1. ACRP 4-09 Risk Assessment Method to Support Modification of Airfield Separation Standards
Period:
Jun 2009 to Feb 2011
Developed by:
Applied Research Associates, Inc.
Robert E. David & Associates
University of Oklahoma
ACRP 4-09 project is part of the Airport Cooperative Research Program and was initiated in June The project was finalized in February 2011 when the final report was delivered.

2. Project Panel Chair Ms. Laurie Cullen – HNTB Corporation
ACRP Staff Representatives
Ms. Marci A. Greenberger – Program Officer
Mr. Joseph J. Brown-Snell – Program Associate
Members
Mr. Gary C. Cathey - California Department of Transportation
Mr. Chad A. Gunderson - TKDA
Mr. Paul Herrera - Los Angeles World Airports
Mr. Scott McMahon - Morristown Municipal Airport
Jorge E. Panteli - MacFarland-Johnson
Liaison Representatives
Mr. John Dermody - Federal Aviation Administration
Mr. Chris Oswald - Airports Council International - North America
Christine Gerencher – Transportation Research Board
Project Panel, TRB Staff Representative and Liaisons.

3. Project Team Principal Investigator
Jim Hall – Applied Research Associates
Co-Principal Investigator
Richard Speir – Applied Research Associates
Project Manager
Manuel Ayres – Applied Research Associates
Team Members
Hamid Shirazi – Applied Research Associates
Robert E. David – RED & Associates
Yih-Ru Huang – University of Oklahoma
Regis Carvalho – Applied Research Associates
Arun Rao – Consultant
Samuel Cardoso – Applied Research Associates
Edith Arambula – Applied Research Associates
The project team was comprised by three organizations: Applied Research Associates (prime), Robert E David & Associates (RED), and University of Oklahoma (OU). Mr. Arun Rao worked as consultant to ACRP 4-09.

4. Briefing Outline Background Study Objectives Project Tasks
Rationale of Airfield Separations
Accident and Incident Data Collected
Basis of Approach Used
Risk-Based Analysis Methodology
Case Studies and Validation
Plan to Gain Industry Support
Limitations and Conclusions
This briefing will describe the project background, how the project was phased and the approach developed, describe the methodology and the results achieved.

5. Background Many airports were built before current standards were set
There is a need to increase airport and aviation capacity, and operation of larger aircraft may be required in existing airfields
In many cases there are physical and environmental restrictions to increase existing separations
Available analysis alternatives are prescriptive and not based on risk
Approximately 20% of ground (commercial aviation) accidents in the U.S. are collisions during taxiing or parking
More than 50% of fatal accidents occur during landing and takeoff operations
Many of the existing airports were built before the current standards were established or when smaller aircraft were operating. With the increase for demand and the need to increase capacity of the aviation system, larger aircraft may be required to operate at these airports. With larger ADG, airfield separations should be larger; however airports face physical and environmental restrictions to accommodate such larger aircraft. A risk-based methodology to evaluate airfield separations is a useful tool to evaluate if the separation is acceptable from a safety perspective, particularly when considering that many aviation accidents and incidents occur during landing, takeoff, taxiing and parking operations at the airport.

6. Modification of Standards (MOS) AC 150/5300-13 (FAA, 1989)
Modification to standards means any change to FAA design standards other than dimensional standards for runway safety areas.
Unique local conditions may require modification to airport design standards for a specific airport.
The request for MOS should show that the modification will provide an acceptable level of safety, economy, durability, and workmanship.
According to AC 150/ an MOS may be requested to the FAA if there are unique conditions that justify the non-standard situation, and that an acceptable level of safety can be met.

