February 2013 

NASA/TM-2013-217804 

The Simplified Aircraft-Based Paired Approach 

with the ALAS Alerting Algorithm 

Raleigh B. Perry, Michael M. Madden, Wilfredo Torres-Pomales, and Ricky W.Butler 

Langley Research Center, Hampton, Virginia 
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February 2013 

NASA/TM-2013-217804 

The Simplified Aircraft-Based Paired Approach 

with the ALAS Alerting Algorithm 

Raleigh B. Perry, Michael M. Madden, Wilfredo Torres-Pomales, and Ricky W. Butler 

Langley Research Center, Hampton, Virginia 
Available from: 

NASA Center for AeroSpace Information 

7115 Standard Drive 

Hanover, MD 21076-1320 

443-757-5802 

Acknowledgments 

The authors want to thank Cesar Munoz for his important contributions to the design of 

the ALAS algorithm. He developed the ALAS_lines and ALAS_circle functions in the 

PVS theorem prover and established some key mathematical properties of these 

functions. These functions are conflict probes that are of primary importance to the 

algorithm. He also contributed to the design of the control structure of this algorithm. 

The authors also want to thank Jeffrey Maddalon for the technical insight he provided 

that helped us create a practical alerting algorithm. 
iii

Abstract 

This paper presents the results of an investigation of a 

proposed concept for closely spaced parallel runways called the 

Simplified Aircraft-based Paired Approach (SAPA). This 

procedure depends upon a new alerting algorithm called the 

Adjacent Landing Alerting System (ALAS). This study used both 

low fidelity and high fidelity simulations to validate the SAPA 

procedure and test the performance of the new alerting 

algorithm. The low fidelity simulation enabled a determination 

of minimum approach distance for the worst case over millions 

of scenarios. The high fidelity simulation enabled an accurate 

determination of timings and minimum approach distance in the 

presence of realistic trajectories, communication latencies, and 

total system error for 108 test cases. The SAPA procedure and 

the ALAS alerting algorithm were applied to the 750-ft parallel 

spacing (e.g., SFO 28L/28R) approach problem. With the SAPA 

procedure as defined in this paper, this study concludes that a 

750-ft application does not appear to be feasible, but 

preliminary results for 1000-ft parallel runways look promising. 
iv

Table of Contents 

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

2. The SAPA Procedure .................................................................................................................. 1

2.1. SAPA Concept ..................................................................................................................... 1

2.2. Applying SAPA Concept to San Francisco International Airport ....................................... 4

2.3. Initial Separation at the FAF ................................................................................................ 5

2.4. Options for Loosening Separation Contraints .................................................................... 13

3. The ALAS Alerting Algorithm ................................................................................................. 15

3.1. Structure of the Intrusion Detection Algorithm ................................................................. 15

3.1.1. Mathematical Definitions of Key Components of Algorithm ................................................... 16

3.2. Runway Conformance Tests .............................................................................................. 19

3.3. ALAS Interface (The Programmer‘s API)......................................................................... 19

4. ALAS Parameters ...................................................................................................................... 20

5. Example Runs Using ALAS ...................................................................................................... 20

6. The tALAS Simulator ............................................................................................................... 21

7. Low-Fidelity Simulation Results ............................................................................................... 24

7.1. Tuning of ALAS algorithm parameters ............................................................................. 26

7.1.1. Performance as a Function of the Escape Pilot Delay ............................................................... 26

7.1.2. Performance as a Function of Algorithm Parameter ln_T_red ................................................. 28

7.1.3. Performance as a Function of Algorithm Parameter absDistRed .............................................. 29

7.1.4. Performance as a Function of Algorithm Parameter numPtsTrkRate ...................................... 30

7.2. Blunder Trajectory Without Vertical Level-Out ................................................................ 30

7.2.1. Performance as a Function of Escape Vertical Acceleration .................................................... 30

7.2.2. Performance as a Function of Peak Trajectory Error ................................................................ 31

7.2.3. Performance as a Function of Maximum Bank Angle of Intrusion .......................................... 31

7.3. Blunder Trajectory With Vertical Level-Out ..................................................................... 32

7.3.1. Performance as a Function of Escape Pilot Delay .................................................................... 33

7.3.2. Performance as a Function of Vertical Acceleration ................................................................. 34

7.3.3. Performance as a Function of Maximum Bank Angle of Intrusion .......................................... 34

8. Performance of Runway Conformance Test ............................................................................. 35

8.1. ALAS Algorithm Without Runway Conformance Test ..................................................... 35

9. Performance of Yellow Alerting ............................................................................................... 36

10. Performance of the Tangent Fan Algorithm ............................................................................ 36

11. Preliminary Performance in a High-Fidelity Simulation ......................................................... 37

11.1. Modeling the SAPA Procedure ........................................................................................ 37

11.2. Modeling Blunders ........................................................................................................... 38

11.3. Modeling the Evasive Manuever ..................................................................................... 38

11.4. Scenarios .......................................................................................................................... 38

11.5. Errors and Latencies ......................................................................................................... 39

11.5.1. Total System Error .................................................................................................................. 39

11.5.2. Uncertainty of Position and Velocity Inputs to ALAS ............................................................ 39

11.5.3. Latency of Evasive Maneuver ................................................................................................. 41

11.6. Increasing Bank Rate of Automated Manuevers .............................................................. 41
v

11.7. Measuring Time to Alert .................................................................................................. 42

11.8. Results .............................................................................................................................. 42

11.8.1. Blunder Type .......................................................................................................................... 42

11.9. Rate of Turn ..................................................................................................................... 42

11.9.1. Modeled Avionics ................................................................................................................... 43

11.10. Indicated Collisions ........................................................................................................ 43

11.11. Worst Case ..................................................................................................................... 47

12. Comparing the Low Fidelity and High Fidelity Results .......................................................... 49

13. Future Work ............................................................................................................................ 51

13.1. ALAS Trigger Function ................................................................................................... 51

13.2. Kinematic Analysis in the Presence of ADS-B Latency and Position Errors .................. 51

13.3. A Kinematic Study Using Double-Turn Blunder Model ................................................. 51

13.4. A Better False Alarm Analysis ........................................................................................ 51

13.5. Develop a Better Yellow Alert ......................................................................................... 51

13.6. Tune/Test Algorithm for Other Runaway Spacings ......................................................... 52

13.7. Enhanced Automated Flight Modes ................................................................................. 52

13.8. An Intelligent Evasive Maneuver ..................................................................................... 52

13.9. New Wake Studies and Trades ........................................................................................ 52

13.10. Procedure Modifications to Improve Safety .................................................................. 52

14. Conclusions ............................................................................................................................. 53

15. References ............................................................................................................................... 54

Appendix A. Estimating False Alarm Rate ................................................................................... 56

A.1. Option 1: Any Deviation From Normal ............................................................................ 56

A.2. Option 2: Protection Zone ................................................................................................. 56

A.3. Option 3: Parametric Family of Blunder/Non-Blunder Trajectories ................................. 57

Appendix B. Simulation Results Using Double-Turn Blunder ..................................................... 58

B.1. Double-Turn Blunder Without Altitude Level-out ............................................................ 59

B.2. Double-Turn Blunder With Altitude Level-out................................................................. 61
vi 

Acronyms 

ADS-B Automatic Dependent Surveillance - Broadcast

AGL Above Ground Level

AILS Airborne Information for Lateral Spacing

ALAS Adjacent Landing Alerting Systems

ALT HLD Altitude Hold mode

API Application Programming Interface

APP Approach mode

ATC Air Traffic Control

CAS Calibrated Air Speed

CMF Cockpit Motion Facility

DOT Department of Transportation

EPU Estimated Position Uncertainty

FAA Federal Aviation Administration

FAF Final Approach Fix

GPS Global Positioning System

HDG Heading mode

HDG SEL Heading Select mode

HITL Human-in-the-Loop

IGE In-Ground Effect

ILS Instrument Landing System

IMC Instrument Meteorological Conditions

IS Intruder Ship

LOC Localizer mode

MASPS Minimum Aviation System Performance Standard

MCP Mode Control Panel

MOPS Minimum Operational Performance Standard

NACp Navigation Accuracy Category for Position

NACv Navigation Accuracy Category for Velocity

NGE Near Ground Effect

OGE Out-of-Ground Effect

OS Ownship

PRM Precision Runway Monitor

RNAV Area Navigation

RTCA Radio Technical Commision for Aeronautics

RWY Runway

SAP Stabilized Approach Point

SAPA Simplified Aircraft-based Paired Approach

SFO San Francisco International Airport

SPD Speed mode

tALAS Test simulator for ALAS

TAS True Air Speed

TCA Time of Closest Approach

TCAS Traffic Collision Avoidance System

TSE Total System Error

UTC Coordinated Universal Time

VERT SPD Vertical speed mode

VMC Visual Meteorological Conditions

WAAS Wide Area Augmentation System
1

1. Introduction 

The Simplified Aircraft-based Paired Approach (SAPA) is a proposed concept for the 

operation of closely spaced parallel runways in Instrument Meteorological Conditions (IMC). 

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

that approaches the arrival rate achievable under Visual Meteorological Conditions (VMC) 

[Johnson2010]. Based on constant-width navigation performance, the SAPA concept leverages 

advanced navigation and flight-guidance technology and Automatic Dependent Surveillance –

Broadcast (ADS-B) to share precise position and velocity data between the paired aircraft. The 

SAPA concept allows one aircraft to pass the other aircraft during the approach segment, while 

keeping the paired aircraft within a defined conformance zone to avoid any wake vortex 

encounters. 

Using the SAPA concept, aircraft are initially established on final approach with a minimum 

of 1000 ft of vertical separation and with Air Traffic Control (ATC) responsible for initially 

pairing the aircraft with appropriate relative longitudinal positioning. During SAPA operations, 

the paired aircraft utilize onboard flight guidance speed cues to maintain longitudinal alignment 

within the conformance zone, and an escape maneuver (climbing turn away from the paired 

aircraft) is required when either lateral or longitudinal position error is beyond tolerance, or there 

is a loss of ADS-B or other required flight-navigation capability. 

Conducted under FAA reimbursable funding, this study (Phase I) focused on the development 

and validation testing of an on-board algorithm that alerts intrusions from paired aircraft. The 

new algorithm, called the Adjacent Landing Alerting System (ALAS), was developed and tested 

for the SAPA concept adapted to the close parallel runway spacing (750 ft) of Runways 28L and 

28R at San Francisco International Airport (SFO), and it is adaptable to other parallel runway 

spacings with greater separations. The alerting algorithm was tested using low fidelity 

(kinematic) and high fidelity simulation capabilities. Phase I deliverables consist of the alerting 

algorithm software source code, and this report documenting the alerting algorithm development 

and validation testing. 

An optional future study (Phase II) is defined that would further refine the alerting algorithm 

for operational robustness. Phase II would examine in more detail the system architecture 

required to support SAPA operations including ATC/pilot procedures and the escape maneuver. 

A preliminary simulation plan would also be developed with input from the DOT/FAA Team for 

a future Human-in-the-Loop (HITL) simulation study that leverages experience from earlier 

Airborne Information for Lateral Spacing (AILS) and other closely spaced operations simulation 

studies. 

2. The SAPA Procedure 

2.1. SAPA Concept 

The Simplified Aircraft-based Paired Approach (SAPA) procedure allows two aircraft to 

perform parallel approaches under instrument conditions on runways spaced as close as 750 ft 

apart. Johnson et al. [Johnson2010] describe the basic concept of the SAPA procedure as 

illustrated in Figure 2-1. SAPA allows the paired aircraft to have different approach speeds. The 

―faster‖ aircraft has the higher approach speed; the ―slower‖ aircraft has the lower approach 

speed. If both aircraft have the same approach speed, the aircraft that begins the procedure in the 
2

trailing position assumes the ―faster‖ aircraft role. The ―faster/slower‖ designation applies only 

to approach speed; prior to establishing approach speed, the ―faster‖ aircraft may flight at or 

slower than the speed of the ―slower‖ aircraft to conform to the procedure. 

Figure 2-1: SAPA Concept 

From the start of the procedure until touchdown, the relative along-track positions of the 

aircraft must remain within a forward and rear boundary that avoids wake vortex encounters. The 

rear boundary represents the furthest trailing distance where the faster aircraft avoids the wake 

from the slower aircraft. The forward boundary represents the furthest leading distance where the 

slower aircraft avoids the wake of the faster aircraft. The faster aircraft begins the procedure in a 

trailing position relative to the slower aircraft. The faster aircraft maintains this trailing position 
3

until the slower aircraft reaches the final approach fix (FAF) and begins to decelerate to final 

approach speed. The faster aircraft is then permitted to pass the slower aircraft before landing. 

The procedure can be divided into four segments: 

 Initiation – Air traffic control (ATC) vectors each aircraft onto the approach. One aircraft is 

placed at a 1000-ft vertical separation from the other. The higher aircraft can be the faster or 

slower aircraft. The faster aircraft must establish initial along-track separation before the 

higher aircraft begins to descend on the glidepath. This segment was not evaluated in the 

study. 

 Constant Speed – The slower aircraft maintains a constant airspeed as assigned by Air Traffic 

Control (ATC) until reaching the FAF. The faster aircraft adjusts speed to maintain 

separation. In this segment, both the high and low aircraft will initially travel straight and 

level on the runway approach course until each aircraft intercepts their glidepath. Each 

aircraft will then descend on the glidepath. The aircraft initially use Traffic Collision 

Avoidance System (TCAS) for collision avoidance until the vertical separation drops below 

800 ft. At that point, the aircraft suppress TCAS alerts and rely on the ALAS algorithm for 

collision avoidance. 

 Deceleration – At the FAF, the slower aircraft begins its deceleration to final approach speed. 

The faster aircraft performs one of two actions: 1) continue at constant speed until reaching 

the FAF and then decelerate to final approach speed or 2) continue to match the ground speed 

of the slower vehicle as the slower vehicle decelerates until the faster vehicle reaches its final 

approach speed. Johnson et al. [Johnson2010] refer to the latter option as  ̳speed 

management‘; the former option will be referenced as ―no speed management‖. Each aircraft 

is expected to stabilize on its final approach speed before the stabilized approach point (SAP), 

a point on the glidepath at 1000 ft above ground level (AGL). 

 Approach Speed – Both aircraft fly their final approach speed to the runway threshold. Then 

they decelerate and flare to landing speed. The ALAS algorithm deactivates alerts once 

decision height is reached. 

The speed schedule of the last three segments to decision height is depicted for the speed 

management option in Figure 2-2. 
4

2.2. Applying SAPA Concept to San Francisco International Airport 

San Francisco International Airport has a pair of runways, 28L and 28R, that are 750 ft apart 

and currently used for parallel approaches under Precision Runway Monitor (PRM) procedures. 

In this study, the SAPA concept is applied to these runways. The first consequence of the SAPA 

concept is that the low altitude vehicle needs a long level segment prior to capturing its 

glideslope. Using a 3 glidepath and 1000-ft vertical separation, the low altitude aircraft must be 

at least 3.14 NM from its glidepath when the higher-altitude aircraft intercepts the glidepath. 

Additional distance will be required to allow the faster aircraft to stabilize its separation prior to 

intercepting the glidepath. This study does not examine the ATC scheduling of SAPA aircraft 

onto the approach. Instead, scenarios begin with the aircraft stabilized on initial separation. 

Nevertheless, the existing area navigation (RNAV) approaches on runways 28L and 28R provide 

a long level segment that extends at least 5.4 NM [FAA2012]. Figure 2-3 shows the vertical 

profile of the SAPA procedure for runways 28L and 28R with the higher aircraft assigned to 28L 

for illustration; the slower or faster aircraft can be assigned to 28L. (The low altitude of 4100 ft

was chosen to be coincident with the ILS approach for RWY 28R since the high fidelity 

simulation conducts the approach using ILS.)

