 

simple as placing each pair at the extreme boundaries of the OGE SAPA-CZ because the 

difference between the speed profiles of the two aircraft is what determines the in-trail separation 

at the runway. 

It is well understood that, except for unusual atmospheric conditions, wakes typically descend 

rather than rise. In VMC in-trail approaches where wakes are a concern, pilots are cautioned to 

maintain a flight path that is at or above the path that was taken by the aircraft they are 

following. In very closely spaced parallel-runway operations, the two aircraft typically turn onto 

the final approach course with 1000 ft altitude separation. Once the aircraft are confirmed to be 

longitudinally spaced within the SAPA-CZ, the higher-altitude aircraft is cleared to descend until 

the two aircraft are approximately co-altitude. If the trailing aircraft is the higher altitude 

aircraft, then the trailing aircraft would potentially remain above any wakes generated by the 

lower-altitude leading aircraft during the descent. However, if the leading aircraft is the one that 

is initially at the higher altitude, then the leading aircraft’s descending wake vortices are more 

likely to encounter the trailing aircraft as the two aircraft near co-altitude position. 

This concern of higher likelihood of wake encounter could be mitigated by requiring that the 

initially trailing aircraft be assigned to the higher altitude. However, the trailing aircraft must be 

the faster of the pair. Requiring that the faster aircraft in each pair be routed to the parallel 

runway with the higher final approach altitude may be a significantly inconvenient operational 

restriction for the controller. 

In order to investigate whether altitude variations significantly affect minimum wake-encounter 

distances, a tall rectangular wake detection surface was used in a second set of simulations for 

the cases of the OGE scenario to detect wakes at any possible vertical separation distance 

between the leading and trailing aircraft. The circular detection surface is only effective for 

simulating wake encounters when the two aircraft are approximately co-altitude. The results for 

the tall detection rectangle vs. the detection circle are summarized in Table 12. As shown in the 

table, the closest in-trail distances resulting in wake encounters were only slightly affected by 

changing the detection circle to a tall rectangle. Because such a large circular detection surface 

was used in the simulation, there were very few wakes that passed just below (or above) the 

detection surface. This indicates that accommodating either the trailing or the leading aircraft to 
34

be at the higher initial altitude would likely result in a performance penalty of only 

approximately 200 ft in the size of the OGE conformance zone. 

Table 12. Comparison of Closest Initial In-Trail Distance Resulting in a Wake Encounter for 

Detection Circle vs. Tall Detection Rectangle, for Each Lateral Runway Spacing 

Detection Surface Lateral Runway Spacing

750 ft 1000 ft 1400 ft

Detection Circle 3044 ft 6026 ft 11,171 ft

Tall Detection Rectangle 2886 ft 5800 ft 10,550 ft

The results for the runway threshold offset cases show that runway threshold offset can have a 

significant impact on the wake interaction between a pair of aircraft conducting SAPA 

operations. The mixed nature of the results corroborate the need for additional simulation 

studies to understand the impact that various runway threshold offsets will have on wake-safe 

boundaries for SAPA operations, particularly as the pair of aircraft move longitudinally and 

hence vertically in relation to each other. 
35

Conduct of SAPA Operations 

The results of the Monte Carlo wake study yielded several key design parameters for SAPA 

operations. The pair of aircraft should initially be positioned such that the aircraft with the faster 

approach speed is trailing the aircraft with the slower approach speed. When the pair of aircraft 

are initially established by ATC on the final approach course separated vertically by 1000 ft, 

either the trailing or the leading aircraft can initially be placed at the higher altitude. 

This section presents issues related to how SAPA operations would be conducted. The following 

section describes in more detail the controller and flight crew processes for initiating the SAPA 

procedure and attaining the initial pair positioning. 

The primary means for safely conducting SAPA approaches to very closely spaced parallel 

runways are through approach procedure design, highly accurate navigation sources, and flight 

operations requirements to include the use of autopilot and SAPA-unique speed management, 

where the combination of these practices greatly reduces the probability of one aircraft intruding 

into the approach path of another or creating a wake vortex encounter. In support of these 

considerations, it is expected that the SAPA procedure will be conducted with the use of 

autopilot and auto-thrust for flight-path control for both aircraft in the SAPA pair. The 

characteristics of an effective speed management algorithm for SAPA operations are explored in 

the first sub-section. Issues related to initial vertical separation are presented in the second subsection, followed by a discussion of missed approach procedures. SAPA relies on the 

participating aircraft pair broadcasting via ADS-B critical flight information, such as position, 

ground track, ground speed, the PFAS, and SAPA status information, and these requirements are 

discussed in the final subsection. 

Speed Management 

ATC will sequence the aircraft pair 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. Once the aircraft are 

established on the final approach course, the clearance to conduct a SAPA procedure may then 

be issued, with both aircraft requiring the SAPA clearance. Upon issuance of the SAPA 

clearance, the SAPA paired aircraft will fly the necessary speed profiles required to perform the 

SAPA operation. 

To avoid a possible “race” condition where each paired aircraft keeps changing speed to catch up 

with the other, the aircraft with the slower PFAS will be assigned as the reference aircraft. After 

the SAPA clearance is issued, the slow aircraft will maintain the last ATC assigned speed until it 

reaches the FAF or the position designated in the procedure where the deceleration to the PFAS 

begins. This is consistent with single runway in-trail operations at high traffic volume airports. 

At this point, the slow aircraft will decelerate to its PFAS using a linear deceleration schedule 

such that it will just reach its PFAS at a distance from the runway touchdown point 
36

corresponding to 1000 ft AGL on the glide slope (Figure 22). It will then maintain that speed 

until landing. 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. 

Figure 22. Generic Calibrated Air Speed (CAS) profile. 

After the SAPA clearance is issued, the fast aircraft will adjust its speed to obtain the correct 

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

the SAPA pair. The computation for this correct position is provided in Appendix A. 

A speed management algorithm was developed to support SAPA operations. The basic 

operational requirements for the speed management algorithm are as follows: 

 aircraft will be at their final approach speed at a position no closer to the runway touchdown 

point than a distance that corresponds to an altitude of 1000 ft AGL on the glide slope; 

 aircraft will fly their aircraft / operator specified final approach speeds; and 

 speed guidance will provide speeds that are operationally reasonable relative to current 

operations. 

In exploring various speed management algorithms, it was determined that varying the speed 

schedules for when each aircraft slows to its respective PFAS can have a significant impact on 

what speed differences between the two aircraft can be allowed as well as on the operational 

acceptability of the speed management to pilots. Four different models or techniques for speed 

management were examined. From the four models that were considered, the one that had the 

greatest merit relative to simplicity and operational robustness was a station-keeping technique. 

In this technique, the fast aircraft, once properly positioned, would then remain in a fixed relative 

position by matching the ground speed of the leading aircraft. Once it reached a speed equal to 

its PFAS, which would naturally occur due to the leader decelerating, it would then maintain that 

speed until landing. This technique allows the spacing aircraft to make minor spacing 

adjustments to compensate for wind prediction or other similar errors until it reaches its PFAS 

while still achieving a smooth deceleration to that speed. After the fast aircraft reaches its PFAS, 

the relative aircraft positions will begin to change, with the fast aircraft beginning to overtake the 

slow aircraft. Assuming that the initial pair positioning criteria are met at the beginning of the 

procedure and that there are no large, unaccounted errors (e.g., large wind prediction errors), 

then the aircraft should remain within the SAPA-CZ until both aircraft land. An example of the 

generic ground speed profile for this station-keeping technique is provided in Figure 23. 

Distance from touchdown point (increasing)

FAF

1000' altitude

(stable point)

Runway

threshold

0

CAS

profile
37

Figure 23. Generic ground speed profile for station-keeping. 