7. Study Objectives
Develop simple and easy to use methodology to evaluate risk of collisions associated with non-standard airfield separations.
Obtain quantitative assessment for decision making when standard cannot be met.
The methodology should serve as a screening tool to evaluate the feasibility of submitting to the FAA a request for Modification of Standards.
The goal of this study was to develop a simple and practical methodology to help airport operators evaluate the level of safety associated with non-standard separations to support MOS requests and to evaluate the feasibility of submitting an MOS to the FAA

8. Project Tasks Literature review and rationale of airfield separations
Collection of veer-off accident and incident data
Modification of Standards (MOS) survey
Develop proposed risk assessment methodology
Perform airport survey for selected MOS cases
Develop risk assessment methodology
Develop plan to gain industry support
Prepare project report
There were eight basic tasks in this project, as listed in the slide. The survey of airports was conducted to identify MOS cases involving airfield separations and to collect data that could be used to validate the methodology.

9. Rationale for Standards - FAA
Taxiways and Taxilanes: probability distribution of lateral deviations plus a safety buffer of 10 ft
TWY/TWY: 1.2 x WS + 10 ft (between centerlines)
TWY/OBJ: 0.7 x WS + 10 ft (axis to object)
TXL/TXL: x WS + 10 ft (between centerlines)
TXL/OBJ: x WS + 10 ft (axis to object)
Runways: probability distributions of lateral and vertical deviations during final approach and initial climb, as well as probability of veer-offs during landing and takeoff
Indication that standards were developed based on best engineering judgment and experience from WW II
The rationale used by the FAA to establish current airfield separation standards is based on lateral aircraft deviations, when the separation is associated with taxiways and taxilanes. When a runway is involved, the risk of excessive lateral and vertical deviations during the final approach and the initial climb is combined with the probability distribution of runway veer-off events. The actual separation standards were mostly based on engineering judgment rather than using actual data or risk models, and basic separations were defined based on the experience gained during WW II.

10. Rationale for Standards - ICAO
Taxiway/Taxiway and Taxiway/Object:
Wingtip Clearance = clearance (C) between the outer main gear wheel and the taxiway edge plus safety buffer (Z).
Runway/Taxiway
Distance to accommodate potential veer-offs and provide sterile area free of obstacles for aircraft executing a missed approach or balked landing maneuver.
The rationale used by ICAO is also based on lateral aircraft deviations. For taxiways and other taxiways or objects, the separation is the maximum deviation possible before the aircraft leaves the paved area plus a safety buffer that depends on the the category of the aircraft. For runways, the distance to a parallel taxiway was defined to protect from collision an aircraft veering-off the runway or executing a missed approach or balked landing. The actual separation standards were mostly based on data presented during the 8th Air Navigation Conference in 1974. 10

11. Veer-off Data Collection
Veer-off accidents and incidents occurring in several countries from 1980 to 2009
Taxiway/Taxilane veer-offs
Identified 300 incidents in straight segments of taxiways
Only 6 relevant incidents were identified in taxilanes
Identified 679 runway veer-off accidents and incidents during landing and takeoff
Aircraft veer-off data was collected for both runway and taxiway operations. The information was collected for the period ranging from 1980 to 2009. 11

12. Taxiway Veer-offs – Some Conclusions
Taxiing airplanes are at lower speeds (normal 20 knots, max 30 knots) when compared to runway operations.
The edge of the paved area is a discontinuity and the pilot is able to stop as soon as the aircraft departs the taxiway.
The model for lateral deviation can be truncated for taxiways outside the ramp area.
The collisions occurred in curves or when other aircraft and equipment were inside the taxiway/taxilane OFA.
Based on the information collected for taxiway and taxilane veer-off accidents and incidents, the following conclusions were obtained.
1 – Taxiing aircraft have smaller lateral deviations compared to runway operations. This is due to smaller taxiing speeds.
2 – In most cases during taxiing operations the pilot was able to stop the aircraft as soon as he/she felt the aircraft departed the paved area and this has also to do with the small taxiing speeds. Therefore, it can be assumed that aircraft will stop as soon the veer-off occurs
3 – The majority of collisions occured in curves or when another aircraft/equipment was located or moving inside the taxiway/taxilane Object Free Area 12