The scenario starts with the high aircraft at PONKE. The low aircraft is positioned at the 

appropriate initial separation between waypoints DUMBA and MEHTA. The scenario starts in 

the constant speed segment of the procedure. Both aircraft travel straight and level on the runway 

approach heading. The high aircraft will intercept the glidepath at approximately 15.7 NM from 

the threshold. At approximately 15.0 NM from the runway threshold, each aircraft will switch 

from TCAS to ALAS for collision avoidance. At approximately 12.5 NM, the slower aircraft will 

intercept its glidepath and begin to descend. From this point forward, the vertical separation of 

the two aircraft will be less than 160 ft due to glidepath geometry. The aircraft continue on the 

glidepath at constant speed until the slower aircraft reaches its FAF at AXMUL. This begins the 

Figure 2-2: Velocity Profile of SAPA Procedure
5

deceleration segment of the SAPA approach. The slower aircraft will then decelerate to its final 

approach speed. Under speed management, the faster aircraft will begin its deceleration either 

upon recognizing the deceleration of the slower aircraft or upon reaching its FAF at DUYET, 

whichever occurs first. With no speed management, the faster aircraft will begin its deceleration 

at DUYET. The final approach segment of the SAPA procedure begins when both aircraft 

stabilize on their final approach speed. When the aircraft reach the decision height, the ALAS 

algorithm deactivates alerts. Both aircraft proceed at their final approach speed to the runway 

threshold, then decelerate and flare to landing speed. 

2.3. Initial Separation at the FAF 

The SAPA procedure requires that the along-track separation of the participating aircraft 

remain within wake-safe boundaries throughout the procedure. Ground effects influence the 

transport of wake vortices. Therefore, the wake-safe boundary changes with altitude. Johnson et 

al. [Johnson2010] defined the wake-safe boundary for three altitude regions: in-ground effect 

(IGE), near-ground effect (NGE), and out-of-ground effect (OGE). These regions are depicted in 

Figure 2-4. Note that distances are measured from the glideslope intercept and not the runway 

threshold. At SFO runways 28L and 28R, the intercept of the ILS glideslope occurs at the 

standard distance of ~1000 ft down the runway. 

DUYET

(5.4)

FAF 4100 4100 3300 1800

AXMUL

(5.3)

FAF

CEPIN

(9.9)

DUMBA

(14.0)

MEHTA

(19.4)

IAF

<1800> <3200> <4100>

PONKE

(17.8)

WETOR

(14.9)

ROKME

(12.0)3900 4800 5100

HEMAN

(9.6) 3100 1800

<1800> <3100> <4000> <4000> <4000>

(4.6) (4.2) (5.4)

(4.2) (2.4) (2.9) (2.8)

(15.7)

(12.5)

Scenario

Start

Scenario

Start

(17.5 to 17.8)

Estimated

ALAS Start

(15.0)

RWY 28L

RWY 28R

Legend

vertical text – flight path altitude (ft)

(x.x) distance from runway threshold (NM)

(x.x) distance between waypoints (NM)

<xxxx> minimum safe altitude between waypoints (ft)

DIVEC

(20.6)

(2.8)

GS 3.00°

TCH 57

GS 3.00°

TCH 55

Figure 2-3: Vertical Profile of SAPA Approaches at SFO
6

The wake study of Johnson et al. [Johnson2010] looked at the worst-case condition of two 

Boeing 747-8 aircraft traveling with a 15 KT adverse crosswind to parallel runways with 750 ft

spacing. Under these conditions, the wake-free boundary for each region is given in Table 2-1. 

The boundary is defined as a longitudinal distance of the faster aircraft from the slower aircraft. 

The boundary extends fore (slower aircraft avoids wake of faster aircraft) and aft (faster aircraft 

avoids wake of slower aircraft).

Table 2-1: Wake-Safe Boundaries 

Region IGE NGE OGE

Wake Safe Boundary (ft) 1000 2600 3000

In dependent operations, the aircraft could use the full length of the conformance zone. 

However, within the SAPA procedure, the aircraft transition to independent operation within the 

deceleration segment, i.e. at or shortly after reaching the FAF. Therefore, the aircraft must be 

positioned at the FAF such that their independent operations do not cause either aircraft to exceed 

the wake-safe boundary before touchdown. Prior to the FAF, the SAPA concept requires the 

trailing aircraft to maintain speed (and, therefore, along-track separation) with the lead aircraft; 

thus, the along-track separation at the FAF is also the initial separation. Since the SAPA concept 

defines the velocity profile of each aircraft, determining the FAF separation window simply 

requires a kinematic back-trace from touchdown to the FAF. Under ideal conditions, the FAF 

separation window is a function of the wake-safe boundaries, the final approach speeds of the 

aircraft, the assigned speed for the constant speed segment, the glidepath angle, and, in the speed 

management case, the latency of dependent operations. This study looked at final approach 

speeds for the slower aircraft ranging from 110 KT to 155 KT. The faster aircraft declares an 

approach speed equal to or greater than the slower aircraft. How much faster the faster aircraft 

could fly the approach would be answered by this exercise. SFO uses the standard 3 glidepath 

for runways 28L and 28R. The assigned speed for the constant speed segment was set at 170 KT. 

To define a latency of operations, a 1 s ownship response was added to the allowable 2 s latency 

for ADS-B OUT under the FAA Rule [FAA2010]; this results in a total latency of 3 s. 

Additionally, the kinematic back-trace was simplified. True and calibrated airspeeds (TAS and 

CAS) were treated as equal from the SAP to touchdown. Aircraft speed in the deceleration 

segment was modeled as a constant deceleration from the true airspeed at the FAF to the 

approach speed at the SAP for the slower aircraft and, when speed management was not used, for 

the faster aircraft. Under the speed management case, the faster aircraft matched the deceleration 

Figure 2-4: Regions with Different Wake-Safe Boundaries 
7

of the slower aircraft. Deceleration and flare to landing speed were not modeled. All cases were 

run with zero winds. Accommodating constant winds is a straightforward addition but is reserved 

for future work. The separation error that the TAS = CAS simplification injects is very small 

since the contribution from each aircraft partially cancels out. For differences in speed between 

fast and slower aircraft of up to 20 KT, the injected error is estimated to be less than 10 ft of 

separation in the no speed management case and less than 25 ft in the speed management case. 

The separation error for not modeling the flare is less than 16 ft. Otherwise, the remaining 

separation error in the kinematic model depends on the ability of real aircraft to fly the speed 

schedule in the model; therefore, the remaining separation error is a function of the total system 

error (TSE) for each aircraft. 

Figure 2-5 depicts the constraints that define the forward and rear edge of the FAF separation 

window. The forward edge of the FAF separation window is constrained by two conditions: 1) 

the faster aircraft cannot be ahead of the slower aircraft by more than the IGE wake-safe 

boundary at touchdown and 2) the faster aircraft cannot be ahead of the slower aircraft by more 

than the OGE wake-safe boundary at the FAF. The rear edge of that window is constrained by 

two conditions: 1) at the IGE/NGE transition, the faster aircraft can be no further back than the 

IGE wake-safe boundary and 2) the faster aircraft can be no further back than the OGE wake-safe 

boundary at the FAF. Due to the geometry of the wake-safe boundaries with distance, the NGE 

wake-safe boundary does not impact the FAF separation window. A faster aircraft near the NGE 

wake-safe distance at the OGE/NGE transition would have to fly so much faster than the slower 

aircraft to meet the IGE wake-safe distance at the IGE/NGE transition that the faster aircraft 

would have to begin beyond the OGE wake-safe distance at the SAP. The constraints above are 

conservative because they do not take into account that one of the aircraft will be downwind in an 

adverse crosswind. For example, the forward edge constraint that the faster aircraft cannot be 

ahead of the slower aircraft by more than the IGE wake-safe distance at touchdown and the rear 

Touchdown IGE FAF

FAF Separation Window

Faster Aircraft

Slower Aircraft < 1000’

< 1000’

Faster aircraft lands first. Forward constraint on FAF Separation Window.

Slower aircraft to IGE region first. Rear constraint on FAF Separation Window.

< 3000’ < 3000’

Wake-free distance between aircraft.

Figure 2-5: FAF Separation Window
8

edge constraint that the faster aircraft cannot be behind the slower aircraft by more than IGE 

wake-safe distance at IGE/NGE transition apply the same 15 KT adverse cross-wind wake-safe 

boundary to both aircraft. This application of the constraints enables execution of the procedure 

without introducing into the decision process a new variable of aircraft position relative to wind. 

To treat one aircraft as downwind in the constraints would require an additional wake study to 

establish the wake-safe boundaries for light and variable winds (to handle the worst case of no 

defined down-wind side). 

Table 2-2 and Table 2-3 show the results for the case of the slower aircraft approaching at 110 

KT for the cases without and with speed management respectively. The 110 KT scenario was 

selected because it provides worst-case results for large speed differences. This data is also 

depicted pictorially in Figure 2-6 and Figure 2-7. 

Table 2-2: FAF Separation Window w/ No Speed Management – 

Slower Aircraft at 110 KT Approach Speed 

Increase in Final Approach Speed for 

Faster Aircraft (KT)

FAF Separation Window

Forward 

Edge (ft)

Rear Edge 

(ft) Length (ft)

ErrorAdjusted 

Rear Edge 

(ft)

0 +1540 -1536 3103 -1350

5 -58 -2888 2830 -2702

10 -1560 -3000 1440 -2814

15 -2957 -3000 43 None

20 None None None None

Figure 2-6: FAF Separation Window w/ No Speed Management – 

Slower Aircraft at 110 KT Approach Speed

2000 1000 0 -1000 -2000 -3000

0

5

1 0

1 5

2 0

Distance from Slower Aircraft (feet)

Approach Speed Delta (KT)

2 x TSE Buffer

Usable separation window

Legend
9

Table 2-3: FAF Separation Window w/ Speed Management –

Slower Aircraft at 110 KT Approach Speed 

Increase in Final 

Approach Speed 

for Faster 

Aircraft (KT)

FAF Separation Window

Forward Edge (ft) Rear Edge (ft) Length (ft)

Error-Adjusted 

Rear Edge (ft)

0 +676 -1318 1994 -1132

5 -58 -2029 1971 -1843

10 -998 -2783 1786 -2597

15 -1903 -3000 1097 -2814

20 -2852 -3000 148 -2814

Three conclusions can be drawn from the data. First, the scenario without speed management 

provides a larger separation window when the approach speeds of the two aircraft differ by less 

than 10 KT. Second, speed management is necessary to make the procedure available to aircraft 

pairs with a speed difference greater than 10 KT while still providing an adequate window for 

low speed differences. Therefore, to keep the SAPA procedure simple, speed management can be 

used as the sole mode of operation. Third, even within speed management, there is no single 

separation window that accommodates all speed differences. The initial separation must be 

tailored for the approach speeds of both aircraft. 

As described above, this data is still subject to the TSE of each aircraft. TSE values are 

defined as the 95% (2) expected deviation of the aircraft true position from the flight plan. A 

Figure 2-7: FAF Separation Window w/ Speed Management –

Slower Aircraft at 110 KT Approach Speed 
10

simple means of incorporating TSE into the FAF separation window is to reduce the forward and 

rear edge by √ x TSE1 each to produce a separation window that provides 99.99% confidence of 

remaining in the conformance zone to touchdown; thus, the window must be at least 2√ x TSE 

in length to be feasible. Johnson et al. [Johnson2010] argue, on the basis of maintaining 

sufficient cross-track separation, that the maximum allowable TSE for the SAPA procedure with 

750-ft runway spacing is 131 ft (40 m). Therefore, the FAF separation window must be at least 

371 ft (113 m) long to be feasible. The kinematic back-trace indicates that the largest speed 

difference the SAPA procedure can accommodate based on this criteria is 18 KT. The last 

column of Table 2-2 and Table 2-3 show the rear edge of the window after applying √ TSE 

error. Figure 2-6 and Figure 2-7 also show a forward and rear buffer of √ TSE in red and the 

remaining usable FAF separation window in blue. In all cases where the SAPA procedure is 

feasible, the error-adjusted rear-edge of the window provides an initial horizontal separation of at 

least 1000 ft. 

However, TSE may not be the best metric for adjusting the separation window to account for 

flight technical error and navigation error. TSE is a position metric. Along-track separation from 

FAF to touchdown is determined by the speed schedule. Therefore, the separation window 

should be most sensitive to errors in following the speed schedule. The results in Figure 2-6 and 

Figure 2-7 can be used to illustrate this. Take the case where the declared speed difference is 10 

KT. If the aircraft errors in following the speed schedule produce a ±5 KT uncertainty in the 

speed difference, then the FAF separation window that can accommodate this error is 

approximately the window formed from the overlap of the windows for the 5 KT, 10 KT and 15 

KT speed differences. The overlap produces a range of -1903 ft to -2029 ft, which has a length of 

126 ft. This is an order of magnitude smaller than the windows for an exact speed difference of 

10 KT. For a declared difference of 15 KT, an uncertainty of ±5 KT cannot be accommodated 

because no overlap exists with the 10 KT and 20 KT separation windows. To examine the effect 

of velocity tracking errors on the FAF separation window, the kinematic model was modified to 

compute the FAF separation for each declared speed difference from the overlap of three 

windows: error low, no error, and error high. In the error low case, the initial speed at the start of 

the deceleration segment and the final approach speed for the faster aircraft are reduced by the 

defined velocity bias, and the initial speed at deceleration and the final approach speed for the 

slower aircraft are increased by the defined velocity bias. In the no error case, no errors are added 

to the speeds of the slow and faster aircraft. In the high error case, the defined error is added to 

the initial speed at deceleration and to the final approach speed of the faster aircraft, and the 

velocity error is subtracted from the initial speed at deceleration and the final approach speed of 

the slower aircraft. The low error scenario for the 0 KT declared speed difference requires an 

additional adjustment. In this scenario, the  ̳fast‘ aircraft will actually be slower than the  ̳slow‘ 

aircraft. This opens the possibility that the  ̳fast‘ aircraft can actually fall beyond the IGE wake 

safe difference in the IGE region. This necessitates a third constraint for computing the rear edge 

of the FAF separation window: if the slower aircraft touches down first, then the trailing aircraft 

must be no further behind than the IGE wake-safe boundary. 

The revised kinematic model employs the mean velocity error (i.e., velocity bias), averaged 

over the distance from FAF to touchdown. It assumes contributions from the remaining 

instantaneous random fluctuations are negligible. The velocity bias includes contributions from 

 

1

 The TSE of one aircraft is not correlated with the other. Therefore, the error in the along track separation 

of the two aircraft for a given confidence value is the square root of the sum of the squares of the TSE for 

each aircraft. 
11

navigation error and flight technical error. Establishing a realistic value, however, is a challenge. 

The only FAA requirement on velocity performance is the minimum NACv requirement for 

ADS-B OUT. But NACv only identifies the upper bound of the 95% navigation accuracy for the 

reported velocity, and navigation accuracy is often assumed to have a mean of zero 

[Mohleji2010]. Any significant bias must, therefore, come from flight technical error, i.e., the 

difference between the commanded velocity and the estimated flown velocity. No sources were 

found that define flight technical error for velocity in modern aircraft. However, Johnson et al. 

[Johnson2010] used a ±2 KT uniform uncertainty for velocity in the Monte-Carlo simulation that 

established the wake-safe boundaries. This uncertainty was randomly computed for each aircraft 

and persistently applied throughout the run. Once the velocity bias is selected, navigation 

uncertainty for position is used to assess the feasibility of the separation window. The separation 

window must still be longer than the uncertainty in separation due to the position navigation 

accuracy of each aircraft. The chosen navigation accuracy was 33 ft (10 m) which is the 

estimated position uncertainty (EPU) of the largest NACp category in ADS-B reports that is 

compatible with the maximum eligible TSE of 40 m for the SAPA procedure. The 99.99% 

uncertainty bounds are therefore 2√ EPU = 93 ft (28.3 m). Using this criteria, the SAPA 

procedure is infeasible at all velocity differences for a velocity bias of ±2 KT. The maximum 

velocity bias to retain a feasible SAPA procedure for approach speed differences up to 15 KT is 

±1.45 KT for each aircraft (i.e., a speed difference uncertainty of ±2.9 KT). 