Initial Vertical Separation 

When ATC initially establishes the SAPA aircraft pair on the final approach course, the pair is 

initially separated vertically by 1000 ft, and ATC may place either the fast or the slow aircraft on

the "high-side" approach. The aircraft are likely not yet longitudinally positioned within the 

SAPA-CZ early on the final approach course, but ATC will have positioned them such that the 

actively spacing trailing aircraft can catch up to the appropriate SAPA-CZ positioning. After the 

SAPA clearance is issued and prior to the paired aircraft losing vertical separation, the aircraft 

must be longitudinally positioned within the SAPA-CZ. This within-the-window requirement 

must occur prior to the high-side aircraft reaching its glide-slope-intercept point. The worst case 

situation for the two possible aircraft / altitude combinations is when the slow aircraft is the highside aircraft. In this situation, the SAPA-CZ requirement must be met at a point where both 

aircraft are farther from the runway threshold, as compared to the situation where the fast aircraft 

is the high-side aircraft. If the slow aircraft is the high-side aircraft, and assuming a 4000 ft 

high-side intercept on a 3o

 glide slope, the pair of aircraft must be within the SAPA-CZ window 

before the slow aircraft reaches 12.3 nm from the runway. If the SAPA-CZ requirement is not 

met at the point that the high-side aircraft intercepts its glide slope, then the SAPA operation 

cannot continue and the high-side aircraft will conduct a missed approach. In this instance, the 

high-side aircraft will maintain its altitude, tracking the final approach course, and notify ATC of 

the situation. 

It is expected that ATC will issue a SAPA clearance to the relevant SAPA aircraft pair 

sufficiently early in the final approach such that they may maneuver in the manner described in 

the Speed Management subsection to obtain the appropriate relative position prior to the highside aircraft reaching its glide-slope-intercept point. 

FAF

1000' altitude

(stable point)

Runway

threshold

Distance from touchdown point (increasing)

0

Ground

speed

profile

Slow aircraft

Fast aircraft
38

Missed Approach 

Once SAPA procedures are being used for separation, i.e., after the high-side aircraft begins its 

descent on the glide slope, several situations could lead to one or both of the SAPA paired 

aircraft needing to conduct a missed approach or "breakout" maneuver. The most obvious 

situation requiring a missed approach would be the physically trailing aircraft lagging outside of 

the SAPA-CZ. In this instance, a traditional missed-approach procedure would be adequate, i.e., 

a climb until passing the missed-approach point, located near the runway threshold, followed by 

any turn that is required by the procedure. The second situation that would require a missed 

approach or "breakout" maneuver would be if one aircraft inadvertently turned toward the other 

aircraft and created a collision risk. The development of an alerting algorithm and blunder 

avoidance maneuver will be the subject of a future study. The third situation, which eventually 

drove the missed-approach requirement in this concept analysis, is the case where the physically 

leading aircraft must conduct a go-around maneuver due to other non-SAPA operational 

problems; e.g., a blocked landing runway for the leading aircraft. This situation is likely to occur 

at a sufficiently high probability that it must be addressed by the SAPA procedure. Because the 

leading aircraft will be immediately climbing during this situation, the trailing aircraft cannot 

conduct a traditional missed approach and expect to avoid the wake vortex of the leading aircraft. 

This condition will require the trailing aircraft to both climb and turn to avoid the wake and to 

reduce the collision risk. Therefore, the recommended SAPA missed approach procedure is an 

immediate climbing turn in a direction away from the parallel approach path for the other 

runway. Previous research has shown that a 45oclimbing turn away from the opposite aircraft's 

approach path provides an operationally acceptable maneuver while producing a rapid separation 

in relative aircraft positions [Winder2001]. 

SAPA Data Requirements and SAPA Status Information 

Due to the unique aircraft-to-aircraft cooperative characteristics of the SAPA concept, data that 

are not part of the current ADS-B message set will be required to support SAPA operations 

(noting that these data could potentially be provided by other means). These data would be used 

to derive the SAPA status information used by both the SAPA aircraft and ATC. These data 

would also be used for the SAPA-CZ and speed guidance calculations by the SAPA aircraft. 

Data in addition to ADS-B state data would include the PFAS for each aircraft and the last ATC 

assigned speed for the slow aircraft. Database information would also be required that could 

include runway locations and elevations. SAPA status information that would be derived within 

each aircraft and broadcast could include: 

 SAPA failure: derived from navigation performance, auto-flight system status (to include 

engagement status), and SAPA equipment status; 

 SAPA-CZ status: derived from the computed SAPA-CZ and the relative aircraft positions; 

 SAPA speed conformance: derived from the SAPA speed guidance and the aircraft's actual 

CAS; and 

 SAPA go-around status: derived from the go-around status of the auto-flight system. 
39

The SAPA failure and SAPA go-around status would be used by the other aircraft in the SAPA 

pair as part of its go-around requirement derivation. The SAPA failure and SAPA go-around 

status could also be used by ATC as an alert to a SAPA go-around, prior to observing it on a 

radar display or before receiving a radio call from the aircraft. The SAPA-CZ status and SAPA 

speed conformance information could be used by ATC as a trigger for required ATC 

intervention. 

To set up the aircraft to perform the SAPA approach, the controller must deliver the aircraft on 

approach with spacing such that the pilots can achieve the longitudinal spacing within the SAPACZ window before reaching the point where they must begin the SAPA procedure. The 

requirements for onboard guidance and the controller and pilot procedures for setting up, 

initiating, and conducting the SAPA procedure are examined in detail in the next section.
40

Set Up and Initialization of SAPA Procedure 

This section focuses on procedures to support the SAPA operation from the groundside. For the 

SAPA concept to be operationally acceptable, several procedural issues require resolution. The 

SAPA concept will require procedures significantly different than those used today for the 

conduct of instrument approaches and approaches to closely spaced parallel runways. In 

response to these differences, new procedures are described in this section. While these new 

procedures demonstrate the operational feasibility of the SAPA concept, further investigation is 

needed to finalize procedures for SAPA operations. 

The following description is provided as background information for discussions regarding the 

integration of SAPA procedures into air traffic operations. Note that airspace configurations, 

procedures, and techniques vary among facilities; this description is based on a “common 

denominator” of operations. 

Current Operations

Airport acceptance rates are based on many factors, including the types of operations that are 

being performed on parallel runways. For instrument approaches to closely spaced parallel 

runways, the pair of runways are treated as a single runway when the distances between 

centerlines are less than 2500 ft. The airport acceptance rate translates into in-trail separations at 

“metering” fixes at the terminal boundary. Metering fixes are usually 30 - 40 miles from the 

airport. Aircraft are “handed-off” (transfer of control responsibility) from the Air Route Traffic 

Control Center (ARTCC) to the Terminal Radar Approach CONtrol (TRACON) prior to the 

TRACON boundary. In general, the initial controller in the TRACON responsible for arrival 

aircraft is the “feeder” controller (note that terminology with respect to positions in TRACONs 

may vary). Upon verifying that the flight crew has received the current Automatic Terminal 

Information Service (ATIS) broadcast, the feeder controller separates and sequences arrival 

traffic such that a manageable flow is provided to the “final” controller. The final controller 

accepts hand-offs from the feeder controller and is responsible for transitioning aircraft to the 

final approach course, sequencing of aircraft (usually to one runway), and providing the requisite 

separation5

. Transfer of communications to the tower normally takes place at or outside the 

FAF. Specific procedural requirements for the aforementioned tasks are found in FAA Order 

7110.65T [FAA7110.65].

 

5 The distance between aircraft outside the final approach fix does not normally reflect the separation 

required between a specific aircraft pair. Additional spacing is normally applied to allow for 

“compression” which occurs when the lead aircraft decelerates to its final approach speed at which time 

separation would reduce. This is not necessarily the case when the final approach speed of the leading 

aircraft is higher than the following aircraft. 
41

SAPA Procedures 

The SAPA procedures impose additional requirements for aircraft entering the terminal area and 

landing. Critical to the procedure is knowledge by the flight crews and ATC of the planned final 

approach speeds of both aircraft and identities of both aircraft in the pair. Aircraft not 

participating in the SAPA procedure have to be identified early in the process of spacing and 

sequencing aircraft so that spacing adjustments can be made. Positioning of the SAPA pair has 

to be accomplished with reasonable precision such that both aircraft are established in the SAPACZ prior to loss of 1000 ft vertical separation at the SAPA Out of Conformance Missed 

Approach Point (SOCMAP). Upon acceptance of a clearance for the SAPA procedure, the flight 

crew assumes certain responsibilities for successful execution of the procedure. 