13. Taxiway Veer-offs – More Conclusions
Taxiway veer-offs in straight segments occured due to poor visibility or low surface friction (e.g. Icing conditions).
Two-part models based on frequency and location were not appropriate for the methodology.
Only two fatal accidents due to taxiway veer-offs were identified; neither was relevant to this study.
4 – The cases of taxiway veer-offs identified were related to poor visibility or low surface friction conditions
5 – There was very little information on the exact location where aircraft stopped after veering-off the taxiway; however they were all close to the pavement edge. This evidence led the research team conclude that a two-part model involving frequency and location functions was not applicable to model taxiway/taxilane veer-offs
6 – There were only two fatal accidents involving large lateral deviations on taxiways. One occurred in a curve after the aircraft departed the paved area and sank in the adjacent terrain. The second case was a pilot DUI who became airborne when taxiing and collided with a hangar. Neither case involved random lateral deviations during the taxi operation.

14. Basis of Approach Used
Probability distributions of lateral and vertical deviations during operations
Boeing/FAA Taxiway Deviation Studies at ANC and JFK (Scholz, 2003 and 2005)
Airborne risk during landing derived from Collision Risk Model (CRM) runs
Ground roll risk of veer-off derived from models developed in this project (landing and takeoff)
The risk plots incorporated into the methodology are based on the following models:
1 – Lateral deviation and collision risk models developed by Boeing/FAA presented in the studies developed by Scholz in 2003 and 2005
2 – Risk of collision based on the FAA/ICAO Collision Risk Model (CRM)
3 – Runway veer-off models for landing and takeoffs developed in this study

15. Taxiways and Taxilanes Separation Probability Distribution of Lateral Deviations
WS1 X
WS2
d = wingtip separation
The probability of wingtip collisions when two aircraft are taxiing in parallel taxiways or taxilanes depends on the lateral deviation probability distributions for both aircraft. The chance of collision is given by the probability that both aircraft deviate enough to overlap the wingtips when they cross the same location.
d = CS – (WS1 + WS2) / 2
centerline separation (CS)

16. wingtip lateral deviation probability distribution
Taxiway or Taxilane to Object Separation Probability Distribution of Lateral Deviations
aircraft semi wingspan
wingtip lateral deviation probability distribution X
obstacle
When the separation involves a taxiing aircraft in a taxiway or taxilane and an object, only one lateral deviation probability distribution is involved. The likelihood of collision is given by the orange area in the probability distribution illustrated in the figure.

17. RWY/TWY Separation Risk of collision during airborne phase Landing
Final Approach
Missed Approach
Rejected Landing
Takeoff – Initial Climb
Risk of collision during ground roll
Takeoff
When evaluating the separation between a runway and a taxiway, there are multiple risks. Basically there is the risk of collision when the aircraft operating on the runway is airborne (landing or taking off) due to lateral and vertical deviations relative to the nominal flight path. In addition, there is the risk of collision during a potential lateral runway excursion (veer-off).

18. Deviations in Airborne Phase
In the airborne phase there is a probability distribution associated with lateral and vertical deviations from the nominal flight path. The distribution is represented by the shadow area in the figure. The darker the area the higher is the probability. Therefore it is more likely that the aircraft will be close to the center of the nominal flight path. The shadow area can be viewed as a probability distribution in 3D. If the deviations are large enough so that the center of the aircraft when crossing an obstacle is within the red zone shown in the figure, a collision between the aircraft and the obstacle will occur.

19. Runway Veer-off Landing (or Takeoff) 1 2 3 x
The second possibility of collision for aircraft operating on the runway is when a veer-off occurs during landing or takeoff. The figure represents a veer-off during landing. After touchdown and during the ground roll, the pilot may loose directional control and have a runway excursion. If the veer-off lateral distance is large enough, there is a chance of collision with an aircraft located on a parallel taxiway.