Table 2-4 and Table 2-5 show the separation window that results from applying a velocity bias 

of 1.45 KT and adjusting the rear edge of the separation window by √ EPU. Figure 2-8 and 

Figure 2-9 provide a pictorial view of the data. This separate application of velocity and position 

errors produces a more tightly constrained window for the initial separation. Moreover, the 

window for the 0 KT difference scenario positions the aircraft with less than 400 ft of along-track 

separation before the FAF. 
12

Table 2-4: FAF Separation Window w/ No Speed Management –

±1.45 KT Velocity Bias and 10 m EPU 

Increase in Final 

Approach Speed 

for Faster 

Aircraft (KT)

FAF Separation Window

Forward Edge (ft) Rear Edge (ft) Length (ft)

Error-Adjusted

Rear Edge (ft)

0 +428 -423 851 -376

5 -1166 -1904 738 -1857

10 -2646 -3000 354 -2954

15 None None None None

20 None None None None

Figure 2-8: FAF Separation Window w/ No Speed Management –

±1.45 KT Velocity Bias and 10 m EPU
13

Table 2-5: FAF Separation Window w/ Speed Management –

±1.45 KT Velocity Bias and 10 m EPU 

Increase in Final 

Approach Speed 

for Faster 

Aircraft (KT)

FAF Separation Window

Forward Edge (ft) Rear Edge (ft) Length (ft)

Error-Adjusted 

Rear Edge (ft)

0 +428 -423 851 -376

5 -943 -1302 359 -1256

10 -1816 -2047 231 -2001

15 -2733 -2836 103 -2790

20 None None None None

2.4. Options for Loosening Separation Contraints 

Though the conformance zone has a length of 6000 ft at the FAF, kinematic back-trace of each 

aircraft from touchdown to the FAF shows that the usable portion of this zone is much smaller, 

sometimes as little as about 100 ft. In addition, some of these separation windows place the 

aircraft pair in close proximity for the 10+ NM stretch from loss of vertical separation to the FAF. 

The constraint with the greatest influence on the separation window is the 1000 ft IGE wake-safe 

boundary. Here are some examples of how extending the IGE wake-safe boundary impacts 

results. 

 An IGE wake-safe boundary of 1275 ft opens the procedure to aircraft with a velocity bias of 

± 2 KT (and approach speed differences of up to 15 KT). 

 An IGE wake-safe boundary of 1450 ft allows the trailing aircraft to begin with an alongtrack separation greater than 1000 ft when the aircraft pair has the same approach speed (and 

2000 1000 0 -1000 -2000 -3000

0

5

1 0

1 5

2 0

Distance from Slower Aircraft (feet)

Approach Speed Delta (KT)

2 x EPU Buffer

Usable separation window

Legend

Figure 2-9: FAF Separation Window w/ Speed Management –

±1.45 KT Velocity Bias and 10 m EPU
14

velocity bias is within 1.45 KT). 

 An IGE wake-safe boundary of 1650 ft opens the procedure to approach speed differences of 

20 KT (and velocity bias of each aircraft is within 1.45 KT). 

Adjustments other than IGE wake-safe boundary can be made to the procedure to make better 

use of the separation windows that result from the wake-safe boundaries. Listed below are 

options to extend the usable separation window or to increase the procedure‘s tolerance of 

velocity or position errors. 

 Position aircraft based on predicted winds and customize separation as appropriate. As stated 

in the previous section, the kinematic back-trace computes the separation windows as if both 

aircraft experience adverse crosswind; this allows the procedure to be performed without 

adjusting for wind direction. However, the procedure could include the predicted winds into 

decision-making and tailor the separation appropriately. For example, controllers could be 

required to place the faster aircraft on the downwind runway or the SAPA avionics on the 

faster aircraft could compute separation based on predicted winds along the path. Placing the 

faster aircraft on the downwind runway will extend the forward edge of the FAF separation 

window. Though this could open the procedure to 20 KT approach speed differences or 

increase tolerance to a ±2 KT velocity bias, it does not allow the faster aircraft to position 

itself any further back in those cases where the rear edge of the separation window is less 

than 1000 ft. Placing the slower aircraft downwind would allow the faster aircraft to position 

itself further back but, unless the faster aircraft remains downwind from initiation to 

touchdown, the faster aircraft can still start no further back than the OGE wake-safe boundary 

of 3000 ft. Thus, placing the faster aircraft downwind can fail to open the procedure to

higher velocity differences or significantly improve tolerance to velocity bias. 

 Decrease allowable adverse wind speed. This option trades availability with respect to time 

against availability with respect to aircraft pairings. To assess this trade will require wake 

studies with different adverse winds. 

 Add a segment prior to the FAF to maneuver into the preferred separation at the FAF. The 

trailing aircraft would initiate separation near the OGE wake-safe boundary and maintain this 

separation until some specified distance before the FAF. The faster aircraft would then 

accelerate to the preferred separation at the FAF. This would allow the faster aircraft to 

remain well behind the slower aircraft for much of the 10+ NM distance between procedure 

initiation and the FAF. 

 When aircraft request the same approach speed, the controller directs the trailing aircraft to 

increase its planned approach speed by 5 KT. An approach speed difference of 0 KT 

becomes a troublesome case because the velocity bias causes the trailing aircraft to be the 

 ̳slow‘ aircraft. This forces the separation window to remain in close proximity to the abeam 

position. Requiring the trailing aircraft to have a greater planned approach speed should 

guarantee that the trailing aircraft will remain the faster aircraft in the presence of velocity 

bias. 
15

3. The ALAS Alerting Algorithm 

ALAS (Adjacent Landing Alerting System) is an alerting algorithm designed to detect 

intrusions on closely spaced parallel runways. It employs a mechanism for detecting imminent 

intrusions into a protection zone (analogous to AILS [Abbott2002]) and a mechanism for 

detecting lateral deviations from the runway centerline in a manner similar to the Precision 

Runway Monitor system [Shank1994]. The algorithm is highly configurable through a set of 

user-specifiable parameters. 

3.1. Structure of the Intrusion Detection Algorithm 

The intrusion detection algorithm in ALAS uses a trigger mechanism based on the rate of 

change of the track angle of the intruder to initiate a sweep of potential intrusions as shown in 

Figure 3-1. This mechanism is augmented with two additional tests. The first test is a simple 

conflict probe. The conflict probe detects if the current velocity vector will intersect the 

ownship‘s front or back buffer. For the second test, illustrated in Figure 3-2, the horizontal twodimensional distance between the aircraft is checked to see if it is less than the distances for a red 

alert (absDistRed) and a yellow alert (absDistYellow). A red alert is issued if the distance is less 

than absDistRed. If the distance is between absDistYellow and absDistRed, a yellow alert is 

issued. 

Figure 3-1: ALAS Algorithm Sweep 

Figure 3-2: ALAS Algorithm distAway Check
16

The basic structure of the algorithm is as follows. The parameters of the algorithm are defined 

in Section 4. 

 if (Math.abs(so.z - si.z) < initHeight) { 

 alertLevel_ lines = alas_lines(so, vo, si, vi, 

 ln_T_red, ln_back_buffer_red, ln_front_buffer_red); 

 omega = estimateOmega(traffic);

 double tau = tau(so - si, vo, vi) 

 if (omega > trackRateThreshold && tau >= 0) { 

 for (double phi = phiIncr; phi <= maxPhi; phi = phi + phiIncr) { 

 double R = turnRadius(vi.groundSpeed(), phi); 

 alertLevel_circle = alas_circle(so, vo, si, vi, R, ln_back_buffer_red, 

 ln_front_buffer_red, ln_T_red); 

 } 

 } 

 alertLevel_distAway = checkabsDistAway(so, si, to); 

 alertLevel = max(alertLevel_ lines, alertLevel_circle, alertLevel_distAway) 

 } 

The following subfunctions are used.

alas_lines Projects the trajectory ln_T_red seconds into the future using current position and 

velocity vectors for the ownship (so, vo) and intruder (si, vi). Returns true if at the 

time that the intruder intersects the path of the ownship, it falls within the front 

and back buffers (ln_front_buffer_red and ln_back_buffer_red).

estimateOmega Estimates the angular velocity (i.e. track rate) omega of the intruder based on the 

past numPtsTrkRateCalc data points.

tau Calculates the time of closest approach (TCA). The function tau is negative if the 

trajectories are divergent.

turnRadius Calculates the turn radius R given a ground speed and bank angle phi.

alas_circle Projects the trajectory ln_T_red seconds into the future using a circular trajectory

with radius R. Returns true if at the time that the intruder intersects the path of the 

ownship, it falls within the front and back buffers (ln_front_buffer_red and 

ln_back_buffer_red).

checkabsDistAway Calculates the horizontal distance between the aircraft and checks if it is less than 

absDistRed and absDistYellow.

Note that the circular trajectory search is not performed unless omega > trackRateThreshold

and the trajectories are convergent (tau > 0). This reveals that the performance of the algorithm is 

sensitive to the value of the trackRateThreshold parameter and the mechanism used to compute 

the angular velocity omega. This same technique was used in the AILS algorithm 

[Samanant2000]. Currently, we are using a simple averaging function to calculate omega. This 

provides some filtering of noise on the velocity vector. In the future, we would like to develop a 

filter based on real data obtained from parallel landings at several airports. 

3.1.1. Mathematical Definitions of Key Components of Algorithm 

In this section, the letter s is used to denote positions and v to denote velocities. The subscript 
17

o indicates ownship and subscript i indicates traffic (i.e., intruder) vectors, e.g., si and vi. Vector 

variables are written in boldface and their components are referenced by subscript indices, e.g., 

vx, vy, and vz. 
18
19

3.2. Runway Conformance Tests 

In addition to the intrusion search algorithm, the Alas object provides a runway conformance 

test that measures the perpendicular distance from the centerline, as illustrated in Figure 3-3. 

This test can be performed on both the ownship and the intruder aircraft. The Boolean parameter 

runwayConformance is true if the test is to be applied to the ownship: 

 int ownConformance = alas.runwayConformance(true); // test ownship 

 int trafConformance = alas.runwayConformance(false); // test intruder 

Figure 3-3: Runway Conformance Test Technique 

3.3. ALAS Interface (The Programmer’s API)

The interface to ALAS is simple. The user of ALAS first creates an Alas object. In Java: 

 Alas alas = new Alas(); 

In C++: 

 Alas alas = Alas(); 

Next, the aircraft id of the ownship is entered: 

 alas.setOwnship("Own"); 

The location of the runways and their orientation are entered as follows: 

 alas.setOwnRunway(37.61352, -122.35713, 13.1, 298.332); // 28R 

 alas.setTrafRunway (37.61170, -122.35641, 12.7, 298.326); // 28L 

Then the following is called in the execution loop of the simulation: 

alas.update("Own“ , lat1, lon1, alt1, trk1, gs1, vs1, time);

 alas.update("Traf1", lat2, lon2, alt2, trk2, gs2, vs2, time); 

 int alert = alas.alasAlert(); 

The alasAlert() function calls both the intrusion-detection sweep algorithm and the runway 

conformance check on the intruder aircraft. 
20

The return value alert indicates the level of the alert: 

 0 = No Alert

 1 = Yellow Alert 

 2 = Red Alert 

4. ALAS Parameters 

The ALAS parameters fall into three broad categories: (1) parameters that define the linebased conflict detection region, (2) parameters that control the algorithm, and (3) parameters used 

by the runway conformance tests. 

Line-based Detection Parameter Meaning Default Value

ln_front_buffer_red Length of the red-alert buffer in front of aircraft 10,000 ft

ln_back_buffer_red Length of red-alert buffer in back of aircraft 800 ft

ln_T_red Lookahead time for red alert 15 s

ln_front_buffer_yellow Length of yellow-alert buffer in front of aircraft 10000 ft

ln_back_buffer_yellow Length of yellow-alert buffer in back of aircraft 1400 ft

ln_T_yellow Lookahead time for yellow alert 35 s

Internal Parameters Meaning Default Value

useAbsDistAwayAlg True if additional distance test is used true

initHeight Altitude difference where algorithm turns on MAX_VALUE

numPtsTrkRateCalc Number of data points used in track rate 

calculation

3 

maxPhi Highest bank angle used in search 40

phiIncr Bank angle increment in search 5

trackRateThreshold Track rate threshold that triggers the bank-angle 

sweeep search

1/s

absDistRed Mininimum horizontal distance that triggers a red 

alert

486.5 ft

absDistYellow Mininimum horizontal distance that triggers a 

yellow alert

545.4 ft

Runway Conformance

Parameter

Meaning Default Value

redRunwayDist Distance from centerline that triggers a red alert 170 ft

yellowRunwayDist Distance from centerline that triggers a yellow alert 132 ft

5. Example Runs Using ALAS 

The Cockpit Motion Facility (CMF) Desktop simulator was used to produce high-fidelity 

trajectories on SFO runways 28L and 28R. The trajectories were recorded in a comma-separated 

values file that contains geodesic positions and velocities for both aircraft every 0.5 s. The trace 

of a single run is illustrated in Figure 5-1. These trajectories were used to tune the ALAS 

parameters and configure the trajectory error model used in the tALAS simulator. 
21

(a) Horizontal View 

(b) Vertical View 

Figure 5-1: High Fidelity Run from CMF Desktop Simulator 

6. The tALAS Simulator 

The test simulator for ALAS (tALAS) is based on simple kinematic models of the trajectory 

of the ownship and intruder aircraft. Figure 6-1 illustrates these models and identifies their key 

parameters. 
22

The flight trajectory for the ownship is a straight-line descent with an optional escape 

maneuver. The trajectory for the intruder can be either a normal or a blundered landing approach. 

The trajectories are independently specified and the only interaction between the trajectories is 

when a red alert from ALAS triggers an escape maneuver by the ownship. The following 

paragraphs describe the trajectory parameters for the ownship and intruder aircraft. 

A test trajectory has independent forward and lateral components. The forward trajectory 

component specifies the point-to-point desired path of travel. The lateral component simulates 

the tracking error with a simple sinusoidal oscillation model. As shown in Figure 6-2 this lateral 

oscillation is superposed on the forward trajectory and is specified by three parameters: peak 

amplitude, time period, and phase offset. The trajectories for the ownship and intruder have 

independently specified lateral oscillations. For the current version of tALAS, the oscillation 

parameters are constant for each test case. Vector addition is used to combine the forward and 

lateral components for a trajectory‘s position and velocity.

 The ownship landing trajectory is specified relative to the position and heading of the runway. 

The position of the runway is specified by the touchdown point, denoted SOSRunway in Figure 6-1. 

The runway heading is denoted OSRunway in Figure 6-1. The landing trajectory is a straight line 

from the initial position SOSInitial to the touchdown point following a specified descent angle flown 

Figure 6-1: Top View of Test Scenario with Relevant Runway and Trajectory 

Parameters

Figure 6-2: Lateral Oscillation Superposed on the Forward Trajectory 

Ownship

Intruder

T3

SOSInitial(x, y, z)

T2

T1

IS

SISInitial(x, y, z)

VOSInitial(x, y, z)

VISInitial(x, y, z)

SOSRunway(x, y, z)

SISRunway(x, y, z)

OSRunway

ISRunway

Xviolation

Dviolation

Ownship 

Runway 

Intruder 

Runway 

escape

escape

Tcrossing

 Peak

Period

Phase 

Offset

Reference

lateral 

oscillation 

(No offset) 

Forward 

trajectory Lateral 

movement S(x, y, z)
23

at a constant ground speed. The escape maneuver is an optional feature of the ownship flight 

trajectory consisting of a climbing turn away from the intruder with bank angle escape and 

constant vertical acceleration. The turn continues until a specified heading escape is reached, and 

the vertical acceleration is sustained until the vertical speed reaches a specified value. After 

completing the turn and vertical acceleration, the ownship continues in a straight line. The escape 

maneuver is triggered by a red alert from ALAS with a specified ―pilot delay‖ from the time of 

the alert to the beginning of the maneuver. Since the position and velocity components of the 

lateral oscillation are superposed onto the forward landing trajectory, the actual initial position 

and velocity are the result of the vector addition of the forward and lateral trajectory components. 