The following subsection discusses an element to the conduct of SAPA operations–the 

positioning of aircraft such that they are within acceptable bounds to conduct the procedure. In 

considering the feasibility of controllers placing aircraft within acceptable bounds, an operation 

currently in practice with similarities is noted. At San Francisco International Airport (SFO), 

simultaneous visual approaches are routinely conducted to parallel runways separated by 750 ft 

(centerline-to-centerline). Due to aircraft departing between arrivals on a set of perpendicular 

parallel runways, arrivals need to be paired and the pairs separated by 4 1/2 to 5 miles. Although 

these arrival aircraft are conducting visual approaches, there are parallels to be drawn between 

these operations and those proposed for SAPA. The simultaneous visual approaches in use at 

SFO today were used as a starting point for developing an initial set of candidate SAPA 

procedures. 

Positioning Task 

The SAPA concept requires establishment of a geometrical relationship within a pair of 

participating aircraft; this relationship must be maintained within defined acceptable bounds. 

Separation of the pair of aircraft from the next pair of aircraft ahead and behind must also be 

maintained. In the case of separation from another pair of aircraft conducting the SAPA 

procedure, separation must be maintained from the aft-most permissible position of the lead set 

of aircraft. 

In the conduct of approaches to two runways, there are normally two controllers each providing 

spacing and sequencing to their respective runways. 

There are several basic assumptions upon which the procedures discussed in this document with 

respect to the pairing process are based. They are as follows: 

 Aircraft with the faster final approach speed will be initially positioned as the trailing aircraft

in the pair; 

 Standard procedures for final approach course turn-ons used for simultaneous instrument 

approaches to parallel runways are employed; and 
42

 Vertical separation requirements for turning aircraft onto parallel final approach courses will 

be observed; specifically, 1000 ft vertical separation will be maintained during turn-on. 

The required actions for positioning the aircraft in the SAPA-CZ are described below. Initial 

positioning is accomplished by ATC; this positioning sets up the flight crew for the final 

positioning phase. The final phase is accomplished by the flight crew and positions the aircraft 

in the SAPA-CZ. 

Initial Positioning of Aircraft 

Both the feeder controller and the final controller are responsible for initial positioning of aircraft 

for the SAPA procedure. The process is as follows. The ATIS broadcast includes verbiage that 

SAPA is in use for a runway pair and that flight crews that will not be participating shall notify 

ATC upon initial contact with the TRACON6

. A slot in the flow will be created to provide for 

aircraft that will not be paired with another aircraft. Upon entering the terminal area, the flight 

crew advises the feeder controller of their PFAS. PFASs are critical information for the final 

controllers; they can be transmitted to the final controller verbally or through alternative 

methods. One potential method is having the feeder controller enter the PFAS into the data 

block scratch pad (see Figure 24). Note that in the data block on the left, the airspeed field 

shows “24” for a current groundspeed of 240 kt. A scratch pad entry could permit display of the 

PFAS, in this case “S” indicating “speed” and “134” indicating a PFAS of 134 KIAS (example 

on right). The PFAS would “time share” with the current ground speed. This information 

display could serve as coordination of the PFASs, which in turn will indicate which aircraft will 

be vectored onto the final approach course in the lead position. 

Figure 24. Depiction of PFAS (S134) input by controller 

in standard data block using time-sharing capability. 

The feeder controller is responsible for positioning the aircraft, through issuance of headings and 

speeds, such that the final controller can achieve the objective of further positioning the aircraft 

for the SAPA procedure. A handoff to the final controller will be accomplished using standard 

handoff procedures. 

 

6

 Note that as the Traffic Flow Management process is refined, knowledge of non-participating aircraft should be 

known much earlier than terminal area entry. This information could perhaps be part of the flight plan. If SAPA 

procedures are not in use upon arrival at the destination, it would not matter. 
43

The leading and trailing aircraft are handled differently with respect to the SAPA procedure. 

Recall that the aircraft with the slower PFAS will always be positioned by the final controller as 

the leading aircraft in the SAPA pair. This aircraft with the slower PFAS will be vectored onto 

the final approach course and given a speed to maintain to the FAF that will provide adequate 

separation from the lead pair of aircraft ahead. This will be described in detail in the following 

Separation Requirements subsection. 

The leading aircraft, once established on the final approach course at the assigned speed, 

basically serves as the “reference” aircraft. The trailing aircraft, which is the active-spacing 

aircraft, will be positioned such that the appropriate position in the SAPA-CZ can be achieved 

through speed management. 

Ultimately, the objective of the final controller is to position the trailing aircraft such that the 

flight crew can achieve the appropriate position in the SAPA-CZ through application of speed 

management. There are several strategies for appropriate positioning of the trailing aircraft, and 

these will need to be investigated further in future studies. The feeder controller hands off the 

aircraft in a position that will permit the final controller to provide a transition to the final 

approach course within reasonable proximity to the leading aircraft. The final controller issues 

appropriate control instructions (discussed in the following paragraph) to establish the trailing 

aircraft on the final approach course followed by a speed command that permits positioning in 

the SAPA-CZ prior to the SOCMAP. 

Conduct of the SAPA procedure requires authorization from ATC and this will be issued in the 

form of a clearance. This clearance could be incorporated in the approach clearance; however, 

current approach clearance phraseology is fairly long. For the current iteration of SAPA 

procedures development, clearance for the SAPA procedure will be a separate clearance. Recall 

that both aircraft in the pair will be issued the SAPA clearance. 

Candidate phraseology for the SAPA procedure is: 

“(A/C ID), PAIRED TRAFFIC (A/C ID of paired aircraft), CLEARED SAPA PROCEDURE” 

For the leading (reference) aircraft, the clearance for, and acceptance of, the SAPA procedure 

implies the following: 

 Flight crew will maintain the last speed assigned from ATC until reaching the FAF; 

 Altitude shall be maintained as assigned by ATC until glide slope intercept; and 

 Once the aircraft intercepts the glide slope and the 1000 ft vertical separation is no longer in 

effect, the flight crew assumes responsibility for monitoring parallel traffic for blunders 

using onboard alerting. 

For the trailing (active-spacing) aircraft, the clearance for, and acceptance of, the SAPA 

procedure implies the following: 
44

 Flight crew will maintain the last speed assigned from ATC until deceleration is required to 

assume the appropriate position in the SAPA-CZ; 

 When the trailing aircraft is established in the SAPA window, speed may be adjusted as 

necessary; however, the aircraft must remain within the confines of the SAPA-CZ window; 

 Altitude shall be maintained as assigned by ATC until glide slope intercept; and 

 Once either aircraft intercepts the glide slope and the 1000 ft vertical separation is no longer 

in effect, the flight crew assumes responsibility for monitoring parallel traffic for blunders 

using onboard alerting. 

Upon acceptance of the SAPA clearance, ATC shall assume a monitoring role with respect to the 

two aircraft. Figure 25 provides a plan view of the aircraft positioning scheme for the SAPA 

procedure. 

Final Positioning of Aircraft 

Final positioning refers to actions taken by the flight crew of the trailing aircraft to appropriately 

position their aircraft relative to the leading aircraft. At this point, both flight crews have 

accepted the SAPA clearance, and the final controller will have assigned speeds to each aircraft 

that will permit a closure rate allowing for final positioning to occur in advance of the aircraft 

reaching the SOCMAP. The flight crew of the leading aircraft will maintain the assigned speed 

until just prior to the Stabilized Approach Point, then decelerate to the final approach speed by 

the time the SAP is reached. The trailing aircraft will maintain the speed assigned by the 

controller until deceleration is necessary for maintaining appropriate position within the SAPACZ. It is assumed that there is automation onboard the aircraft to assist with the positioning task. 

Figure 25. Positioning of SAPA aircraft on final approach course. 
45

Separation Requirements 

There are two considerations with respect to separation requirements for the SAPA procedure. 

First is the existing requirement to provide separation between all aircraft on the final approach 

courses. 

Within a pair of aircraft conducting SAPA operations, vertical separation is applied until the 

higher-altitude aircraft intercepts the glide slope and begins its descent. So for the SAPA 

procedure, separation responsibility ends for ATC when the higher-altitude aircraft initiates the 

descent.