20. Risk-Based Analysis Methodology
Taxiway to Taxiway or Taxilane
Taxiway to Object
Taxilane to Taxilane
Taxilane to Object
Runway to Taxiway/Taxilane/Object
Landing
Airborne phase
Ground rolling phase
Takeoff
The methodology to evaluate airfield separations is based on risk plots. The user enters the exitsing or planned separation and obtains an estimate of risk of collision. There are different sets of plots and they are selected according to the type of separation to be evaluated. When a runway is involved, it is necessary to evaluate the risk during the airborne phase of landing, as well as during the ground roll for both landing and takeoff operations.

21. Taxiway Lateral Deviation Studies
FAA/Boeing (Scholz, 2003 and 2005)
Collision risk models were developed by Boeing/FAA based on B-747 taxiway deviation studies at ANC and JFK
The objective was to evaluate the risk of collision for B operations
Data was collected during one year
In both cases, lateral deviation data was collected in straight segments with taxiway centerline lights
FAA/Boeing conducted lateral deviation studies using B-747 data collected over one year at Anchorage and JFK airports. In both cases data was collected for B-747 aircraft taxiing in straight taxiway segments with centerline lights. The studies led to the development of risk of collision models by Scholz.

22. Assumptions
Lateral deviation for smaller aircraft are similar or smaller than those of the B-747
The taxiway or taxilane centerline is conspicuous and visible to the pilot under any operational conditions
The FAA separation standards for taxiways and taxilanes are based on similar probability of aircraft departing the lane during taxiing operations
The risk estimated with the CRM is more restrictive compared to the risk under visual conditions
The following assumptions were used to develop the methodology presented in the ACRP 4-09 study. These assumptions are associated with the type and availability of data and models used to develop the risk plots.

23. ACRP 4-09 Methodology Example of Risk Plot for Taxiway/Taxiway Separation – ADG I
This is an example of risk plot included in the methodology. In this case, the risk plot is for taxiway/taxiway separation of ADG I aircraft. The user enters the centerline separation to be evaluated and the risk is estimated. For this example, for a separation of 65 ft the risk of collision is 8.0E-07 or one collision in 1.25 million operations.

24. Lateral Deviation Models for Taxilanes
Wingtip Separation
ADG - Distances in ft I II
III IV V VI
Taxiway/Object 20 26 34 44 53 62
Taxilane/Object 15 18 22 27 31 36
Ratio
0.75
0.69
0.65
0.61
0.58
Taxilane
No models were identified for lateral deviations during taxilane operations. The models for taxiways were adjusted and used in the methodology. The basis for the adjustments was the assumption that the wingtip distances used to develop the standards are related to a similar risk of collision when comparing taxiways and taxilanes. The wingtip clearance ratios between taxiways and taxilanes were used for the adjustment of each ADG.
Similar Probability
Taxiway

25. Analysis Procedure Taxiways/ Taxilanes/Objects
Identify the type of separation
Identify the ADG or aircraft types involved
Characterize the separation (between centerlines, between centerline and object, or wingtip clearance)
Identify the appropriate risk plot to use
Use the centerline or wingtip clearance to estimate risk of collision
The procedure for analysis involving taxiways, taxilanes and objects is simple and includes the steps presented in this slide. First it is necessary to characterize the type of separation (e.g. separation between a taxiway and a taxilane). The second step is to identify the Aircraft Design Group or the two types of aircraft involved. If the analysis is for a specific design group, the plots involving centerline separations can be used. Otherwise, when the analysis involves specific types of aircraft, the wingtip clearance is characterized. The next step is to identify the correct risk plot and a table is presented to facilitate the identification. Using the separation measure selected, the user can estimate the risk of collision using the risk plot selected.

26. Example - Taxiway/Taxiway Separation
The slide shows an example involving the analysis of ADG V aircraft. In this case the separation is 233 ft rather than the standard separation of 267 ft. The risk of collision is estimated to be 2.3E-08 or one collision in approximately 43 million operations. It should be noted that both centerlines should be lighted or be very conspicuous to achieve this level of risk for such small separation.