The intruder trajectory is a straight-line descent with an optional blunder. In Figure 6-1, the 

runway touchdown point and heading are denoted SISRunway and ISRunway. The initial position 

SOSInitial and velocity VOSInitial are specified to match the desired descent profile with a specified 

glideslope angle. The intruder ground speed remains constant throughout the descent. A blunder 

consists of a constant bank angle IS turn from time T1 to T2, after which the intruder continues 

in a straight line. The turn can be to either the left or the right of the forward direction of travel. 

The blunder may also include leveling out to a constant altitude at or after T1. 

All the parameters for the ownship and intruder trajectories are real valued, except for the 

binary discrete variables specifying whether the intruder will execute a normal or blundered 

descent, whether the intruder will level out after T1, the turn direction for a blunder, and whether 

the ownship is allowed to execute the escape maneuver. A run on tALAS consists of a series of 

test cases, each with specific parameter values. During a run, the discrete parameters are constant 

and each real-valued parameter is assigned evenly spaced values over a specified range starting 

with the minimum value and continuing with constant increments as long as the value is within 

the specified range. A run generates test cases until all the sweeps of all the real-valued 

parameters are complete or until a specified number of trials have been completed. 

The simulator tALAS is instrumented to collect data on false alarms, missed alerts, and the 

distance of closest approach. A safety protection zone defined around the ownship is used to 

assess false alarms and missed alerts. Figure 6-1 illustrates the protection zone, which is defined 

as a moving open quadrant with the corner point X ft behind the current position of the ownship 

along its runway centerline, and D ft inward toward the opposite runway measured relative to the 

ownship‘s runway centerline. For each test case, tALAS determines: the time of closest approach 

and the corresponding distance in 3D space, as well as the horizontal and vertical distances; 

whether there was an intrusion into the safety violation zone; whether the intruder crossed the 

ownship‘s centerline and the corresponding time of crossing; the time of the first yellow (i.e., 

level 1) alert; the time of first red (i.e., level 2) alert; whether there was a loss of separation, 

which is determined with respect to dedicated intrusion envelopes around the aircraft; whether a 

given red alert was not preceded by a yellow alert; the elapsed time from the yellow alert to the 

red alert; the elapsed time from the red alert to the time the intruder crossed the ownship‘s 

centerline; whether there was a red alert without a blunder (i.e., a false alarm); and whether there 

was a violation of the protection zone without a red alert (i.e., a missed alert). For a set of test 

cases, tALAS can also identify the case with the overall minimum approach distance and present 

a complete analysis for it. The simulator tALAS generates the test trajectories as time-indexed 

state sequences. These sequences are processed by the instrumentation, and they can also be 

written to output files for post-run visualization and analysis. 
24

7. Low-Fidelity Simulation Results 

The tALAS simulator was used to evaluate the performance of the ALAS algorithm and 

software implementation. Each data point in the tables below was obtained from 1,889,568 

simulated landings. Different trajectories for the ownship and intruder were obtained by varying 

the parameters listed in Table 7-1. 

Table 7-1: tALAS Trajectory Parameters 

Parameter Meaning Min Value Max Value Step Size

T1 Start Time of Intrusion (s) 10 20 5

T2 Duration of Intrusion Turn (s) 2 10 1

T3 Duration after turn (s) 20 20 20

bankAngle Bank Angle of Intrusion () 5 30 5

Peak Max Trajectory error (ft) 131 131 10

Period Period of Trajectory error (s) 60 70 10

Phase Phase of Trajectory error () -180 +180 45

ownshipInitialSx Distance from runway (NM) 5.0 5.4 0.2

intruderInitialSx Distance from runway (NM) 5.0 5.4 0.2

ownshipInitialGs Ground speed (KT) 160 170 10

intruderInitialGs Ground speed (KT) 160 170 10 

The horizontal profile of a blunder trajectory is illustrated in Figure 7-1. 

Figure 7-1: Blunder Trajectory (Horizontal View) 

The program tALAS can create blunders where the intruder‘s altitude levels out at some point 

or where it continues to follow its normal vertical profile. If a vertical level-out is specified then 

the vertical profile is as shown in Figure 7-2. 
25

Figure 7-2: Vertical Profile for a Blunder Trajectory with a Level-out Component 

The TLevel parameter, which specifies the time the level-out begins, can appear anytime after 

T1, the beginning of the intrusion. If a vertical level-out is not specified, then the vertical profile 

is as shown in Figure 7-3. 

Figure 7-3: Vertical Profile for a Blunder Trajectory without a Level-out 

Component 

The parameters in Table 7-2 characterize the escape maneuver that was used. 

Table 7-2: Escape Maneuver Parameters 

Parameter Meaning Nominal Value

escapePilotDelay Time for pilot to react (s) 0

escapeTrack Target track delta () 45

escapeBankAngle Bank Angle of Escape Turn () 30

escapeGoalVs Target vertical speed (fpm) 2000

escapeVsAccel The vertical acceleration (m/s2

) 2.0

We will first present some of the major results obtained while tuning the ALAS algorithm. 

Then, we present the results for the two types of intrusions: blunder without a vertical level-out 

and blunder with an altitude level-out. In the millions of test cases generated by tALAS there 

were no cases where the ALAS algorithm failed to issue an alert. 
26

7.1. Tuning of ALAS algorithm parameters 

We present in this section the results of experimentation to evaluate the performance of ALAS 

as a function of algorithm parameters and procedural characteristics. 

7.1.1. Performance as a Function of the Escape Pilot Delay 

The parameter escapePilotDelay is the time between first red Alert and the initiation of the 

escape maneuver. It has a very significant impact on the minimum distance obtained. The results 

in Table 7-3 were obtained without an altitude level-out. TCA stands for Time of Closest 

Approach. 

Table 7-3: Performance as a Function of Pilot Delay 

escape

PilotDelay 

(s)

Worst-Case

Minimum

Distance (ft)

Horizontal

Distance at TCA 

(ft)

Vertical

Distance at 

TCA (ft)

T1 

(s)

Time of 

Red Alert 

(s)

TCA 

(s)

0 448 448 7 20.0 26.5 28.0

1 379 370 82 20.0 20.5 26.5

2 260 208 155 10.0 10.5 19.0

3 163 114 116 10.0 10.5 19.0

4 122 89 83 10.0 10.5 19.0

The worst-case run for escapePilotDelay = 0 is shown in Figure 7-4. The Euclidean distance 

(i.e. 3D) at closest approach was 448.28 ft, which occurred at 28.0 s, or 8 s after the start of the 

intrusion. The horizontal distance at closest approach (indicated by a circle) was 448.22 ft and 

the vertical distance was 7.38 ft. The first red alert was issued at 26.5 s, or 1.5 s before the time 

of closest approach (TCA) and 6.5 s after the start of the intrusion. 

The worst-case run for escapePilotDelay = 1 results in a minimum distance of 379.24 ft at 

26.5 s. The horizontal distance at closest approach was 370.27 ft and the vertical distance was 

82.02 ft. 
27

(a) Horizontal View 

(b) Vertical View 

Figure 7-4: Worst Case for escapePilotDelay = 0

The worst case run for escapePilotDelay = 3 is shown in Figure 7-5. The closest approach, 

indicated by a circle, is 163.15 ft at 19.0 s. The horizontal distance at TCA was 114.67 ft. The 

vertical distance at TCA was 116.05 ft. This example clearly shows that even with an alert that 

occurs 0.5 s after the start of the intrusion, a pilot delay of 3 s leads to an unacceptable minimum 

distance. Interestingly, as the escape pilot delay is increased, the vertical separation at TCA 

increases, but the horizontal distance is much closer. The circle indicates the point of closest 

approach and the orange dot indicates the point of the first red alert. 
28

(a) Horizontal View 

(b) Vertical View 

Figure 7-5: Worst Case for escapePilotDelay = 3

7.1.2. Performance as a Function of Algorithm Parameter ln_T_red 

The parameter ln_T_red is the look-ahead time for the red alert. All projected conflicts that 

will occur later than this parameter are ignored. If this parameter is too large, there can be 

nuisance alarms from normal trajectories. The data in Table 7-4 shows that this is not a problem 

for ln_T_red < 20 s. The % False Alarms column was calculated using only normal trajectories 

(i.e., without a blunder). 

Table 7-4: Performance as a Function of ln_T_red

ln_T_red 

(s)

Worst-Case Minimum

Distance (ft)

Horizontal Distance at 

TCA (ft)

Vertical Distance at 

TCA (ft)

% False 

Alarms

10 417 416 20 0.00

15 448 448 7 0.00

18 455 455 7 0.00

20 461 461 3 0.02

30 465 464 11 21.48
29

7.1.3. Performance as a Function of Algorithm Parameter absDistRed 

The performance of the ALAS algorithm is strongly dependent upon the absDistRed

parameter. This parameter sets the threshold for the red alert that is issued based on the 

horizontal distance between the aircraft. A peak value of 131 ft was used for the trajectory error. 

Therefore, two ―normal‖ trajectories can get as close as 488 ft (i.e., 750 - 2*131) when they are 

abeam and 180 out of phase with each other. The absDistRed parameter is currently set at 486.5 

ft, so normal trajectories never invoke this part of the algorithm. However, for values larger than 

488 ft, an alert will sometimes occur for normal trajectories. This can be thought of as a 

preemptive alert. The minimum distance over all the runs can be increased by increasing this 

parameter. However, this comes at the cost of false alarms being issued for ―normal‖ trajectories. 

The results in Table 7-5 were obtained for intrusions without an altitude level-out. 

Table 7-5: Performance as a Function of absDistRed 

absDistRed (ft) Worst-Case Minimum Distance (ft) % False Alarms

0.00 (i.e. Off) 419 0.0

300 .00 419 0.0

450.00 419 0.0

486.50 448 0.0

500.00 458 3.6

525.00 471 7.8

550.00 477 10.7

In this work, we have explored the idea of false alarms in the presence of abnormal 

trajectories. This is discussed in Appendix A, but we do not have any solid statistical results at 

this time. 

We note that for absDistRed = 0, the horizontal distance check in the algorithm is effectively 

turned off. For that case, the time of the first red alert was 27.5 s, or 7.5 s after the beginning of a 

5 bank angle intrusion. With the horizontal distance check active (using default 486.5 ft), the 

alert occurs at 26 s, which is 1.5 s earlier. The importance of this part of the algorithm can be 

seen by examining the horizontal distance for 5 bank angle intrusion as a function of time as 

shown in Table 7-6. 

Table 7-6: Horizontal Distance as a Function of Time for a 5 Bank Angle Intrusion

Time (s) Horizontal Distance (ft)

23.5 538

24.0 526

24.5 514

25.0 503

25.5 490

26.0 478

26.5 465

27.0 452

27.5 438

Obtaining the alert 1.5 s earlier has a large impact on the minimum distance at the point when 

the escape maneuver begins. This data is for a very gradual intrusion caused by a 5 bank turn. 

In a sharp turn, the distances drop at a much faster rate. 
30

7.1.4. Performance as a Function of Algorithm Parameter numPtsTrkRate 

The ALAS algorithm‘s bank-angle sweep is guarded by a function estimateOmega that 

estimates the track rate. If this estimate is smaller than the parameter trackRateThreshold, then 

this sweep is not performed. The function estimateOmega performs a simple averaging using the 

latest numPtsTrkRate of data. This parameter influences the detection time and hence the 

minimum distance as shown in Table 7-7. 

Table 7-7: Performance as a Function of numPtsTrkRate 

numPtsTrkRate

(s)

Worst-Case

Minimum

Distance (ft)

Horizontal

Distance at 

TCA (ft)

Vertical

Distance at 

TCA (ft)

T1 

(s)

Time of 

Red 

Alert (s)

TCA 

(s)

2 451 451 7 20.0 43.5 47.0

3 448 448 7 20.0 26.5 28.0

4 446 446 7 20.0 36.5 38.0

5 437 425 99 15.0 16.0 21.5

6 427 419 82 20.0 21.0 26.0

7 427 419 82 20.0 21.0 26.0

10 379 370 82 20.0 21.5 26.5

7.2. Blunder Trajectory Without Vertical Level-Out 

The tests analyzed in this section were run with the intruder aircraft following a descending 

vertical profile without an altitude level-out. 

7.2.1. Performance as a Function of Escape Vertical Acceleration 

The minimum distance results in Table 7-8 were obtained for escapePilotDelay = 0. 

Table 7-8: Performance as a Function of Escape Vertical Acceleration 

Vertical Acceleration (m/s2) Worst-Case Minimum Distance (ft) Vertical Distance at TCA (ft)

1.0 448 3.69

2.0 448 7.38

3.0 448 11.07

5.0 448 18.45

Surprisingly, the vertical acceleration has almost no effect. But for escapePilotDelay = 2, the 

following results in Table 7-9 were obtained. 

Table 7-9: Performance as a Function of Escape Vertical Acceleration 

Vertical 

Acceleration (m/s2)

Worst-Case Minimum

Distance (ft)

Horizontal Distance 

at TCA (ft)

Vertical Distance 

at TCA (ft)

1.0 199 144 137

2.0 260 208 155

3.0 293 226 186

5.0 317 292 122
31

The vertical acceleration has a major effect on the minimum distance when escapePilotDelay

is greater than 0. When escapePilotDelay is 0, the horizontal turn is initiated soon enough to 

achieve adequate separation. 

7.2.2. Performance as a Function of Peak Trajectory Error 

As Table 7-10 shows, the worst-case minimum distance decreases as the peak value of the 

trajectory error increases. 

Table 7-10: Performance as a Function of Peak Trajectory Error 

Peak Trajectory Error (ft) Worst-Case Minimum Distance (ft)

10 533

40 500

80 461

90 453

121 448

131 448

7.2.3. Performance as a Function of Maximum Bank Angle of Intrusion 

The results are shown in Table 7-11. Notice that an increase in the maximum bank angle 

allowed on an intrusion has a significant effect. 

Table 7-11: Performance as a Function of Maximum Bank Angle of Intrusion

Maximum Bank Angle () Worst-Case Minimum Distance (ft)

10 448.28

20 448.28

30 448.00

35 410.90

40 301.67

The worst-case run with maxBankAngle = 40 resulted in an unacceptably close encounter. 

This run is shown in Figure 7-6. 
32

(a) Horizontal View 

(b) Vertical View 

Figure 7-6: Worst-Case Run Using High Bank Angle Intrusion

The closest approach, indicated by a circle, is 301.67 ft at 18.5 s. The horizontal distance at 

TCA was 201.58 ft and the vertical distance was 224.43 ft. In this case, the vertical separation 

was much more important than in the lower bank angle intrusions. This shows the high 

sensitivity of the minimum distance to the basic assumptions about the trajectory of the intrusion. 

Low bank angle intrusions lead to later alerts, but because the closure rate horizontally is slower, 

the horizontal turn of the escape maneuver is effective. If the intrusion has a high bank angle, 

then the horizontal closure rate is faster. Fortunately, the ALAS algorithm detects these earlier 

than the 30 blunder, so adequate separation is maintained. But if the blunder has a bank angle of 

40 or higher, the protection zone can be penetrated. 

7.3. Blunder Trajectory With Vertical Level-Out 

In this section, we look at the effect of allowing the blunder trajectory to level out vertically at 

some arbitrary point during the intrusion. The level-out is controlled by two parameters listed in 

Table 7-12: TLevel and blunderVSAccel. To keep the sample size manageable, we set stepT2 = 2 

s for the level-out runs. 
33

Table 7-12: Parameters for Blunder Trajectory with Vertical Level-Out 

Parameter Meaning Min value Max Value Step size

TLevel Time of Level Out (added to T1) (s) 0 .0 15.0 1.0

blunderVsAccel Vertical Acceleration [m/s2

] 2.0 2.0 2.0

7.3.1. Performance as a Function of Escape Pilot Delay 

The parameter escapePilotDelay is the time between first red alert and the initiation of the 

escape maneuver. The results are given in Table 7-13. 