In the case of separation between two SAPA aircraft pairs, in-trail separation is applied between 

the SAPA-CZ boundaries. For the following discussion, refer to Figure 26. Separation is 

essentially being applied between the forward-most and aft-most positions in the SAPA-CZ; this 

is necessary because aircraft pairs are permitted to use the entire longitudinal bounds of the 

SAPA-CZ. Since the aircraft that is initially leading is the reference aircraft, the SAPA-CZ 

boundaries are defined with respect to the position of the leading aircraft. The candidate SAPA 

procedures enhance the controller’s ability to effectively monitor and maintain appropriate 

separation between aircraft pairs by having the reference aircraft maintain the speed assigned by 

the controller until just prior to the SAP. Rather than trying to anticipate the movement of the 

active-spacing aircraft, the controller simply ensures that the entire SAPA-CZ defined around the 

reference aircraft is safely separated from the SAPA-CZ for the pair ahead and the SAPA-CZ for 

the pair behind. 

To support the controller in separating SAPA-CZ boundaries, display of those boundaries is 

likely required. In the example below, 4 nm is required at the threshold between aircraft pair A 

and pair B and between pair B and pair C. Note that there is currently 5 nm between pair B and 

pair C to allow for compression as the lead pair decelerates. Absent presentation of the SAPACZs, an allowance is required to ensure appropriate separation is applied. One way this could be 

accomplished is through adding spacing to allow for what to the controller is an unknown. The 

determination of maximum bounds could provide for the establishment of appropriate additions 

to current standards. A reasonable analogy is the additional separation required for formation 

flights from other aircraft; an additional mile is required when a formation flight is involved 

[FAA7110.65]. 
46

Figure 26. In-trail separation requirements between pairs of aircraft.

Roles and Responsibilities 

Previous discussions have alluded to responsibilities through description of procedures. The 

following list summarizes responsibilities of ATC and the flight crews. Note that the procedures 

listed are limited to those unique to the SAPA procedure. 

Responsibilities of ATC 

 Advertise the following on an ATIS broadcast: use of SAPA procedures, runways, and 

advise ATC on initial contact of PFAS, or if unable to participate in SAPA procedure 

 On initial contact verify PFAS; if advised by flight crew that they will not accept the SAPA 

procedure, initiate coordination for appropriate handling of aircraft 

 Coordinate SAPA pairing of aircraft as appropriate 

 Provide appropriate control instructions to position aircraft for the SAPA procedure 

 Issue clearance for SAPA procedure (this includes a traffic call pointing out appropriate 

traffic) 

 Monitor position of aircraft conducting the SAPA procedure7

 

7

 “Monitoring” in this case refers to the responsibility that rests with controllers for scanning their area for situations 

requiring corrective action. As mentioned, display of the SAPA window would be of benefit to the controller. 

Detecting deviations from acceptable bounds for aircraft pairs would not be feasible with traditional terminal 

displays. Further, the current concept does not require staffing of monitor positions as required for simultaneous 

independent instrument approaches or a Precision Runway Monitoring (PRM) position, or similar capability. 

Aircraft Pair A Pair B Pair C 
47

Responsibilities of the flight crew 

 Flight crew obtains ATIS broadcast prior to contact with TRACON; advises TRACON on 

initial contact of: PFAS (indicates that will accept SAPA procedure) or non-participation in 

SAPA Procedure 

 Acceptance of SAPA procedure; implies that flight crew will perform positioning task 

associated with the SAPA procedure 

 In the event of deviation from SAPA-CZ, flight crew will initiate a missed approach using 

SAPA-specific missed approach procedure and advise ATC 

Airspace Requirements 

To facilitate the conduct of the SAPA procedure, adequate distance is required along the final 

approach course. The required distance would have to reside in the final controller’s airspace. 

The final determination of the distance required is the subject of further investigation focused on 

techniques applied by ATC and the flight deck. In the previous section, Conduct of SAPA 

Operations, it was determined that the aircraft would have to be positioned within the SAPA-CZ 

as much as 12.3 nm from the runway threshold. A preliminary assessment of distances available 

for aircraft positioning in the final controller’s airspace at four major air traffic facilities suggest 

that adequate airspace is available. 

Traffic Flow Management Considerations 

The SAPA procedure is designed to increase capacity at airports with closely spaced parallel 

runways during periods when instrument approach procedures are in use. The use of instrument 

approach procedures (as opposed to the use of visual approaches) requires the application of 

separation standards. The use of the SAPA procedure permits operations to two runways where 

current procedures would limit arrival operations to one of the parallel runways. The airport 

acceptance rate would reflect use of the procedure. Note that there may be limitations on use of 

the procedure for reasons such as maximum crosswind limitations. There will also have to be 

accommodations made for non-participating aircraft. Finally, airport capacity considerations 

based on availability of the procedure could influence selection of runway configurations. 

Role of Automation 

There are several areas where the SAPA procedure will require tools or displays aids. Those 

mentioned here support the role of ATC in the conduct of the procedure. To minimize workload 

and coordination, metering of the aircraft to fixes at the terminal boundary is useful; whether or 
48

not this is a requirement is the subject of future research. Current traffic management tools such 

as the Center-TRACON Automation System (CTAS) Traffic Management Advisor (TMA) could 

provide the needed functions, including identities of the paired aircraft. A depiction of the 

SAPA-CZ, although not currently considered a requirement, would be highly beneficial and 

would assist the controller in applying the appropriate in-trail separation between aircraft pairs 

and other aircraft pairs or single aircraft. If the SAPA-CZ is not displayed, additional separation 

would be required as the controller could not be certain where the bounds of acceptable 

longitudinal maneuvering could occur. 
49

Concluding Remarks 

At the request of the FAA, NASA has performed an initial assessment of the potential 

performance and feasibility of the SAPA concept. This initial assessment included an initial 

system study of the SAPA concept to assess operational benefits and constraints plus a 

preliminary technical feasibility analysis. While this assessment is preliminary, it has shown the 

basic SAPA concept to be technically and operationally feasible. An example implementation of 

the concept was developed in some detail, including identification of potential pilot/controller 

roles and responsibilities. Several design and implementation issues were explored, which 

should be useful to aid the FAA in the development and refinement of the SAPA concept. 

The assessment has revealed that the SAPA concept has significant operational advantages in 

allowing the pairing of aircraft with dissimilar final approach speeds for dependent operations, if 

blunder safety, which was not addressed in the current study, can be shown to be acceptable. 

The use of abeam positioning rather than echelon positioning will require higher lateral 

navigation performance to reduce the likelihood of blunders, but this requirement appears 

achievable in the mid-term timeframe. A preliminary look at lateral navigation requirements 

revealed that the required navigation and FTE accuracy are aggressive, but seem to be within the 

realm of existing state-of-the-art avionics. 

The requirements for longitudinal positioning were investigated through a Monte Carlo 

simulation. The use of a two-phased conformance zone, with a larger longitudinal conformance 

zone defined for higher altitudes out of ground effect and another conformance zone defined for 

altitudes near and in ground effect, has the potential to allow even larger speed differentials. Onboard speed management, such as station-keeping, is probably required to achieve robust 

performance in a realistic operational environment.

The current study was only a preliminary assessment of the basic SAPA concept, and many more 

studies will be required before an implementation decision could be supported. There are a 

number of studies that need to be conducted related to the breakout maneuver, including 

identification of the appropriate triggering conditions and alerting mechanisms, the operational 

aspects of using coupled autopilot for the maneuver, and developing operational procedures for 

the breakout and missed-approach maneuvers. 

There are a number of operational issues that need to be studied related to the SAPA procedure 

design. Onboard speed management requirements and algorithms must be developed. The 

design of SAPA operational procedures that are robust to off-nominal and non-normal situations 

must be investigated. Also, because the SAPA operation will be performed in conjunction with 

other current and future planned procedures, such as time-based metering and merging and 

spacing, the initial pairing of aircraft needs to be studied with consideration for other current and 

future planned ATC equipage and procedures. 