27. Risk Analysis during Landing
Airborne Phase
Ground Roll Phase
When the separation involves a runway, it is necessary to evaluate the risk of collision during the airborne phase of landing and during possible runway veer-offs (landing and takeoffs).

28. Collision Risk Model (CRM) Runs
The risk of collision plots for the airborne phase were developed based on runs using the FAA/ICAO Collision Risk Model. The CRM is a tool developed to estimate the risk of collision with obstacles during missed approaches for precision instrument approaches of categories 1 and 2. It was assumed that the risk estimated with the CRM is conservative when considering visual conditions. Several CRM runs were conducted assuming a taxiing aircraft was located at different distances along the taxiway and that the taxiway was located at different separations from the runway. Based on the risk obtained with these runs, risk plots were developed.

29. Development of Risk Curves Airborne Phase
The risk plots are based on the highest risk estimated for each taxiway separation. Again this is considered a conservative approach because it takes into consideration only the most critical location of the taxiing aircraft for each separation evaluated. The dashed line is the envelope of the most critical situation for each taxiway separation and was used to define the risk plots used in the methodology.

30. Runway Veer-off Incident Rates (U.S.) (1980-2009)
Type of Incident
Number of Incidents
Incident Rate per Operation
Incident Rate in Operations per Incident
LDVO
512
1.195E-06
837,000
TOVO
111
2.590E-07
3,861,000
Although frequency models to estimate the likelihood of runway veer-offs for specific conditions were developed in this study, the requirement for simplicity led to the use of risk rates for these types of incidents. Based on historical records and the air traffic volume from 1980 to 2009 in the United States, average rates were calculated for both landing and takeoff veer-offs, as presented in the table.

31. Location Model – Landing Veer-off
Based on the largest distance from the runway edge during the veer-off path, location models were developed. The models provide an estimate of the probability that the aircraft exceeds a given distance from the runway edge during the veer-off. The combination of veer-off rates with the respective location model provides an estimate of the likelihood that an aicraft veers-off the runway and that the lateral deviation exceeeds a given distance from the runway edge. Using the taxiway separation distance it is possible to estimate the risk of collision during the veer-off. It should be noted that this is a conservative approach, given the aircraft may veer-off at any location along the runway and a taxiing aircraft may not be in the taxiway at that specific location during the incident.

32. Analysis Procedure – Runway/Taxiway
Identify the ADG
Identify type of approach (Cat I or Cat II)
Characterize the separation between the runway and taxiway axes
Identify plots for specific ADG (landing)
Airborne phase (lateral and vertical deviations)
Ground roll phase (frequency and location)
Use axes separation to estimate risk of collision for each phase
Repeat process for takeoffs
The analysis procedure involving runways has two steps. In the first step the risk of collision during the airborne phase is estimated. In the second step the risk of collision during runway veer-offs is estimated. The risk of collision during landing veer-offs is combined with the risk of collision during the final approach. For takeoffs, only the risk of collisions during veer-offs is evaluated.

33. Risk Criteria – FAA Risk Matrix
Risk estimated is compared to risk criteria to check for acceptability
Taxiway/Taxilane/Object Separation
Criteria for
The risk estimated using the plots should be compared to existing criteria. The FAA recommends the risk matrix shown in this slide. For runway/taxiway separation, the worst credible consequences are catastrophic and the only likelihood level is “extremely improbable”, and an accident is expected to occur less than once every 100 years. For taxiway accidents, based on historical records, the worst credible consequence is major and in this case the acceptable likelihood is one accident expected to occur once every year, or once for every 2.5 million departures, whichever occurs sooner.
Criteria for
Runway/Taxiway Separation