Table 7-13: Performance as a Function of Escape Pilot Delay 

Escape Pilot

Delay (s)

Worst-Case

Minimum Distance

(ft)

Horizontal

Distance at TCA 

(ft)

Vertical Distance

at TCA (ft)

Time (intrusion, 

alert, TCA) (s)

0 448 448 5 20.0, 26.5, 28.0

1 340 313 133 10.0, 10.5, 20.5

2 177 144 102 10.0, 10.5, 21.0

3 68 56 38 10.0, 10.5, 20.5

The worst case run with escapePilotDelay = 0 is shown in Figure 7-7. 

(a) Horizontal View 

(b) Vertical View 

Figure 7-7: Worst-Case Run with Level-out Blunder 
34

The TCA occurs at 28.0 s and the Euclidean distance at that time is 448.26 ft. The horizontal 

distance at TCA is 448.22 ft. The vertical distance at TCA is 5.74 ft. Interestingly, the level-out 

aspect of the intrusion has no significant effect on the distance at closest approach. This shows 

that the horizontal component of the escape maneuver is what is providing the needed separation. 

However, as the pilot delay increases, the minimum distance decreases faster in the level-out case 

compared with the blunder without a level-out in altitude. 

7.3.2. Performance as a Function of Vertical Acceleration 

The results are shown in Table 7-14. The minimum distance at TCA was not improved by 

increasing the escape vertical speed for the level-out blunder. The reason for this is that the 

horizontal turn is providing all of the separation. 

Table 7-14: Performance as a Function of Vertical Acceleration 

escapeVSAccel (m/s2) Worst-Case Minimum Distance (ft)

1.0 448

2.0 448

3.0 448

5.0 448

7.3.3. Performance as a Function of Maximum Bank Angle of Intrusion 

Table 7-15 shows the effect of varying the bank angle (and hence the turn radius) of the 

intrusion. 

Table 7-15: Performance as a Function of Maximum Bank Angle of Intrusion 

MaxBank () Worst-Case Minimum Distance (ft)

10 448

20 448

30 448

35 336

40 208

There is no effect up to the standard blunder bank angle (i.e. 30), but the effect is significant 

for very sharp turns. 
35

8. Performance of Runway Conformance Test 

The runway performance test measures the perpendicular distance from the runway centerline. 

In this section, we analyze the effectiveness of this test when it is used alone. 

Table 8-1: Performance as a Function of redRunwayDist 

redRunwayDist

(s)

Worst-Case

Minimum Distance 

(ft)

Horizontal

Distance at TCA 

(ft)

Vertical

Distance at TCA 

(ft)

Times (T1, Alert,

minDist) (s)

132 142 131 52 (20, 25.5, 29.5)

140 142 131 52 (20, 25.5, 29.5)

150 140 129 52 (20, 26.5, 30.5)

180 125 117 40 (20, 26.0, 29.5)

200 118 115 29 (15, 22.0, 25.0)

As Table 8-1 shows, this test alone does not provide an alert early enough for the escape 

maneuver to maintain adequate separation. Even for the most aggressive case, redRunwayDist = 

132, the alert occurs 5.5 s after the beginning of the intrusion. Nevertheless, when used in 

conjunction with the ALAS triggering algorithm, it helps for very slow intrusions. 

8.1. ALAS Algorithm Without Runway Conformance Test 

If the ALAS algorithm is run without the runway conformance test, the minimum separation at 

TCA drops from 448 ft to 421 ft for escapePilotDelay = 0. The worst-case run is shown in Figure 

8-1. The intrusion started at time 20 s, but the red alert was not issued until 53.5 s. The intrusion 

was so gradual that it did not trigger the omega threshold until 33.5 s after the slow intrusion 

began. The TCA is 56.5 s. When the runway conformance test is run, a red alert is issued at time 

26.0 s, only 6 s after the start of the intrusion. The horizontal separation at TCA was 420 ft and 

the vertical separation was 16 ft. 

Figure 8-1: ALAS Algorithm without Runway Conformance Test (Horizontal View) 
36

9. Performance of Yellow Alerting 

In phase I of this work, we have only performed a rudimentary analysis of yellow alerting. In 

fact, we have still not optimized the values of the algorithm parameters associated with yellow 

alerting. We have obtained some statistics on the timing of the yellow alerts relative to the red 

alert. A histogram of the time between yellow and red alerts is shown in Figure 9-1. In this 

histogram, we can see that there are a high percentage of cases where there is no time between the 

yellow and red alert. We also see that there are a large percentage of cases with 10 or more 

seconds between the alerts. This suggests that there is much room for improving the yellow 

alerting in the future. 

10. Performance of the Tangent Fan Algorithm 

A new implementation of the Tangent Fan algorithm, which is similar to the AILS algorithm 

[Abbott2002], has been developed. The ALAS software provides easy access to several variants 

of the Tangent Fan algorithm. Each of the variants uses a different conflict probe. There are 

three choices: a circular protection zone, an elliptical protection zone, and a linear protection 

zone. The worst-case minimum distances shown in Table 10-1 were measured for the case where 

the Tangent Fan algorithm was not augmented by a runway conformance monitor. 

0.00

5.00

10.00

15.00

20.00

25.00

30.00

35.00

40.00

45.00

50.00

0 1 2 3 4 5 6 7 8 9 10+

Frequency (Percentage)

Time-Interval Bin (sec) 

(A bin is denoted by the lower bound of its time-interval range.)

Red Alert Time - Yellow Alert Time

Figure 9-1: Histogram of Red Alert Times - Yellow Alert Times
37

Table 10-1: Performance of the Tangent Fan Algorithm 

Algorithm Worst-Case Minimum

Distance (ft)

Horizontal Distance at 

TCA (ft)

Vertical Distance at 

TCA (ft)

1 (circle) 136 19 135

2 (ellipse) 201 174 99

3 (lines) 96 96 1

We present these numbers with some caution because the parameters used in the algorithm 

have not been tuned as carefully and fully as the ALAS parameters. In addition, the elliptical 

conflict probe is different than the one explored in earlier AILS studies. The probe used in the 

ALAS object allows for a front buffer larger than the back buffer. 

Table 10-2 shows that a significant improvement is seen if we add a runway conformance 

monitor to the Tangent Fan algorithm. 

Table 10-2: Tangent Fan Performance with Runway Conformance Test 

Algorithm Worst-Case Minimum

Distance (ft)

Horizontal Distance at 

TCA (ft)

Vertical Distance at 

TCA (ft)

1 (circle) 279 249 125

2 (ellipse) 201 174 99

3 (lines) 315 283 138

11. Preliminary Performance in a High-Fidelity Simulation 

The ALAS algorithm with runway conformance test was exercised under limited scenarios in 

a high-fidelity simulation of a large transport-class aircraft that is used in Langley‘s CMF. This 

CMF simulation provides an environment that can subject ALAS to system latencies and errors 

and can assess the effectiveness of the escape maneuver with high-fidelity dynamics. 

11.1. Modeling the SAPA Procedure 

In the CMF simulation, the SAPA procedure was automated using mode control panel (MCP) 

settings and event-driven pilot actions. The CMF simulation does not have the capability to 

automate dependent operations, and each participating aircraft performed the SAPA procedure 

independently from initiation to touchdown. However, since the implementation is automated 

and dependent operation occurs under constant speed conditions, the independent operation does 

approximate the outcome of a dependent operation under the same condition. The CMF 

simulation did require an adjustment to the initial separation of the two vehicles to account for the 

larger true airspeed of the higher vehicle (for the given constant calibrated airspeed) that exists 

until the high aircraft descends along the glidepath to a near co-altitude position with the low 

aircraft. The transport aircraft model in CMF simulation is designed to use ILS for coupled 

approaches and does not have RNAV capability. This does not influence the planned path of the 

vehicle in the SAPA procedure, but does introduce differences in modeling total system error as 

described in Section 11.5.1. In the CMF simulation, the speed schedule of the procedure is 

primarily automated using the autothrottle speed (SPD) mode. Horizontal position is controlled 

using localizer (LOC) mode. Vertical position is initially automated with altitude hold (ALT 
38

HLD) until near the glideslope where the MCP is switched to approach (APP) mode. The 

simulated aircraft requires pilot action to drop the landing gear, set the flaps, and deploy the speed 

brake (if needed). These actions were scheduled based on events with at least 2 s between 

consecutive actions. Deceleration of the lead aircraft was scheduled to occur at the FAF. 

Deceleration of the trailing aircraft was programmed to start 3 s after the lead aircraft began 

deceleration or upon reaching the FAF, whichever came first. The autoland feature of the 

autopilot performed flare and touchdown. 

11.2. Modeling Blunders 

The CMF simulation was modified to simulate three types of blunders: a side-step, a 30

heading change while descending, and a 30 heading change while leveling-off altitude. The 

latter two are consistent with FAA guidance for modeling blunders [Massimini2006]. In the sidestep, the blundering aircraft switches runways, crossing to the approach path of the ownship 

runway. The 30 heading change was automated using heading select (HDG SEL) mode. The 

heading change while descending used vertical speed mode (VERT SPD) to continue the descent 

as dictated by the glidepath and speed schedule. The heading change while leveling-off used 

altitude hold (ALT HLD) mode to maintain altitude. To provide results comparable with the 

tALAS simulation, only results from the 30 heading change blunders (descending and level) are 

presented here. 

11.3. Modeling the Evasive Manuever 

The escape maneuver was modeled as a go-around with a heading change. Prior to initiating 

the go around, target speed on the mode control panel is changed to 250 KT and target altitude is 

set to the SFO go-around altitude of 3000 ft for runways 28L and 28R or to 1700 ft of climb 

whichever is higher. The 1700 ft is double the vertical collision zone for TCAS II and allows the 

aircraft to switch from ALAS to TCAS for collision avoidance after the SAPA pair separate 

vertically by more than 800 ft. The go-around is initiated by automated toggling of the go-around 

switch. The heading change follows and is executed as a 60 heading change away from the 

intruding aircraft, using heading select (HDG SEL) mode. A change of 60 elicits the most 

aggressive turn from the autopilot. Section 7.1.1. shows that pilot delays of 1 s or more in 

executing the escape maneuver can lead to intrusions of the collision zone. Therefore, the escape 

maneuver is auto-executed after an ALAS alert with the latency described in Section 11.5.3. 

11.4. Scenarios 

The ALAS algorithm was run in the CMF simulation using aircraft pairs with 7 to 8 KT 

differences in approach speed which are in the midrange of what the SAPA procedure can 

support as shown in Section 2.3. The paired approach speeds were 114 KT / 122 KT, 122 KT / 

130 KT, 130 KT / 138 KT, 138 KT / 145 KT, 145 KT / 153 KT, and 153 KT /160 KT. Each step 

of approach speed in the above pairs corresponds to a 20,000-lb step in aircraft weight. The 

aircraft weight ranged from 140,000 lb to 260,000 lb to approximate the variety of aircraft sizes 

that may participate in the procedure. A run without blunders (not presented) was used to verify 

that the initial conditions would produce a successful SAPA approach and adjust those conditions 

as needed. Then, for each pair, four blunders were run. One pair of blunders was executed 10 

NM from the threshold in the constant speed segment. A second pair of blunders was executed at 

2.5 NM from the threshold in the final approach segment. In each blunder scenario, the ownship 

would conduct an escape maneuver when ALAS issued an alert. For each blunder, two runs were 
39

made, an ―ideal‖ case and a case with the ―modeled avionics‖. The ―ideal‖ case runs the scenario 

without noise, latency, or precision applied to the ALAS input data; however, the total system 

error in following the approach path is still modeled through induced ILS noise. The ―modeled 

avionics‖ case models GPS noise, end-to-end latency, and GPS and ADS-B OUT output 

precision. These are applied along with latency compensation to the ownship and traffic states 

that are input into ALAS. 

11.5. Errors and Latencies 

Next, we consider the total system error, the uncertainty in the position and velocity inputs to 

ALAS, and the latency between the red alert and the beginning of the escape maneuver. 

11.5.1. Total System Error 

There are multiple avionics contributors to total system error. To adjust total system error 

with one control variable, each for horizontal error and vertical error, the standard deviations of 

the localizer and glideslope receiver noise were used. The noise in both receivers is modeled 

using a first-order Gauss-Markov process. The noise causes the autopilot to meander about the 

ILS or glideslope beam with an error on the order of the standard deviation of the noise. The 

standard deviation of the localizer noise was set to 0.036 which corresponds to a 2 navigation 

error of 40 m (131 ft) at the distance where the ALAS algorithm activates. The standard 

deviation of the glideslope noise was left at its default setting of 0.035 and thus produces a nearequal error in the vertical. Because the standard deviation is expressed as a small angle, the 

corresponding position error decreases linearly as the aircraft approach the ILS transmitters. At 

200 ft AGL (when ALAS deactivates), the corresponding position error has a 2 value of 18 ft

(5.5 m). This differs from GPS-based RNAV, which is expected to exhibit a constant error with 

distance from the runway threshold. However, the ILS-induced total system error, as modeled 

above, is adequate to model worst-case performance for the aircraft fleet in the 5 to 10+ year 

deployment horizon for the procedure. A side effect of injecting glideslope error for this 

simulated aircraft is that the meandering climbs and descents also cause deviations away from the 

commanded airspeed. These deviations contributed to an uncertainty in velocity difference of 

less than ±3 KT in the constant speed segment and of less than ±3.9 KT in the approach. This is 

larger than the estimated maximum uncertainty for the SAPA procedure that was computed in 

Section 2.3. The default noise for the glideslope is indicative of older ILS receivers and could be 

reduced. However, since all aircraft pairs can be positioned to successfully execute the SAPA 

procedure with these uncertainties, establishing more realistic vertical deviations was left to 

future work. 

11.5.2. Uncertainty of Position and Velocity Inputs to ALAS 

ALAS is expected to receive updates of the traffic aircraft state via ADS-B IN. The 

uncertainty of the input is a function of the uncertainty in the traffic aircraft‘s GPS-sensed 

position, the truncated precision of the GPS and ADS-B transmissions, the end-to-end latency 

from measurement to ALAS input, and the compensated latency. Ownship state updates are 

expected to be received directly from the on-board GPS unit. Both data paths begin with the onboard GPS unit and end with the avionics executing ALAS. The full data path for the ownship 

state is simply GPS  ALAS. The full path for the traffic data is GPS  ADS-B OUT 

ADSB-IN  ALAS. 
40

This study assumes that WAAS GPS will be the minimum equipage for aircraft participating 

in the SAPA procedure. RTCA DO-229D defines minimum operations standards for WAAS 

GPS receivers. However, the allowable position uncertainty of 32 m and update rate of 1 Hz are 

inadequate to support the SAPA requirement for a NACp of 10 or better [RTCA2006]. RTCA 

DO-242, the MASPS for ADS-B, defines, in Appendix J, the expected 1  position error of 

WAAS GPS to be 1.8 m (2 = 3.6 m) in steady flight [RTCA2002]. 2

 Moreover, Garmin 

advertises WAAS GPS units for aviation with a 1 m RMS position error and 5 Hz update rate 

[Garmin2012]. For velocity, RTCA DO-260B, the MOPS for ADS-B OUT, identifies the 

expected 95% velocity accuracy of GPS units to be 0.2 m/s per axis (or 0.5 m/s for ground speed 

magnitude assuming a Rayleigh distribution) in stable flight [RTCA2009]. For GPS unit latency, 

RTCA DO-242 Appendix K defines the expected latency from measurement to data transmission 

to be 0.3 s for high NACp applications [RTCA2002]. This latency is divided into 0.1 s for 

receiver operation and 0.2 s for state data transmission to destination. GPS units transmit the 

UTC time of applicability for the state data, which enables compensation of latency by the 

receiving unit; the time of applicability coincides with the time mark pulse emitted by the GPS 

unit, which is normally also the start of transmission. RTCA DO-260B implies that in high 

NACp applications, the ADS-B avionics must be synchronized on the time mark of the GPS unit 

and the GPS unit must be configured to emit time marks at a UTC sub-epoch (a 0.2 UTC subsecond) [RTCA2009]. Based on the information above, the GPS units were modeled with a 3 m 

position noise, a 0.5 m/s ground-speed noise, a 5 Hz update rate on the UTC sub-epoch, and a 

300-ms latency from truth state through transmission. In addition, the data transmitted has 

numerical precision as defined in RTCA DO-229D Appendix H: 8.38E-8 for latitude and 

longitude, 0.125 ft for altitude, 0.125 KT for ground speed, and 0.0055 for true track 

[RTCA2006]. 