A detailed examination of the feasibility, safety, and operational details of the blunder avoidance 

maneuver is needed. Blunder detection and reaction times are minimal at the targeted lateral 

runway spacings, and the feasibility of meeting the timing requirements should be explored, 
50

including the timing required to initiate a pre-programmed breakout maneuver. The SAPA 

concept includes the use of ADS-B to share position and status information between the aircraft, 

and the sharing of additional SAPA-specific information via ADS-B intent to aid in timely 

onboard blunder alerting should be explored. 

Additional studies are required to define the requirements for the SAPA-CZ. The current study 

explored wake encounters only for a specific pair of aircraft, and wake studies need to be 

conducted to define safe zone parameters across the full range of possible SAPA use scenarios, 

including aircraft pairs, landing weights, approach speeds, runway lateral spacings, runway 

threshold offsets, crosswind velocities and directions, etc. The behavior of wakes at various 

altitudes on approach as they transition into ground effect also needs to be characterized in order 

to define effective altitude-based conformance zones. These studies are especially problematic 

because of the lack of observational data and understanding of wake movement and decay in and 

near ground effect. This will be a serious problem in defining a wake-safe conformance zone 

boundary for operational implementation of any dependent closely spaced parallel operation, not 

just for SAPA. 
51

Acknowledgements 

The authors would like to thank the following team members who contributed to this effort: 

Chris Swider (FAA), Lisa O. Rippy, Daniel M. Williams, Vincent P. Schultz, Fernando RicoCusi, Hazari I. Syed (Raytheon), H. Paul Stough, Fred H. Proctor, and Kurt W. Neitzke.
52

References

[Abbott2001] Abbott, Terence S; and Elliott, Dawn M: Simulator Evaluation of Airborne 

Information for Lateral Spacing (AILS) Concept, NASA/TP-2001-210665, March 2001. 

[ATSI2010] ATSI: Available: http://www.atsi.com, 2010. 

[CSPO2009] Closely Spaced Parallel Operations Working Group: Closely Spaced Parallel 

Operations Concept of Operations, draft, FAA, September 2009. 

[DeCleene2008] DeCleene, Bruce; and Swider, Christopher: Simplified Aircraft-Based Paired 

Approach (SAPA), Internal FAA concept paper, July 2008. 

[FAA7110.65] FAA: Order 7110.65T, Air Traffic Control, February 2008. 

[Hammer1999] Hammer, J.: Study of the Geometry of a Dependent Approach Procedure to 

Closely Spaced Parallel Runways, Proceedings of the IEEE 18th Digital Avionics Systems 

Conference, October 1999.

[Landry2000] Landry, S. and Pritchett, A. R.: The Safe Zone for Paired Closely Spaced Parallel 

Approaches: Implications for Procedures and Automation, Proceedings of the IEEE 19th Digital 

Avionics Systems Conference, October 2000. 

[McKissick2009] McKissick, Burnell T.; Rico-Cusi, Fernando J.; Murdoch, Jennifer L.; 

Oseguera-Lohr, Rosa; Stough; H. Paul, III; O’Connor, Cornelius J. and Syed, Hazari I.: Wake 

Encounter Analysis for a Closely Spaced Parallel Runway Paired Approach Simulation; 9th 

AIAA Aviation Technology, Integration, and Operations Conference, September 2009. 

[Proctor1998] Proctor, F.H.: The NASA-Langley Wake Vortex Modelling Effort in Support of 

an Operational Aircraft Spacing System, 36th Aerospace Sciences Meeting & Exhibit, January 

1998, AIAA 98-0589. 

[Proctor1999] Proctor, Fred H.; and Han, Jongil: Numerical Study of Wake Vortex Interaction 

with the Ground Using the Terminal Area Simulation System, 37th Aerospace Sciences 

Meeting & Exhibit, January 1999, AIAA 99-0754

[Proctor2000] Proctor, F.H.; Hamilton, D.W.; and Han, J.: Wake Vortex Transport and Decay in 

Ground Effect: Vortex Linking with the Ground, 38th Aerospace Sciences Meeting & Exhibit, 

January 2000, AIAA-2000-0757.

[Proctor2009a] Proctor, Fred H.: Interaction of Aircraft Wakes from Laterally Spaced 

Aircraft, 47th AIAA Aerospace Sciences Meeting Including The New Horizons Forum and 

Aerospace Exposition, January 2009, AIAA-2009-343.

[Proctor2009b] Proctor, F. H.; and Hamilton, D. W.: Evaluation of Fast-Time Wake Vortex 

Prediction Models, 47th AIAA Aerospace Sciences Meeting, January 2009, AIAA 2009-344. 
53

[Sarpkaya2000] Sarpkaya, T.: New Model for Vortex Decay in the Atmosphere, Journal of 

Aircraft, Vol. 37, No. 1, September 2000. 

[Stone1996] Stone, R., Paired Approach Concept, Proceedings of the NASA Workshop on Flight 

Deck Centered Parallel Runway Approaches in Instrument Meteorological Conditions, NASA 

CP 10191, October 1996.

[Winder2001] Winder, Lee F.; and Kuchar, James K.: Generalized Philosophy of Alerting with 

Applications for Parallel Approach Collision Prevention, AIAA Guidance, Navigation, and 

Control Conference, August 2001. 
54

Appendix A – Speed Management using a Station-Keeping Model 

Definitions and Abbreviations 

CAS Calibrated airspeed. 

CFAGS Corrected final approach speed (CAS) converted to ground speed. 

D The desired distance the trailing aircraft is behind the leading aircraft as the leading 

aircraft crosses the FAF. 

FAF Final approach fix or the final glide slope intercept point from the lowest published 

altitude. 

GAP The glide slope abeam point. This is the point at the centerline of the runway that is 

abeam of the glide slope. This is also the approximate point where the aircraft lands. 

GndSpd Ground speed. 

M The point on the CAS speed profile of the aircraft with the slower PFAS where the CAS 

is equal to the PFAS of the faster aircraft. 

PFAS Planned final approach speed. The CAS the aircraft will fly along the stable approach 

segment. 

SAP Stabilized Approach Point. The distance from the runway where the aircraft must be 

configured for landing and be at its PFAS. This point is typically 3.14 nm from the GAP 

(distance = 1000 ft/ tan 3°), assuming a 3o glide slope. 

TTL Time to land. The time to fly from a given point to the GAP. 

SAPA-CZ The "window" length, either forward or rearward from a nominal aircraft's center point to 

the SAPA wake vortex boundary limit. Note that because ground effect influences wake 

lateral movement, the length of this window will change depending on where it is on the 

approach, e.g., SAPA-CZFAF ≠ SAPA-CZGAP. It is also assumed that above some altitude, 

and therefore beyond some point on the approach, that this value will not change, e.g., 

SAPA-CZFAF = SAPA-CZGlide slope Intercept Point. 

Unique Subscripts 

fast aircraft with the faster PFAS. 

slow aircraft with the slower PFAS. 

General 

This appendix describes a station-keeping model designed to accommodate pairs of aircraft with 

dissimilar final approach speeds performing SAPA approaches. In this model, a station-keeping 

technique is used to control the speed of the fast aircraft, aircraftfast, not only up to the FAF but 

until aircraftfast reaches its PFAS. The final control point for this model is just prior to the SAP; 

after this point no relative positioning control occurs. No winds are assumed in the equations but 

may be used in the ground speed calculations if they are known. In this station-keeping 

technique the ADS-B state data for the slow aircraft, aircraftslow, is used directly by aircraftfast in 

computing its speed command. Aircraftfast will match the current ground speed of aircraftslow

until the CAS speed command for aircraftfast matches its PFAS. Unless the PFAS for both 
55

aircraft are the same, aircraftfast will reach its PFAS at a point farther from the runway than the 

1000 ft SAP. An example of this concept is shown in Figure A1. 

Figure A1. Station-keeping ground speed for aircraftfast.

Maximum Time Criterion 

In order to determine whether a given pair of dissimilar final approach speeds can be 

accommodated, the length of time that the aircraft are traveling at different speeds must be 

calculated. The time of interest in this calculation occurs at the point where aircraftfast reaches its 

PFAS along the CAS profile of aircraftslow. To calculate the maximum time criterion for this 

model, the following equations are used: 

Using the speed profile for aircraftslow, the point M (Figure A2) is the distance along the CAS 

profile of aircraftslow where the CAS value is equal to PFASfast. 

Figure A2. Station-keeping speeds after the FAF. 

Then,

equation 1: TTLfast = (M + SAPA-CZM) / CFAGSfast,

FAF

SAP

GAP

Distance from touchdown point (increasing)

0

Ground

speed

Aircraftslow

Aircraftfast

Aircraftfast

glide slope 

intercept

Ground

speed

FAF

SAP

GAP

Distance from touchdown point (increasing)

0

Aircraftslow

Aircraftfast

M

SAPA-CZM
56

where SAPA-CZM is the window length at position M and CFAGSfast is the average ground 

speed between point M + SAPA-CZM and the GAP. Note that M is the point where aircraftfast 

reaches its PFAS, so CFAGSfast is the corrected PFASfast (CAS) converted to ground speed. 

The maximum time criterion to allow a SAPA operation is: 

TTLslow < TTLfast + t1,

where TTLslow is the time for aircraftslow to fly from M to the SAP plus the time to fly from the 

SAP to the GAP, and t1 is the delta time from aircraftfast reaching the GAP until aircraftslow

reaches the GAP. 

t1 = s1 / CFAGS 

s1 ≤ SAPA-CZGAP. 

where s1 is the relative distance between the two aircraft when aircraftfast reaches the GAP. Then 

let

 t2 = (M – SAP) / ((GndSpdM + CFAGSslow,SAP) / 2) 

where GndSpdM is the ground speed at a distance = M and CFAGSslow,SAP is the ground speed at 

the SAP, noting that for the general case, CFAGSslow is the corrected PFASslow (CAS) converted 

to ground speed. 

 t3 = SAP / ((CFAGSslow,SAP + CFAGSslow,GAP)/2) 

where CFAGSslow,GAP is the ground speed at the GAP. 

 TTLslow = t2 + t3, 

The maximum time criterion test is then: 

 TTLslow < TTLfast + t1

Position Offset at the FAF 

This section examines the desired initial longitudinal offset between the pair of aircraft. Because 

the aircraft will be station-keeping until aircraftfast reaches its PFAS, the position offset at the 

FAF is the same as the position offset at the point that aircraftfast reaches its PFAS. In this 

model, 

SAPA-CZFAF = SAPA-CZM.

The calculation of the desired position offset between aircraftslow and aircraftfast at the point 

where aircraftfast reaches its PFAS along the speed profile of aircraftslow is as follows: 

t1
57

1. Case where aircraftfast does not pass aircraftslow after the FAF 

In some situations where the PFAS differences are not large, aircraftfast may not need to pass 

aircraftslow after the FAF. This situation occurs when TTLslow is less than TTLfast,

TTLslow < TTLfast

In this situation, assuming that abeam landing of the two aircraft is preferred, the desired 

distance, D, that aircraftfast should be behind aircraftslow as aircraftfast reaches its PFAS is based 

on determining a value of D such that 

TTLslow = TTLfast = (M + D) / CFAGSfast,

where from equation 1, D is substituted for SAPA-CZM and CFAGSfast is the average ground 

speed between point M + D and the GAP. 

Then,

D = (TTLslow x CFAGSfast) – M 

2. Case where aircraftfast passes aircraftslow after the FAF 

If the conditions of the previous case do not apply, i.e., if TTLslow is greater than TTLfast, then 

aircraftfast should initially be positioned at the rear of the SAPA-CZ; thus, 

 D = SAPA-CZM. 

Advantages and Disadvantages 

The major advantage for this model is that any ground speed or wind errors prior to aircraftfast

reaching its PFAS are eliminated. The major problem with this model is that differences in the 

headwind speed due to wind forecast or wind prediction errors after this point may result in 

spacing errors during that portion of the approach. However, this distance is by far the smallest 

for any of the models studied. It should be noted that with this model, aircraftfast will begin to 

slow prior to the FAF since it is station-keeping on aircraftslow. 
58

Appendix B – Box Plot of Initial In-Trail Distance Data 

A synopsis of all initial in-trail distance data with recorded wake encounters in the SAPA study 

is shown in Figure B1. This figure provides a quick visual comparison of the minimum in-trail 

distances resulting in wake encounters for all of the cases run in this experiment. 

The notation used in the figure is as follows. The boxes represent the 25% and 75% containment 

limits for the data, the line denotes the median, and the whiskers represent the extreme 

boundaries of the data. The data is grouped by each simulation, with the following notation: 

 Scenario – Runway centerline spacing – Runway threshold offset 

where 

 S1 refers to the OGE scenario with the wake detection circle, 

 S2 refers to the NGE scenario with the wake detection circle, 

 S3 refers to the IGE scenario with the wake detection circle, and 

 S4 refers to the OGE scenario with the vertical wake detection window. 

For example, “S1-750-0” refers to the OGE scenario runs using the detection circle for a 750 ft 

runway centerline spacing with zero runway threshold offset. 
59

Figure B1. Box plot of results for all experiment cases.
60

Appendix C – Asymptotic Confidence Intervals for Minimum Initial InTrail Distances

A necessary part of experiments is statistical inference. This inference extends the relevant 

results for the sample of data collected or produced by or during the experiment to the population 

from which the sample was taken. The Monte Carlo simulation study investigated the minimum 

initial in-trail distance given a vortex encounter for a scenario case. The current study only 

examined a specific set of cases, so the quantified results of the study are only applicable to the 

specified aircraft pair and for a minimal subset of scenario parameters. Before operational 

implementation of the SAPA concept, detailed analyses will be required to accurately quantify 

minimum in-trail distances and develop standards for the boundaries of the SAPA-CZ. This 

appendix presents a statistical analysis of the data from the experiment, defining asymptotic 

confidence intervals for the minimum in-trail distances for each case. These statistical analyses 

are provided as an example of how extreme value theory might be used to define a SAPA-CZ, 

not as recommended values. 

In the Monte Carlo simulation, the range of initial in-trail distances for each case was defined to 

capture the tail of the distribution rather than the mean. While this decision limited the 

applicability of standard statistical techniques that might be used to characterize the overall 

distribution of the data, it enabled the capture of significantly more data points at the tail, which 

is the region of interest. 

Statistical inference was applied to compute confidence intervals for the 18 combinations of 

scenarios (three) and cases (six). An application of extreme value theory and maximum 

likelihood estimation was used to compute the confidence intervals. But, before any 

computations are presented, it may be instructive to know what confidence intervals are and 

what they are not. In this experiment, the confidence intervals are used to evaluate and report on 

the precision of estimates. The meaning of a confidence interval is often misunderstood, as 

described in the textbook Common Errors in Statistics (and How to Avoid Them): 

A common error is to misinterpret the confidence interval as a statement about the 

unknown parameter. It is not true that the probability that a parameter is included in a 

95% confidence interval is 95%. What is true is that if we derive a large number of 95% 

confidence intervals, we can expect the true value of the parameter to be included in the 

computed intervals 95% of the time. (That is, the true values will be included if the 

assumptions on which the tests and confidence are based are satisfied 100% of the time.) 

Like the p value, the upper and lower confidence limits of a particular confidence interval 

are random variables because they depend upon the sample that is drawn [Good2003].

Thus, if we repeated our Monte Carlo experiment and analyses many times, we could expect the 

minimum initial in-trail distance leading to a wake vortex encounter to be within our confidence 

interval 95% of the time. But it is not accurate to say that we have 95% confidence that the 

minimum initial in-trail distance leading to a wake vortex encounter is within our defined 

interval. It is essential to realize that the confidence to be placed in the interval is no greater 

than the confidence we have in the experiment it is based upon. 
61

Extreme Value Theory 

Extreme value theory was used to select the family of distributions that approximated the true 

distributions of the in-trail distance data. Extreme value distributions are the limiting 

distributions for the minimum (or the maximum) from a large set of random observations from a 

distribution. Tables C1 through C11 contain descriptive statistics about the initial in-trail 

distance data collected in the Monte Carlo experiment runs. These statistics were computed using 

the software EasyFit (Standard) [EasyFit2011]. The Beta distribution was selected as 

appropriate due to two of its properties: (1) the distribution of the limit (as the number of random 

variables increase) of the minimum of a sample of properly normalized independent identically 

distributed random variables is the general extreme value distribution and (2) the Beta 

distribution is in the minimum domain of attraction. Property (1) does not hold for all probability 

distributions. But, property (2) means that property (1) does apply to the Beta distribution. The 

Beta distribution family is as follows:

Parameters 

  continuous shape parameter (  > 0) 

  continuous shape parameter (  > 0) 

 continuous boundary parameters ( 

Domain 

 

Probability Density Function 

 ( 

 

 ( 

( 

 ( 

( 

Cumulative Distribution Function 

 ( ( 

where ≡ 

 

 

, B is the Beta Function, and is the Regularized Incomplete Beta 

Function. 