For ADS-B OUT, the FAA rule permits a 2.0 s latency from state measurement to ADS-B 

OUT transmission; 0.6 s of the latency can be uncompensated [FAA2010]. These minimum 

requirements are compatible with the FAA minimum NACp of 8 (92.6 m) but not the SAPA 

minimum NACp of 10 (10 m). RTCA DO-242 Appendix K, again, provides guidance for the 

end-to-end latency of ADS-B OUT in high NACp applications. The latency contribution from 

the GPS unit is as defined in the previous paragraph. The latency expected for the ADS-B 

avionics from reception of GPS data to transmission of the ADS-B report is 0.4 s [RTCA2002]. 

ADS-B units synchronized to the GPS time mark, as defined in DO-260B, should be able to apply 

compensation for the full latency and can assign a UTC sub-epoch as the time of applicability for 

the ADS-B OUT report. This will allow the receiving aircraft to further compensate for latencies 

downstream. ADS-B OUT reports also exhibit the precision defined in RTCA DO-260B. The 

encoding algorithm for position maintains an approximately 5 m precision in the North and East 

axes. Velocity precision is 1 KT in the North and East axes [RTCA2009]. 

Latency for ADS-B IN was taken from RTCA DO-317A, the Minimum Operational 

Performance Standards for Aircraft Surveillance Applications. This RTCA document allocates 

0.5 s to ADS-B IN latency [RTCA2011]. To define the latency for the ALAS avionics, it was 

assumed that the ALAS avionics would run at 5 Hz but would operate asynchronously to both the 

ownship GPS unit and the ADS-B IN avionics. Therefore, ALAS can introduce 200 ms of 

additional latency to the ownship and traffic data. 

 

2

 In the SAPA procedures, ownship and traffic accelerations are expect to remain below 0.1 g prior to an 

alert; therefore, the SAPA scenarios qualify as steady flight. 
41

Because the state data is closely associated with a UTC sub-epoch under both paths, nearly all 

of the latency can be compensated. However, there are some allowances to decrease the 

compensated latency. RTCA DO-260B allows up to a 5-ms deviation in the GPS time mark from 

the actual UTC sub-epoch [RTCA2009]. This deviation plus an additional 1 ms of uncaptured 

time delays is subtracted from the compensated latency for paths within each aircraft. The 5-ms

GPS time mark deviation is also used to establish a worst-case clock difference between aircraft 

of 10 ms to which an additional 1 ms is added for other uncaptured time delays. This 11 ms

uncompensated latency is assigned to the ADS-B IN processing of the traffic data. The resulting 

end-to-end latency of aircraft equipped for the SAPA procedure is summarized below. In this 

summary, the latency of the traffic data is divided between ADS-B OUT and ADS-B IN because 

of the change in precision of the state data in the ADS-B OUT report. 

 Ownship State 

o Total Latency (Measurement to ALAS input): 500 ms 

o Compensated Latency: 494 ms 

 Traffic State

o ADS-B OUT 

 Total Latency (Measurement to Transmission): 700 ms 

 Compensated Latency: 694 ms 

o ADS-B IN 

 Total Latency (Reception to ALAS): 700 ms 

 Compensated Latency: 689 ms 

One side effect of relying on ADS-B synchronized to the GPS time mark is that the 2-persecond transmission of state data cannot occur in equal intervals. The transmission is now 

associated with a 0.2 s sub-epoch. Therefore, the interval alternates between 0.4 and 0.6 s. The 

ALAS algorithm has not been verified and validated for variable intervals. Therefore, the CMF 

simulation calls ALAS at a constant 0.5 s rate and increases total and compensated latency by 100 

ms for both the ownship data and the ADS-B in path of the traffic data. 

11.5.3. Latency of Evasive Maneuver 

The latency between alert and command of the escape maneuver was set at 260 ms. Two 

hundred ms was the assumed worst-case compute time of the ALAS avionics. The autopilot was 

assumed to run at a rate of 50 Hz. A one-cycle delay in receiving the alert and a two-cycle 

compute delay in issuing the escape maneuver command were assumed for the autopilot. 

11.6. Increasing Bank Rate of Automated Manuevers 

Initial runs showed that turns using the autopilot are slow to develop because the autopilot 

takes more than 15 s to bank the aircraft to 30. This is almost three times as long as what is 

achievable in manual flight. Therefore, an option was added to augment the roll rate using roll 

stick commands. A simple feedback loop moves the roll control based on three criteria. At the 

start of the turn, the roll control chases a 10/s roll rate. When the aircraft nears a bank angle of 

30, the roll control is moved toward a normalized value of 1/3, which maintains a bank of ~30. 

As the aircraft nears the desired heading, the roll control is returned to a normalized value of zero 

and augmentation of the autopilot control ceases. The feedback loop was tuned to ensure that the 

blundering aircraft would not intercept the ownship‘s approach path by more than 30. The 

tuning effectively capped the length of time that the blundering aircraft could maintain a high turn 
42

rate corresponding to bank angles between 25 and 30. The ownship, targeting a heading change 

of 60, could sustain a high turn rate longer. However, this advantage had little influence on 

minimum separation after the alert because the minimum occurs within the first 13.5 s following 

the alert. When activated, this roll augmentation was applied to both the blunder and the escape 

maneuver since participating aircraft turning at the same rate should represent a worst case. 

Results with and without this roll augmentation are presented in Table 11-1 through Table 11-4. 

11.7. Measuring Time to Alert 

One aspect of ALAS performance is how quickly it alerts the ownship of the blundering 

traffic. Of particular interest is the effect of avionics latencies and error in the time to alert. In 

this paper, the time to alert is defined as the time between vehicle departure from the approach 

path and the issuing of the alert. ALAS uses rate of turn as the metric to identify suspected 

departures from path. However, aircraft normally perform small turns on approach due to 

navigation and flight technical error. In the CMF simulation, the aircraft exhibit a maximum turn 

rate of 0.36/s under blunder-free conditions. Therefore, the time when the turn rate first exceeds 

0.5/s or greater is the reference used to count time-to-alert. 

11.8. Results 

The outcome of the high-fidelity CMF simulation runs are shown in Table 11-1 through Table 

11-4. Each table shows the time to alert, the magnitude of the 3D distance vector between 

aircraft at the time of the alert, and the 3D distance at closest separation. This data set enables 

analysis of blunder type, rate of turn, and avionics performance on collision avoidance. 

11.8.1. Blunder Type 

Whether the aircraft descends or levels out while blundering does not make a significant 

difference to alert times. Of the 48 case-pairs that differ by blunder type, only one-third (16) 

differ by greater than the 0.5 s time step for the ALAS algorithm; the median difference is 0.02 s. 

However, where the case-pairs do differ, the alert times for level-out blunders are lower (14 out 

of 16). Though blunder type has little influence on alert time, this is not true for efficacy of the 

escape maneuver. The closest approach is lower for the level-out blunder in 46 of the 48 casepairs and the median decrease is 125 ft. The level-out blunder eliminates one advantage of the 

escape maneuver, the climb. In the level-out blunder, the trailing aircraft continues to descend on 

the glidepath while the blundering aircraft remains level. By the time an alert is issued, the 

trailing aircraft is often near or below the altitude of the blundering aircraft. For the level-out 

blunder, the median vertical separation at the alert is -8 ft; for the descending blunder the median 

is +79 ft. Furthermore, for the descending blunder, the blundering aircraft continues to descend 

during the escape maneuver. That leads to a greater vertical separation at closest approach. For 

the descending blunder, the median vertical separation at closest approach is 293 ft; for the levelout blunder, the median is 63 ft. 

11.9. Rate of Turn 

Rate of Turn does not significantly influence time to alert. Of the 48 case-pairs, only 18 show 

a difference greater than the 0.5 s time step of the algorithm. Nevertheless, all 18 are lower for 

the augmented turn and the median difference over all 48 cases is +0.2 s (normal turn higher). 

Increased rate of turn also does not offer consistent improvement in separation. Of the 48 case 
43

pairs, the increased rate of turn produces improved separation greater than 33 ft (10 m) in 23 

cases but also produces reduced separation greater than -33 ft in 12 cases. Overall, little more 

than half (27) of the cases show any improvement in separation from increased turn. The median 

difference between the augmented and normal turns is +30 ft. The primary reason that rate of 

turn has little influence on separation is that the blundering aircraft and ownship use the same turn 

performance, separated only by time. Nevertheless, significant trends do emerge when the case 

pairs are subdivided between constant speed and final approach segments. In the constant speed 

segment, there emerges a tilt toward better separation under the normal turn; however, the 

advantage is not significant. The normal turn produces greater separation in 16 of the 24 cases 

but the median improvement is only 18 ft. On the other hand, the augmented turn produces 

significantly higher separations against blunders in the final approach segment. Separation is 

improved in 19 of 24 cases and the median improvement is 60 ft. What favors the augmented 

turn during final approach is the difference in speed between the two aircraft. The faster turn rate 

applies more of the trailing aircraft‘s faster groundspeed toward cross-track separation more 

quickly; in fact, the median increase in cross-track separation over the normal turn at minimum 

separation is 190 ft. 

11.9.1. Modeled Avionics 

The end-to-end latency of the avionics, though compensated, has a direct effect on time to 

alert. When avionics are modeled, the time-to-alert grows by a median value of 1.5 s, the same as 

the end-to-end latency of the traffic data. Avionics noise and precision almost evenly tilts half 

(23 out of 48) of the case-pairs toward a 1 s (13 cases) or 2 s (10 cases) increase in time-to-alert. 

This delay in time-to-alert directly leads to a decrease in minimum separation during the escape 

maneuver.3

 The median loss of separation is 113 ft, almost a half-wingspan for a Boeing 747-8. 

11.10. Indicated Collisions 

A collision is indicated when the minimum separation between the participating aircraft falls 

below 400 ft. Within the 96 runs presented in Table 11-1 through Table 11-4 there are nine 

collision indications. However, these 96 runs are not all independent; they are variations of 12 

uncorrelated initial conditions. Of these twelve, a collision indication appears for three. All but 

one of the collisions occur with avionics modeled. The one collision in the ideal case occurs for a 

normal turn, which may not be indicative of the escape performance of the aircraft; the 

augmented turn for the same case produces a separation well outside 400-foot collision zone. All 

of the collision indications occur when 3D separation at alert is less than 880 ft. But not all alerts 

at less than 880 ft separation lead to a collision; there are 17 of these runs without a collision 

indication. Nevertheless, the data suggests that an unsafe probability of collision may exist at 

separations of less than 900 ft. 

 

 

3

 The avionics latency and noise modeled here are only applied to ALAS inputs and are not applied to the 

autopilot or other systems that drive vehicle performance. Therefore, changes in seperation between the 

ideal and modeled avionics cases can be attributed solely to delays in ALAS alerts. 
44

Table 11-1: High-Fidelity Simulation Results: 30 Blunder While Descending 

During Constant Speed Segment 

Ideal Input to ALAS

Normal Turn Augmented Turn

Approach 

Speeds of 

Fast/Slow (KT)

Time to 

Alert (s)

3D Distance 

at Alert (ft)

Closest 3D 

Distance (ft)

Time to 

Alert (s)

3D Distance 

at Alert (ft)

Closest 3D 

Distance (ft)

122 / 114 1.52 855 587 1.34 862 613

130 / 122 1.74 1016 833 1.12 1019 861

138 / 130 2.16 915 773 2.14 889 700

145 / 138 1.34 1133 988 1.66 1123 908

153 / 145 1.16 1214 1072 1.50 1203 1039

160 / 153 2.56 967 704 1.88 938 736

Modeled Avionics Input to ALAS

Normal Turn Augmented Turn

Approach 

Speeds of 

Fast/Slow (KT)

Time to 

Alert (s)

3D Distance 

at Alert (ft)

Closest 3D 

Distance (ft)

Time to 

Alert (s)

3D Distance 

at Alert (ft)

Closest 3D 

Distance (ft)

122 / 114 3.02 836 500 2.84 841 448

130 / 122 3.24 1000 732 2.62 1007 730

138 / 130 3.66 907 678 3.14 878 615

145 / 138 3.34 1120 853 2.66 1114 828

153 / 145 3.16 1205 962 3.00 1190 907

160 / 153 3.56 963 677 3.38 921 552
45

Table 11-2: High-Fidelity Simulation Results: 30 Blunder While Level During 

Constant Speed Segment 

Ideal Input to ALAS

Normal Turn Augmented Turn

Approach 

Speeds of 

Fast/Slow (KT)

Time to 

Alert (s)

3D Distance 

at Alert (ft)

Closest 3D 

Distance (ft)

Time to 

Alert (s)

3D Distance 

at Alert (ft)

Closest 3D 

Distance (ft)

122 / 114 1.50 853 410 1.36 859 539

130 / 122 1.70 996 661 1.10 1010 749

138 / 130 1.62 875 441 1.10 866 620

145 / 138 1.30 1108 804 1.14 1107 787

153 / 145 1.58 1186 923 0.94 1187 904

160 / 153 2.46 940 515 1.30 919 624

Modeled Avionics Input to ALAS

Normal Turn Augmented Turn

Approach 

Speeds of 

Fast/Slow (KT)

Time to 

Alert (s)

3D Distance 

at Alert (ft)

Closest 3D 

Distance (ft)

Time to 

Alert (s)

3D Distance 

at Alert (ft)

Closest 3D 

Distance (ft)

122 / 114 3.00 838 353 2.86 835 349

130 / 122 2.70 985 634 2.60 991 601

138 / 130 2.62 868 383 2.60 852 441

145 / 138 2.80 1099 717 2.64 1092 661

153 / 145 3.08 1176 810 1.94 1180 807

160 / 153 3.96 933 456 2.80 904 418
46

Table 11-3: High-Fidelity Simulation Results: 30 Blunder While Descending 

During Final Approach Segment 

Ideal Input to ALAS

Normal Turn Augmented Turn

Approach 

Speeds of 

Fast/Slow (KT)

Time to 

Alert (s)

3D Distance 

at Alert (ft)

Closest 3D 

Distance (ft)

Time to 

Alert (s)

3D Distance 

at Alert (ft)

Closest 3D 

Distance (ft)

122 / 114 1.20 1369 1056 1.00 1375 1157

130 / 122 1.40 1587 1277 1.14 1609 1319

138 / 130 1.24 1592 1289 1.14 1613 1427

145 / 138 1.56 814 500 1.36 828 558

153 / 145 1.56 874 431 1.44 903 491

160 / 153 1.26 1405 1074 1.22 1440 1074

Modeled Avionics Input to ALAS

Normal Turn Augmented Turn

Approach 

Speeds of 

Fast/Slow (KT)

Time to 

Alert (s)

3D Distance 

at Alert (ft)

Closest 3D 

Distance (ft)

Time to 

Alert (s)

3D Distance 

at Alert (ft)

Closest 3D 

Distance (ft)

122 / 114 3.20 1345 883 3.00 1357 1013

130 / 122 2.90 1563 1125 2.64 1589 1213

138 / 130 3.24 1564 1115 2.64 1591 1332

145 / 138 3.06 792 422 2.36 812 463

153 / 145 3.56 830 267 2.44 880 327

160 / 153 2.26 1386 988 2.72 1408 958
47

Table 11-4: High-Fidelity Simulation Results: 30 Blunder While Level During 

Final Approach Segment 

Ideal Input to ALAS

Normal Turn Augmented Turn

Approach 

Speeds of 

Fast/Slow (KT)

Time to 

Alert (s)

3D Distance 

at Alert (ft)

Closest 3D 

Distance (ft)

Time to 

Alert (s)

3D Distance 

at Alert (ft)

Closest 3D 

Distance (ft)

122 / 114 1.38 1355 906 1.00 1364 970

130 / 122 0.94 1581 1169 1.12 1604 1098

138 / 130 1.24 1580 1180 1.16 1612 1358

145 / 138 1.60 822 454 0.90 835 582

153 / 145 1.62 884 241 0.98 919 517

160 / 153 1.34 1411 952 1.28 1449 1033

Modeled Avionics Input to ALAS

Normal Turn Augmented Turn

Approach 

Speeds of 

Fast/Slow (KT)

Time to 

Alert (s)

3D Distance 

at Alert (ft)

Closest 3D 

Distance (ft)

Time to 

Alert (s)

3D Distance 

at Alert (ft)

Closest 3D 

Distance (ft)

122 / 114 2.38 1344 840 2.50 1351 806

130 / 122 2.94 1544 1023 2.62 1578 986

138 / 130 3.24 1545 956 2.16 1594 1294

145 / 138 2.60 810 394 2.40 816 462

153 / 145 2.62 866 177 2.98 871 235

160 / 153 3.34 1369 854 2.78 1413 879

11.11. Worst Case 

In the above discussion, three modeling parameters are shown to have the greatest influence 

on minimum separation after alert. They are a level-turn blunder, avionics latency, and 

separation at the alert below 900 ft. These aspects were combined in two additional runs, for the 

constant speed and final approach segments, to elicit the worst-case performance of the SAPA 

procedure. For the constant speed segment, starting separation was adjusted to produce an alongtrack separation of 130 ft at the time of alert. For the final approach segment, an along track 

separation of 260 ft was targeted for the alert. The increased separation for the final approach 

segment is intended to counter the faster speed of the trailing aircraft. In both cases, adjustment 

of starting separation used the augmented turn as a reference. As a result, the along-track 

separation at alert for the normal turns are smaller, 130 ft and 260 ft, respectively. The results are 

presented in Table 11-5 and Table 11-6. Only one case in the table stays outside of the collision 

zone. Moreover, the minimum separation in half of the cases is less than the wingspan of a 

Boeing 747-8. Such a deficit is unlikely to be overcome by adjustments to the algorithm or the 

SAPA procedure. In fact, the tALAS simulation results point to an inherent obstacle to 

overcoming this deficit. 