The Generalized Extreme Value Distribution is as follows: 

Parameters 

 continuous shape parameter 

 continuous scale parameter ( > 0) 

 continuous location parameter
62

Domain 

 ( 

 

 for 

-∞ < x < +∞ for 

Probability Density Function 

 ( { 

 

 ( ( 

 

 

 ( 

 

 

 

 

 ( ( 

Cumulative Distribution Function 

 ( { ( ( 

 

 ( ( 

where ≡ 

 

 

.

The General Extreme Value distributions have three subclasses: Frechet distributions, Gumbel 

distributions and Weibull distributions. Specifically, the Beta distribution is in the minimum 

domain of attraction of the Weibull distribution. More about extreme value theory can be found 

in [de Haan2006] and also in [Embrechts1997]. 

Maximum Likelihood Estimation 

After selecting the Beta distribution, maximum likelihood estimation was applied to each of the 

18 scenario case combinations. The appropriateness of the fit of the distribution to the data was 

judged visually and also by use of the Anderson-Darling goodness-of-fit test. An example of the 

visual judgment is shown in Figures C1 through C4 for the OGE scenario, 750 ft runway 

centerline spacing with zero runway threshold offset. Figures C2 and C3 contain examples of 

“probability plots.” A probability plot is used to compare two distributions. If the distributions 

are the same then the points of the plot will lay on a straight line. Two types of probability plots 

are used. C2 is a probability-probability (p-p) plot, while C3 is a quantile-quantile (q-q) plot. A 

p-p plot is the plot of the percentages (probabilities) of one distribution on the percentages 

(probabilities) of a second. Similarly, a q-q plot is the plot of the percentiles (quantiles) of one 

distribution on the percentiles (quantiles) of a second. It is important to note that the points on 

the left end of the plots appear to almost lie on a straight line. This indicates that the beta 

distribution is a good choice for estimating the lower end point of this empirical distribution. 

The EasyFit (Standard) software was used to perform the maximum likelihood approximations 

and generate the plots for these figures. The Anderson-Darling test did not reject the hypothesis 

at the 99% level that the empirical distribution and a Beta distribution are the same for the above 

scenario case combination. The Anderson-Darling test was used because it is more sensitive to 

data at the ends of the distributions than other popular tests (for example the χ2test or the 

Kolmogorov-Smirnov test). These are typical results. A maximum likelihood estimate for the 
63

minimum initial in-trail distance is a=0.49774 for the above scenario case combination. More 

about maximum likelihood estimation can be found in [vanderWaerden1969] and also 

[Casella2002]. [D’Agostino1986] is a good source of information about the Anderson-Darling 

test and other goodness-of-fit tests. 

Asymptotic Confidence Interval 

According to van der Waerden, the maximum likelihood estimate for the minimum initial in-trail 

is asymptotically normally distributed. That is, as the number of samples increase, the 

distribution functions of the estimates minus the true value times √ tends to a normal 

distribution with mean zero and standard deviation c 

where n is the number of samples, 

 

 

 ( 

 )

 

,

f is the Beta density, and 

a is the left end point of the Beta density. 

The expected value ( 

 )

 

was computed using the initial in-trail data and estimated value of 

a. In the literature this computation is known as the “observed” information number. By applying 

the above information, the asymptotic confidence interval can easily be generated. 

Let ̂ be the maximum likelihood estimate of the true value a and let x

*

be the minimum of the 

sample of data of interest. Then the asymptotic confidence interval is written as 

( ̂ 

 

√ 

 ̂ 

 

√ 

  

where is given by 

√ ∫ 

 

 

 

 

 

 

 .

This is the ( asymptotic confidence interval. The R project statistical software version 

2.7.2 [RFoundation2011] was used to compute the confidence intervals. Tables C12 and C13 list 

the 99% confidence intervals for this experiment. 

Use of Confidence Intervals 

The confidence intervals constructed are intervals containing the left end points of the 

distributions 99% of the time. Data less than the lower confidence intervals limits are points 

where there were no encounters 99% of the time. The initial in-trail distances less than the lower 

confidence limits are points where there are no encounters 99% of the time. 
64

References 

[Casella2002] Casella, G; and Berger, RL: Statistical Inference (Second Edition). Pacific Grove, 

CA: Duxbury Thomson Learning, 2002. 

[D’Agostino1986] D’Agostino, R. B.; and Stephens, M. A.: Goodness-of-Fit Techniques, New 

York, Marcel Dekker. Inc, 1986. 

[deHaan2006] de Haan, L; and Ferreira, A.: Extreme Value Theory (An Introduction), New York, 

Springer Science+Business Media, LLC, 2006. 

[EasyFit2011] EasyFit (Standard), Mathwave Technologies, www.mathwave.com, 2011. 

[Embrechts1997] Embrechts, P.; Kluppelberg, C.; and Mikosch, T.: Modelling Extremal Events, 

Berlin Heidelberg, Springer-Verlag, 1997. 

[Good2003] Good, P. I.; and Hardin, J. W.: Common Errors in Statistics (and How to Avoid 

Them), Hoboken, NJ, John Wiley & Sons, 2003. 

[RFoundation2011] R Foundation, version 2.7.2, www.r-project.org, 2011. 

[vanderWaerden1969] van der Waerden, B. L.: Mathematical Statistics, Berlin, Springer-Verlag, 

1969.
65

Tables 

Table C1. OGE Scenario, 750 ft Runway Centerline Spacing, No Runway Threshold Offset 

Case: OGE, 750, 0

Statistic Value

Sample Size 1627

Range 0.79884

Mean 0.97469

Variance 0.03889

Std. Deviation 0.1972

Coef. of Variation 0.20231

Std. Error 0.00489

Skewness -0.18746

Excess Kurtosis -1.0055

Initial In-Trail Distance

Percentile Value

Min 0.50091

5% 0.65128

10% 0.7053

25% (Q1) 0.81696

50% (Median) 0.98253

75% (Q3) 1.1546

90% 1.2327

95% 1.2657

Max 1.2998

Table C2. OGE Scenario, 1000 ft Runway Centerline Spacing, No Runway Threshold Offset 

Case: OGE, 1000, 0

Statistic Value

Sample Size 1589

Range 0.80819

Mean 1.4875

Variance 0.03738

Std. Deviation 0.19334

Coef. of Variation 0.12998

Std. Error 0.00485

Skewness -0.20751

Excess Kurtosis -0.97165

Initial In-Trail Distance

Percentile Value

Min 0.99173

5% 1.1709

10% 1.2166

25% (Q1) 1.3314

50% (Median) 1.494

75% (Q3) 1.6547

90% 1.7437

95% 1.772

Max 1.7999
66

Table C3. OGE Scenario, 1400 ft Runway Centerline Spacing, No Runway Threshold Offset 

Case: OGE, 1400, 0

Statistic Value

Sample Size 1425

Range 0.76109

Mean 2.3012

Variance 0.03101

Std. Deviation 0.17609

Coef. of Variation 0.07652

Std. Error 0.00466

Skewness -0.24852

Excess Kurtosis -0.83158

Initial In-Trail Distance

Percentile Value

Min 1.8386

5% 1.9916

10% 2.0572

25% (Q1) 2.1646

50% (Median) 2.3079

75% (Q3) 2.4484

90% 2.5292

95% 2.5683

Max 2.5997

Table C4. NGE Scenario, 750 ft Runway Centerline Spacing, No Runway Threshold Offset 

Case: NGE, 750, 0

Statistic Value

Sample Size 1326

Range 0.69915

Mean 0.84049

Variance 0.0256

Std. Deviation 0.16001

Coef. of Variation 0.19038

Std. Error 0.00439

Skewness -0.18904

Excess Kurtosis -0.91989

Initial In-Trail Distance

Percentile Value

Min 0.43138

5% 0.57233

10% 0.6291

25% (Q1) 0.71217

50% (Median) 0.84766

75% (Q3) 0.97215

90% 1.052

95% 1.0771

Max 1.1305

 
67

Table C5. NGE Scenario, 1000 ft Runway Centerline Spacing, No Runway Threshold Offset 

Case: NGE, 1000, 0

Statistic Value

Sample Size 1445

Range 0.75355

Mean 1.3157

Variance 0.03127

Std. Deviation 0.17682

Coef. of Variation 0.1344

Std. Error 0.00465

Skewness -0.14494

Excess Kurtosis -1.0068

Initial In-Trail Distance

Percentile Value

Min 0.88439

5% 1.0206

10% 1.0725

25% (Q1) 1.1702

50% (Median) 1.3236

75% (Q3) 1.4664

90% 1.5478

95% 1.5808

Max 1.6379

Table C6. NGE Scenario, 1400 ft Runway Centerline Spacing, No Runway Threshold Offset 

Case: NGE, 1000, 0

Statistic Value

Sample Size 1617

Range 0.82455

Mean 2.0741

Variance 0.03612

Std. Deviation 0.19004

Coef. of Variation 0.09163

Std. Error 0.00473

Skewness -0.0487

Excess Kurtosis -1.1008

Initial In-Trail Distance

Percentile Value

Min 1.6132

5% 1.7801

10% 1.8188

25% (Q1) 1.912

50% (Median) 2.0765

75% (Q3) 2.2373

90% 2.3306

95% 2.3689

Max 2.4378

 
68

Table C7. IGE Scenario, 750 ft Runway Centerline Spacing, No Runway Threshold Offset 

Case: IGE, 750, 0 

Statistic Value

Sample Size 1886

Range 0.33101

Mean 0.33214

Variance 0.00581

Std. Deviation 0.07622

Coef. of Variation 0.22948

Std. Error 0.00176

Skewness 0.05449

Excess Kurtosis -1.0144

Initial In-Trail Distance

Percentile Value

Min 0.16587

5% 0.21427

10% 0.22987

25% (Q1) 0.26851

50% (Median) 0.33149

75% (Q3) 0.39502

90% 0.43456

95% 0.45315

Max 0.49688

Table C8. IGE Scenario, 1000 ft Runway Centerline Spacing, No Runway Threshold Offset 

Case: IGE, 1400, 0 

Statistic Value

Sample Size 1292

Range 0.26032

Mean 0.48925

Variance 0.00282

Std. Deviation 0.05307

Coef. of Variation 0.10847

Std. Error 0.00148

Skewness 0.06367

Excess Kurtosis -0.81048

Initial In-Trail Distance

Percentile Value

Min 0.35233

5% 0.40823

10% 0.41974

25% (Q1) 0.44642

50% (Median) 0.48888

75% (Q3) 0.52975

90% 0.5596

95% 0.57689

Max 0.61265

 
69

Table C9. IGE Scenario, 1400 ft Runway Centerline Spacing, No Runway Threshold Offset 

Case: IGE, 1400, 0 

Statistic Value

Sample Size 1037

Range 0.23325

Mean 0.83591

Variance 0.00227

Std. Deviation 0.04769

Coef. of Variation 0.05705

Std. Error 0.00148

Skewness 0.06711

Excess Kurtosis -0.64031

Initial In-Trail Distance

Percentile Value

Min 0.71091

5% 0.75898

10% 0.77585

25% (Q1) 0.80062

50% (Median) 0.8354

75% (Q3) 0.87079

90% 0.90433

95% 0.91827

Max 0.94416

Table C10. OGE Scenario, 750 ft Runway Centerline Spacing, 1500 ft Runway Threshold Offset 

Case: OGE, 750, 1500

Statistic Value

Sample Size 1678

Range 0.8918

Mean 0.95937

Variance 0.04135

Std. Deviation 0.20336

Coef. of Variation 0.21197

Std. Error 0.00496

Skewness -0.14095

Excess Kurtosis -0.95818

Initial In-Trail Distance

Percentile Value

Min 0.40771

5% 0.61485

10% 0.68535

25% (Q1) 0.79594

50% (Median) 0.96516

75% (Q3) 1.1386

90% 1.2368

95% 1.2694

Max 1.2995
70

Table C11. OGE Scenario, 1400 ft Runway Centerline Spacing, 

1500 ft Runway Threshold Offset 

Case: OGE, 1400, 

1500 

Statistic Value

Sample Size 1681

Range 0.84143

Mean 1.4591

Variance 0.04096

Std. Deviation 0.20238

Coef. of Variation 0.1387

Std. Error 0.00494

Skewness -0.203

Excess Kurtosis -0.91215

Initial In-Trail 

Distance

Percentile Value

Min 0.9585

5% 1.1171

10% 1.1789

25% (Q1) 1.3033

50% (Median) 1.4683

75% (Q3) 1.6298

90% 1.7283

95% 1.7615

Max 1.7999

Table C12. 99% Confidence Intervals 

99% Asymptotic Confidence Intervals for In Trail Distances (nm) 

Scenario Runway Centerline 

Spacing (ft)

Runway Threshold 

Offset (ft)

Lower 

Limit

Upper 

Limit

OGE 750 0 0.48841 0.50091

OGE 1000 0 0.98398 0.99173

OGE 1400 0 1.82408 1.8386

NGE 750 0 0.40646 0.43138

NGE 1000 0 0.86645 0.88439

NGE 1400 0 1.60263 1.6132

IGE 750 0 0.16299 0.16587

IGE 1000 0 0.34541 0.35233

IGE 1400 0 0.69973 0.70995

OGE 750 1500 0.39643 0.40771

OGE 1000 1500 0.94053 0.9585

OGE 1400 1500 1.75331 1.7639

NGE 750 1500 0.44558 0.46071

NGE 1000 1500 1.08853 1.0938

NGE 1400 1500 1.53168 1.5442

IGE 750 1500 0.18144 0.18347

IGE 1000 1500 0.36478 0.3715

IGE 1400 1500 0.70988 0.71761
71

Figures 

 

Figure C1. Probability Density Functions (α1=1.7417, α2=1.1613, a=0.49774, b=1.2999) 

 
72

Figure C2. Probability-Probability Plot (α1=1.7417, α2=1.1613, a=0.49774, b=1.2999) 
73

Figure C3. Quantile-Quantile Plot (α1=1.7417, α2=1.1613, a=0.49774, b=1.2999) 
74

Figure C4. Probability Difference Plot (α1=1.7417, α2=1.1613, a=0.49774, b=1.2999) 
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Simplified Aircraft-Based Paired Approach: Concept Definition and Initial 

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Johnson, Sally, C.; Lohr, Gary W.; McKissick, Burnell T.; Abbott, Terence 

S.; Guerreiro, Nelson M.; Volk Paul

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NASA Langley Research Center

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Simplified Aircraft-based Parallel Approach (SAPA) is an advanced concept proposed by the Federal Aviation Administration (FAA) to support dependent 

parallel approach operations to runways with lateral spacing closer than 2500 ft. At the request of the FAA, NASA performed an initial assessment of the 

potential performance and feasibility of the SAPA concept, including developing and assessing an operational implementation of the concept and conducting a 

Monte Carlo wake simulation study to examine the longitudinal spacing requirements. The SAPA concept was shown to have significant operational 

advantages in supporting the pairing of aircraft with dissimilar final approach speeds. The wake simulation study showed that support for dissimilar final 

approach speeds could be significantly enhanced through the use of a two-phased altitude-based longitudinal positioning requirement, with larger longitudinal 

positioning allowed for higher altitudes out of ground effect and tighter longitudinal positioning defined for altitudes near and in ground effect. While this 

assessment is preliminary and there are a number of operational issues still to be examined, it has shown the basic SAPA concept to be technically and 

operationally feasible. 

15. SUBJECT TERMS

Runway operations; Wake vortex spacing

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