As discussed in Section 7.1.1. the tALAS simulation results show that collisions cannot be 

prevented if pilot initiation of the escape maneuver is delayed by 1 s or more after the alert. The 
48

tALAS simulator uses true state data for inputs and, therefore, is equivalent to the ideal cases of 

the CMF simulation. The alert delay in the modeled avionics cases becomes comparable to a 

delay in pilot action. This delay has a median of 1.5 s but is also followed by a 260-ms delay for 

the autopilot to initiate the maneuver. Therefore, the worst-case results of the high-fidelity 

simulation should approach the worst-case result from tALAS with a 2-second delay. Section 

7.3.1. shows the results of pilot delay for the level-out blunder. The program tALAS estimates a 

worst case separation of 177 ft for a 2 s delay. In the CMF simulation, the augmented turn comes 

closest to matching the turn performance that tALAS uses for the ownship and traffic. Under the 

augmented turn, the minimum separation is as low as 146 ft, which is near the 177 ft result from 

tALAS. Therefore, a collision-free procedure is not possible under the latency introduced by 

real-world avionics. The total latency of the traffic state would need to be reduced below 1 s for a 

collision-free procedure at 750-ft runway separation. This is unlikely to be accomplished using 

ADS-B as currently defined. 

Table 11-5: Small Along-Track Separation at Alert: 30 Blunder While Level 

During Constant-Speed Segment 

Modeled Avionics Input to ALAS

Standard Turn Augmented Turn

Approach 

Speeds of 

Fast/Slow (KT)

Time to 

Alert (s)

3D Distance 

at Alert (ft)

Closest 3D 

Distance (ft)

Time to 

Alert (s)

3D Distance 

at Alert (ft)

Closest 3D 

Distance (ft)

122 / 114 2.78 718 51 2.68 700 216

130 / 122 2.56 737 58 2.54 717 241

138 / 130 2.46 754 114 2.44 731 294

145 / 138 2.34 752 119 2.72 722 146

153 / 145 2.90 767 118 2.78 737 174

160 / 153 4.26 802 115 3.06 761 181

Table 11-6: Small Along-Track Separation at Alert: 30 Blunder While Level 

During Final-Approach Segment

Modeled Avionics Input to ALAS

Standard Turn Augmented Turn

Approach 

Speeds of 

Fast/Slow (KT)

Time to 

Alert (s)

3D Distance 

at Alert (ft)

Closest 3D 

Distance (ft)

Time to 

Alert (s)

3D Distance 

at Alert (ft)

Closest 3D 

Distance (ft)

122 / 114 2.36 852 348 2.58 853 418

130 / 122 2.66 828 302 2.36 850 354

138 / 130 3.28 776 114 2.60 818 374

145 / 138 3.16 833 167 2.52 856 342

153 / 145 3.22 811 88 2.58 841 274

160 / 153 3.02 755 65 2.44 800 245
49

12. Comparing the Low Fidelity and High Fidelity Results 

The conclusions from the low-fidelity simulations and the high fidelity simulations are 

different. The low fidelity simulations show that with a zero pilot delay, a minimum separation 

distance greater than 400 ft is achievable. The high-fidelity simulations found several cases 

where the separation was significantly less than 400 ft. A comparison of the results show that the 

difference is caused by the escape maneuver used in these simulations. The low-fidelity 

simulation assumed that a 30 turn immediately begins with a 3.94/s turn rate. In the highfidelity simulation, the automated system produced a very sluggish turn. In fact, it took 5.5 s just 

to reach a turn rate of 1/s. A typical turn is shown in Figure 12-1. 

Figure 12-1: Horizontal Perspective for Typical Large Class Aircraft Turn 

The orange dot indicates the point where the escape maneuver was initiated (e.g. time 323.5 

s). But the rate of change of the aircraft track does not reach 1/s until 5.5 s later at time 329 s. 

This is indicated by the T on the diagram. The vertical perspective is shown in Figure 12-2. 

Figure 12-2: Vertical Perspective For Typical Large Aircraft 
50

By the time the aircraft has reached a robust turn rate (e.g. 2.8/s), 12 s has elapsed and the 

intruder aircraft has covered most of the 750-ft separation. This sluggish turn rate is likely the 

result of tuning for passenger comfort; transport autopilots are not designed with the expectation 

that they will be used to evade potential collisions. When the high-fidelity simulation models 

avionics, communication latencies add between 1 and 1.5 s to the time to alert over what was 

seen in the low-fidelity simulations. 

Late in the study, some additional high-fidelity runs were performed using automated 

movement of the pilot roll control in addition to the autopilot to increase the rate of turn; these are 

referred to previously as the ―augmented turn.‖ The augmented escape maneuver reduced the 

time to reach 1/s to about 3 s, but this is still not fast enough to have a guarantee that there is no 

violation of the protection zone for the worst possible scenarios. 

It appears that the fully general SAPA procedure will not be possible unless an escape 

maneuver can be created that initiates an aggressive turn almost immediately after the alert is 

received. We believe that the false alarm rate can be reduced with further improvements and that 

the yellow alerting can be improved, but in our simulations, the ALAS algorithm detected the 

intrusions as quickly as could be expected. 

We have performed some preliminary studies of the 1000-ft parallel runway case and the 

results are more promising. A pilot delay of 2 s or less resulted in no near collisions. These 

results were obtained without tuning the ALAS parameters for the 1000-ft separation. Table 12-1

provides the results of these simulations. 

Table 12-1: Low Fidelity Kinematic Simulation of 1000 ft Parallel Runway 

escape

PilotDelay 

(s)

Worst-Case

Minimum

Distance (ft)

Horizontal

Distance at 

TCA (ft)

Vertical

Distance at 

TCA (ft)

Start of 

Blunder T1

(s)

Time of 

Red 

Alert (s)

TCA 

(s)

0 649 649 13 15.0 24.0 26.0

1 607 581 177 10.0 10.5 18.5

2 454 393 226 10.0 10.5 20.5

3 306 230 202 10.0 10.5 21.0

4 185 99 157 10.0 10.5 21.0

The blunder was an immediate 30 bank turn with no level out. The worst-case run for 

escapePilotDelay = 0 shows the Euclidean distance (i.e. 3D) at closest approach was 649 ft, 

which occurred 11 s after the start of the intrusion. The horizontal distance at closest approach 

was 649 ft and the vertical distance was 13 ft. Once again the need for a very rapid escape 

maneuver is seen in the results. A pilot delay of 3 s resulted in a near miss while a pilot delay of 

2 s was acceptable. The extra 250 ft enables an extra 2 s to respond. 

The 750 ft parallel spacing problem was especially challenging because the tracking error of a 

normal aircraft could be as large as 131 ft. With this maximum tracking error for both aircraft, 

the paired aircraft can get as close as 488 ft before the intrusion begins. Repeatedly, the fast-time 

kinematic simulator would find that the worst case involved trajectories where the aircraft get 

close to the tracking limits and then the intruder turns abruptly into the ownship. 
51

13. Future Work 

There are several directions that can be pursued to improve the ALAS algorithm and the safety 

analysis for the SAPA procedure. 

13.1. ALAS Trigger Function

The ALAS algorithm uses a sweep of trajectories with different bank angles for the intruder 

aircraft and checks to see if any of these cross the ownship‘s runway centerline too close to the 

ownship. This sweep is guarded by a simple trigger that filters noise on the trajectories. The 

current design relies on a simple averaging function. If real data can be obtained for aircraft 

approaches on parallel runways, then a filter function tailored to this domain can be designed. 

This potentially could improve both the minimum distance and the false alarm rate. 

13.2. Kinematic Analysis in the Presence of ADS-B Latency and Position Errors 

Although we performed some simulations in the CMF laboratory that included ADS-B latency 

and position errors, this occurred late in the study and we were not able to introduce these errors 

into our tALAS low-fidelity simulation. The effect of these latencies on the minimum distances 

could be significant. Future work could improve the low-fidelity simulation to include ADS-B 

latencies and position errors. 

13.3. A Kinematic Study Using Double-Turn Blunder Model 

Appendix B presents our initial simulation results for the double-turn blunder model. The 

double-turn blunder model introduces several new degrees of freedom into the simulation and 

hence the number of test cases that are needed for even a coarse exploration of the input space 

leads to very large test times (e.g. 4+ days). In phase II, we can explore this state space more 

fully. We suspect that a more complex escape maneuver will be needed if the double-turn model 

is selected as the ―standard blunder model‖ for use in the safety analysis. However, the more 

fundamental question that must be answered is what is the appropriate blunder model? 

13.4. A Better False Alarm Analysis 

To accurately forecast a false-alarm rate, one must have an accurate population distribution of 

blunder trajectories. Unfortunately, given the rarity of real blunders, there is very little statistical 

information available. Future work could compare the three different approaches presented in 

Appendix A. 

13.5. Develop a Better Yellow Alert 

Because there are heuristic components to the ALAS algorithm, it is difficult to develop an 

analytical basis for yellow alerting. We would like to have a yellow alert a few seconds before a 

red alert, but it is difficult to predict when a red alert is going to happen because of the 

uncertainty in the future trajectory. Therefore, the only strategy that we know how to pursue is to 

vary the parameters that influence the yellow alert until the statistics improve. Unfortunately, in 

phase I, we have not had adequate time to do this well. In future work, a yellow alerting 

methodology for closely spaced parallel runways could be developed and justified. 
52

13.6. Tune/Test Algorithm for Other Runaway Spacings 

In this study we have limited our experiments to approaches for parallel runways that are 750 

ft apart. We could expand our studies to include other runway spacings. It is likely that the 

optimal values for the algorithm parameters vary with the runway separation. 

13.7. Enhanced Automated Flight Modes 

The escape maneuvers we studied were insufficient for the 750 ft parallel runway case. In 

order for the SAPA procedure to be used at this spacing, a more aggressive escape maneuver is 

required. One could envision a new automated flight mode in the flight guidance system, that 

performs a very aggressive escape maneuver, but this would not be available on existing aircraft 

and would depend upon a costly development life cycle of a new avionics suite. Alternatively, 

modifications to the SAPA procedure may be pursued that limit exposure to the fully abeam 

positioning of the aircraft. 

13.8. An Intelligent Evasive Maneuver 

The escape maneuver used in this study is a simple climb-turn and it is performed regardless 

of the relative position and velocity of the traffic at the time of the alert. In some of the CMF 

simulation cases, the ownship is below the traffic when the alert is issued. This most often occurs 

for the level-turn blunders. In this situation, the climb may only achieve a decrease in vertical 

separation, and it may be best for the ownship to delay the climb until the turn achieves adequate 

horizontal separation. There are also limited cases where an immediate turn or an increase in 

speed will narrow rather than increase separation. An intelligent escape maneuver, could 

improve separation for these select cases. 

13.9. New Wake Studies and Trades 

The current wake assumptions produce a highly constrained procedure. Options to trade 

procedure complexity and availability for increased approach speed differences or robustness to 

system errors will require more wake studies to determine the new wake-free boundaries for each 

trade. 

13.10. Procedure Modifications to Improve Safety 

Some procedure modifications with the potential to improve safety are using an offset 

approach for one or both aircraft and/or using an offset glidepath. The former provides greater 

horizontal separation; the latter provides greater vertical separation. However, under both 

options, the aircraft must still eventually converge on their respective runway centerlines, 

returning to a 750-ft cross-track separation and to an insignificant vertical separation. Therefore, 

these modifications represent solutions only if a blunder-free, along-track separation can be 

achieved prior to the convergence and maintained afterward. This may not be feasible with the 

current IGE wake-safe boundary of 1000 ft, which constrains the along-track separation from the 

centerline convergence to touchdown. Data in this study suggests that the collision-free boundary 

using ALAS may be in the neighborhood of 1000 ft and other options to expand the IGE wakesafe boundary may also be needed. 
53

14. Conclusions 

In this report, we have documented the progress in developing a practical SAPA procedure 

usable in closely spaced parallel approaches (as close as 750 ft apart) along with a tailored 

alerting algorithm. A new algorithm named ALAS was developed that combines features of a 

runway precision monitor and a conflict probe. The ALAS algorithm is highly configurable 

using a set of user-definable parameters. 

A custom fast-time, low-fidelity kinematic simulator (tALAS) was developed to test the 

performance of the algorithm in conjunction with a proposed escape maneuver. The tALAS 

simulator uses kinematic models for the aircraft trajectories. This includes a simple turn model 

and constant ground speed and vertical speed accelerations. In this simulator, the ALAS 

algorithm was tested in the presence of millions of approaches with varying spacings and 

intrusion characteristics. Four different blunder models were pursued: (1) single-turn blunder, (2) 

single-turn blunder with altitude level-out, (3) double-turn blunder (see Appendix B), and (4) 

double-turn blunder with altitude level-out. We never encountered a case where the ALAS 

algorithm failed to issue an alert for a blunder. However, the distance at the closest point of 

approach varied significantly depending upon which blunder model was used. For the single-turn 

blunder, a minimum distance of 448 ft was obtained using the default values of the algorithm 

parameters. Using the double-turn model with altitude level-out, a minimum distance of 212 ft

was obtained (see Appendix B). We expect that this could be improved with future work on the 

algorithm, but a satisfactory solution may require a more sophisticated escape maneuver that, for 

example, takes advantage of knowledge about the altitude of the blundering aircraft. However, a 

more fundamental question needs to be answered: exactly what type of blunder model should be 

used in the safety analysis for parallel runway studies? We are not sure at this time whether such 

a sophisticated blunder model is warranted. 

The high-fidelity simulation returned a more negative result than the low-fidelity studies. The 

primary cause of this result was the sluggish turn response of the high-fidelity aircraft when 

executing the escape maneuver. The high-fidelity aircraft automated turn requires at least 6 s to 

achieve a 1/s turn rate. This corresponds to a 5 s escape pilot delay in the kinematic simulations, 

which also had a very negative result. A more aggressive escape maneuver (the augmented 

maneuver) was explored, that improved the response of the high-fidelity aircraft significantly by 

reducing the latency to about 3 s. Nevertheless, this corresponds to a 3 s pilot delay in the 

kinematic studies that was found to result in collisions as well. This problem was further 

compounded when 1 to 2 s additional delay was added due to end-to-end latency of the traffic 

state in the ADS-B implementation. 

The SAPA procedure for 750-ft parallel runways does not appear to be feasible at this 

juncture. Although we explored several options for an automated escape maneuver using the 

existing capabilities in a modern flight deck, the results indicate that the escape maneuver must be 

more aggressive than what we were able to achieve in the high fidelity simulation. However, 

additional turn performance may require development of a new autoflight mode. Using the 

SAPA procedure for 1000 ft or larger spacings looks more promising, but this has not yet been 

studied in sufficient detail to make any definitive recommendations. 
54

15. References 

[Abbott2002] Abbott, T.S: Flight Test Evaluation of the Airborne Information for Lateral Spacing 

(AILS) Concept. NASA Langley Technical Paper, NASA/TM-2002-211639, 2002.

[FAA2010] FAA. "Automatic Dependent Surveillance—Broadcast (ADS–B) Out Performance 

Requirements To Support Air Traffic Control (ATC) Service; Final Rule." Federal 

Register (National Archive and Records Administration) 75, no. 103 (May 2010): 

30160 - 30195.

[FAA2012] FAA. digital - Terminal Procedures Search. http://aeronav.faa.gov/digital_tpp.asp?

ver=1209&eff=08-23-2012&end=09-20-2012. Washington, D.C., August 23, 2012.

[Garmin2012] Garmin. ―Datasheet for GPS 500W, GPS 400W, and GNC 420.‖ 

https://buy.garmin.com/shop/store/assets/pdfs/specs/gps400w_spec.pdf (accessed 

September 4, 2012)

[Johnson2010] Johnson, Sally C., Terrance S. Abbot, Nelson M. G. Guerreiro, Gary W. Lohr, Paul 

Volk, and Burnell T. McKissick. Simplified Aircraft-based Paired Approach: 

Concept Definition and Initial Analysis. Hampton: NASA Langley Research Center, 

2010 (under review).

[Massimini2006] Massimini, Vincent S. ―Simultaneous Independent and Dependent Parallel Instrument 

Approaches: Assumptions, Analysis, and Rationale‖, MITRE, McLean, VA, 2006.

[Mohleji2010] Mohleji, Satish C., and Ganghuai Wang. Modeling ADS-B Position and Velocity 

Errors for Airborne Merging and Spacing in Interval Management Application. 

McLean: Mitre, 2010.

[RTCA2002] RTCA, Inc: ―Minimum Aviation System Performance Standards for Automatic 

Dependent Surveillance Broadcast (ADS-B)‖, DO-242A, Washington, D.C., June 25, 

2002.

[RTCA2006] RTCA, Inc: ―Minimum Operational Performance Standards for Global Positioning 

System / Wide Area Augmentation System Airborne Equipment‖, DO-229D, 

Washington, D.C., December 13, 2006.

[RTCA2009] RTCA, Inc: ―Minimum Operational Performance Standards for 1090 MHz Extended 

Squitter Automatic Dependent Surveillance – Broadcast (ADS-B) and Traffic 

Information Services – Broadcast (TIS-B)‖, DO-260B, Washington, D.C., December 

2, 2009.

[RTCA2011] RTCA, Inc: ―Minimum Operational Performance Standards (MOPS) for Aircraft 

Surveillance Applications (ASA) System‖, DO-317A, Washington, D.C., December 

13, 2011.

[Samanant2000] Samanant, Paul and Mike Jackson: ―Description of the AILS Alerting Algorithm‖, 

NASA/CR-2000-210109, May 2000.

[Shank1994] Shank, Eric M. and Katherine M. Hollister: ―Precision Runway Monitor‖, The Lincoln 

Laboratory Journal. Vol. 7. No. 2. pp. 329-353, 1994.
55

[Winder2001] Winder, L. W. and James K. Kuchar: "Generalized Philosophy of Alerting with 

Application to Parallel Approach Collision Prevention", AIAA Guidance, 

Navigation, and Control Conference, Montreal, Canada, August 6-9, 2001.
56

Appendix A. Estimating False Alarm Rate 

Intuitively, a false alarm occurs when an alert is issued on a non-blundering intruder 

trajectory. However, a rigorous definition of ―false alarm‖ necessitates a mathematical definition 

of exactly which trajectories are blundering trajectories and which are not. This is surprisingly 

difficult to do. The problem is that there are many approaches and there is no a-priori means of 

deciding which approach is preferable to another. Three options for defining blundering are 

defined below. 

A.1. Option 1: Any Deviation From Normal 

In this approach, no attempt is made to delineate a class of blundering trajectories. Instead, 

normal trajectories are defined and any trajectory outside of this class is a blundering trajectory. 

The simplicity of this approach is appealing but it requires real data from actual landings for this 

approach to have any practical value. In [Winder2001] trajectories are described using several 

two-dimensional plots. 

A.2. Option 2: Protection Zone

In this approach, a protection zone is defined. Only trajectories that carry the intruder into the 

protection zone should be alerted. All trajectories that do not carry the intruder into the 

protection zone yet cause ALAS to issue an alert are false alarms. This approach does provide a 

rigorous definition of a false alarm, but the definition of the violation zone can be non-trivial. 

The use of a circular disk around aircraft is problematic for closely spaced parallel runways. 

Using 400 ft diameter protection zones, there is an immediate loss of separation when the runway 

centerlines are 750 ft apart and the aircraft are abeam. Elliptical zones can help but the choice of 

the major and minor axes is non-trivial. Neither circular nor elliptical zones take the wake vortex 

risk into consideration. There is good reason to never allow the intruder aircraft to pass in front 

of the ownship. The risk of a catastrophic wake encounter or that the intruder might crash on the 

runway in front of the ownship argues for an infinite buffer size in front of the ownship. For 

these reasons we propose a strawman definition. The protection zone is a moving quadrant that is 

D distance away from the ownship‘s centerline (in the direction of the intruder) and X units 

behind the ownship. This is illustrated in Figure A-1.

We tentatively propose the following values of X and D: X = 400 ft and D = 230 ft. A 

violation occurs if the intruder enters the protection zone within the SAPA time interval. The 

SAPA time interval begins when the vertical separation becomes less than 800 ft and ends at 

decision height. 

To estimate the false-alarm rate, one must construct trajectories that do not violate the 

protection zone. However, without any knowledge of the distribution of these trajectories in the 

real world, we do not know how to assign relative weights or probabilities to particular 

trajectories. Certain trajectories, (e.g. a turn right followed by a turn left) are probably more rare 

than a simple turn right. But in the absence of real blunder data, we have no credible way to 

assign these probabilities. 

We will pursue this approach in future work. 
57

Figure A-1: A Strawman Definition of the Protection Zone for Parallel Runways 

A.3. Option 3: Parametric Family of Blunder/Non-Blunder Trajectories 

In this option, we do not seek to define a protection zone. Instead, we divide trajectories into 

two classes: blundering trajectories and non-blundering trajectories. The non-blundering 

trajectories can be defined parametrically in many ways. One attractive definition is all 

trajectories that stay within the runway conformance zone are non-blundering. This definition 

allows some of the non-blundering trajectories to have a short duration, high-bank angle turn 

followed by a corrective turn that never leaves the conformance zone. This type of trajectory 

stresses the trigger function (track-rate estimation) of the alerting algorithm and can be used to 

insure that the trigger is not too sensitive. However, if one seeks to calculate a percentage of false 

alarm statistics using this method, one is confronted with the reality that the statistic is highly 

dependent upon the definition of the parametric families. It is interesting that this approach can 

allow a third class of trajectories that are neither blundering nor normal. 
58

Appendix B. Simulation Results Using Double-Turn Blunder 

Towards the end of our study, we constructed a double-turn blunder trajectory in tALAS. The 

components of this trajectory are shown in Figure B-1. This blunder has two turn components 

defined by two different bank angles Φ1 and Φ2. The beginning and end times of these turn 

components are all variables.

Figure B-1: Components of a Double-Turn Blunder 

Unfortunately, adding this additional degree of freedom leads to enormous simulations times 

(e.g. multiple days), so we have had to be strategic in our experimentation. All of the results here 

are preliminary. 

Table B-1 shows the parameter values for the double-turn blunder scenarios. We first ran a 

series of experiments with a coarse step size for the parameters (e.g. T4 step size of 5 s) and 

noticed that the worst case occurred when the first bank angle was 5 and the second bank angle 

was 30. We then fixed these values and varied the other parameters using a smaller step size. 

Table B-1: Trajectory Parameters for Double-Turn Blunder Scenarios

Parameter Meaning Min Value Max Value Step Size

T1 Start Time of Intrusion (s) 10 20 5

T2 Duration of Intrusion Turn 1 (s) 2 10 2

T3 Duration of straight segment after turn 1 (s) 0 6 2

T4 Duration of Intrusion Turn 2 (s) 2 10 2

bankAngle 1 Bank Angle of Intrusion () 5 5 5

bankAngle 2 Bank Angle of Intrusion () 30 30 5

Peak Max Trajectory error (ft) 131 131 10

Period Period of Trajectory error (s) 60 70 10

Phase Phase of Trajectory error () -180 +180 45

ownshipInitialSx Distance from runway (NM) 5.0 5.4 0.2

intruderInitialSx Distance from runway (NM) 5.0 5.4 0.2

ownshipInitialGs Ground speed (KT) 160 170 10

intruderInitialGs Ground speed (KT) 160 170 10
59

B.1. Double-Turn Blunder Without Altitude Level-out 

First we ran without a vertical level-out and obtained a worst-case minimum distance of 284 

ft. The horizontal distance was 234 ft and the vertical distance was 160 ft at TCA. This run is 

shown in Figure B-2. 

(a) Horizontal View 

(b) Vertical View 

Figure B-2: Worst-Case Run Using Double-Turn Blunder 

This is a particularly difficult scenario. The ownship‘s tracking error is bringing it closer and 

closer to the intruder before the blunder begins. The blunder begins with a very slow and gradual 

turn towards the ownship because the first turn has a bank angle of only 5. Then when the 

aircraft are as close as possible without tripping the runway conformance test, the sharp turn 

begins. The first red alert occurred at time 34.5 s, or 14.5 s after the beginning of the intrusion. 

Interestingly, the yellow alert occurred at 27 s, a full 7.5 s earlier. The reason that the red alert 

was delayed was because of the parameter ln_T_red = 15. Because the first bank angle was 5, 

the estimated time to cross the centerline was greater than 15 s for quite a while. This allowed the 

intruder to slowly get closer to the ownship. Then the intruder trajectory switched to a sharp 30

turn. Table B-2 shows details about the four different subcomponents of the ALAS detection 

algorithm 
60

Table B-2: Performance of the ALAS Component Tests for Worst-Case No LevelOut, Double-Turn Blunder Scenario 

Time

(s)

Alert

Level

Track-rate > 

Threshold

(/s)

alas_lines

test

(time-in) (s)

distAway Test 

(ft)

RunwayConformance

Test (ft)

27.0 1 No [0.84] Yes [34.5] No No

27.5 1 No [0.86] Yes [31.7] No No

28.0 1 No [0.87] Yes [29.9] No No

28.5 1 No [0.89] Yes [27.1] No No

29.0 1 No [0.90] Yes [25.1] No No

29.5 1 No [0.91] Yes [23.3] No No

30.0 1 No [0.93] Yes [21.6] No No

30.5 1 No [0.66] Yes [21.1] No No

31.0 1 No [0.39] Yes [20.6] No No

31.5 1 No [0.12] Yes [20.1] No No

32.0 1 No [0.14] Yes [19.6] No Yes [132]

32.5 1 No [0.12] Yes [19.1] Yes [539] Yes [141]

33.0 1 No [0.16] Yes [18.7] Yes [525] Yes [151]

33.5 1 No [0.17] Yes [18.1] Yes [511] Yes [169]

34.0 1 No [0.19] Yes [17.6] Yes [496] Yes [182]

34.5 2 Yes [2.05] Yes [12.8] Yes [480] Yes [199]

35.0 2 Yes [3.92] Yes [12.3] Yes [461] Yes [212]

We note that the track-rate threshold test prevents the alas_circle probe from executing on 

turns with a small angular velocity. From this table we can see that the first yellow alert (level 1) 

is caused by the straight-line projection probe. This does not result in a red alert because the 

projected time to cross the centerline is 32.6 s, which is below the yellow threshold of 35 s but 

above the red threshold of 15.0 s. This suggests that a way to improve the minimum distance is 

by using a higher value of ln_T_red. But we also notice that the track rate threshold only reaches 

0.89/s for the initial turn. This suggests that a lower value of trackRateThreshold could also 

help, but this could also increase the false alarm rate. 

Increasing the value of ln_T_red to 20 s improved the situation somewhat with a resulting 

minimum distance of 323 ft. However, the yellow alert was still 8 s ahead of the red alert. 

So next we set trackRateThreshold = 0.75, which improved the situation significantly with a 

resulting minimum distance of 364.46 ft. 

Finally, we tried (trackRateThreshold = 0.60, ln_T_red = 20, and redRunwayDist = 150) and 

we were able to improve the minimum distance to a value of 406.04 ft (397 ft horizontal, 82 ft

vertical) These values resulted in a false alarm rate of 0.13%. 

These results with the two-turn blunder model were obtained late in the study, so there was 

insufficient time to investigate the full effect of reducing the trackRateThreshold parameter. A 

value of 1.0/s was more than adequate to detect the single turn blunder used in the body of this 

paper. In future work we will explore the reduction of this parameter to adequately detect the 

more sophisticated double-turn blunder including a level-out in altitude. It may also be necessary 

to design a better track-rate filter than was used in phase I of this study. The availability of real 
61

data for aircraft approaches to parallel runways would greatly help this design. 

B.2. Double-Turn Blunder With Altitude Level-out 

We have only performed a few experiments with a double-turn blunder and an altitude levelout. The minimum distance at TCA was 212 ft, which is deep in the protection zone. The worstcase run is illustrated in Figure B-3. The circle indicates the point of closest approach and the 

orange dot indicates the point of the first red alert. 

(a) Horizontal View 

(b) Vertical View 

Figure B-3: Worst-Case Run with Double Turn and Altitude Level-out 

The altitude at TCA (43 s) was 1214 ft while it was 1088 ft at the time of the red alert at 33.5 

s. The aircraft decelerates and reaches a minimum altitude of 1071 ft. The aircraft then climbs 

back up into the altitude of the level-out blunder. In this case, it would clearly have been better to 

stay at the lower altitude for a short while. In this particularly difficult scenario, the blundering 

aircraft also manages to closely follow the escape path in the horizontal dimension. We believe 

that decision logic could be designed which would select between two different kinds of escape 

maneuvers. In particular, a low altitude escape and a fast climb escape could be chosen 

depending upon the observed characteristics of the intrusion. 
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The Simplified Aircraft-Based Paired Approach with the ALAS Alerting 

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14. ABSTRACT

This paper presents the results of an investigation of a proposed concept for closely spaced parallel runways called the Simplified 

Aircraft-based Paired Approach (SAPA). This procedure depends upon a new alerting algorithm called the Adjacent Landing Alerting 

System (ALAS). This study used both low fidelity and high fidelity simulations to validate the SAPA procedure and test the performance of 

the new alerting algorithm. The low fidelity simulation enabled a determination of minimum approach distance for the worst case over 

millions of scenarios. The high fidelity simulation enabled an accurate determination of timings and minimum approach distance in the 

presence of realistic trajectories, communication latencies, and total system error for 108 test cases. The SAPA procedure and the ALAS 

alerting algorithm were applied to the 750-ft parallel spacing (e.g., SFO 28L/28R) approach problem. With the SAPA procedure as defined 

in this paper, this study concludes that a 750-ft application does not appear to be feasible, but preliminary results for 1000-ft parallel 

runways look promising.

15. SUBJECT TERMS

algorithms; landing aids; runways; simulations

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