34. Case Studies and Validation
Airp.
ADG
Type of MOS
Risk Level
Expected # Yrs
Risk < 1.0E-7
Risk < 1.0E-09
Credible Severity
FAA Risk Classification
Acceptable
PHL
III, IV
Taxilane/Taxilane
<1.0E-9
N/A
Yes
Major
Low
ANC VI
Taxiway/Object
ADS
III
Runway/Taxiway
1.0E-7
> 100 No
Catastrophic
Medium
BDR II
1.1E-7
MFV
Runway/Object
5.9E-8
N07 I
Taxilane/Object
1.2E-9 -
JFK
Taxiway/Taxiway
EWR V
MSP IV
ORD
HYA
8.8E-8
LCI
2.0E-7
SEA
1.6E-6
High*
No*
Taxiway/Taxilane
ASE
9.0E-8
ACK
ILG
2.8E-8
JYO
1.2E-7
TAN
8.0E-8
Several MOS cases involving airfield separations and approved by the FAA were evaluated and 20 cases were selected to validate the methodology. The purpose was to compare the outcome of the analysis with the FAA decision to approve the non-standard separation. For only one case the risk estimated with the methodology was not acceptable; however for the specific approval the FAA established additional restrictions to maintain an acceptable level of safety. The risk criteria used to compare runway/taxiway separations was 1.0E-09 (one accident in one billion ops). For cases not involving runways, the risk criteria used was one collision in ten million operations.

35. Plan to Gain Industry Support
Research Product
Risk assessment methodology to evaluate airfield separations and intended to serve as a screening tool to support the submittal of MOS for FAA approval
Audience
Civil aviation agencies like the FAA, ICAO, military aviation organizations, and civil aviation stakeholders
Main obstacle for implementation
Will require FAA support
Implementation
Actions to present the product in airport conferences and aviation safety meetings (TRB, AAAE, ACC, ACI)
Presentation to the FAA Office of Airports
A plan to gain industry support was also prepared. The plan includes presentations and workshops when the methodology will be described and examples discussed. The objective is to make the industry aware of the methodology to evaluate airfield separations.

36. Limitations
Can only be used to assess risk for straight parallel segments of taxiways and taxilanes.
Taxiway deviations for smaller aircraft were assumed to be equal or smaller than deviations for the Boeing 747 aircraft.
Application of the models for taxiway and taxilane deviations assume the centerline is conspicuous under any weather and light conditions.
Veer-off models were developed based on incidents and accidents of aircraft with MTOW larger than 5,600 lbs.
Assumed the lateral and vertical deviation probability distributions provided by the Collision Risk Model is conservative when considering visual conditions.
There are some limitations associated with the application of the methodology.
1 – It can only be applied to straight parallel segments of airfields. It is not possible to apply the procedure to taxiway or taxilane curves as no models or data were available to use.
2 – Only lateral deviation data for B-747 was collected during the studies at ANC and JFK. It was assumed that smaller aircraft have similar or smaller deviations when compared to those for the B-747.
3 – Taxiway deviation data was collected for segments with centerline lights. This feature provides an extra aid to the pilot to maintain the aircraft near the centerline, even with poor weather, light and visibility conditions.
4 – Historical accident and incident data was collected only for aircraft with MTOW larger than 5,600 lbs and it is assumed that the models are conservative when applying to smaller aircraft.
5 – The airborne risk of collision was estimated using the CRM; however the model is applicable to precision approaches of categories 1 and 2. It was assumed that the risk estimated with the CRM is conservative when compared to the risk for visual or non-precision approach conditions.

37. Conclusions
The methodology developed in this research study provides a practical and simple guide to help airports quantify and evaluate risk associated with non-standard airfield separations.
The risk assessment obtained can be helpful to examine the feasibility of and to support MOS requests to the FAA.
The methodology is based on lateral and vertical deviation studies and models developed in this research as well as in previous studies conducted by the FAA, Boeing, and ICAO.
The methodology was validated using twenty MOS cases approved by the FAA.
The methodology developed is very simple and practical and should help airport operators evaluate the feasibility of submitting an MOS request to the FAA. The methodology is based on the rationale to develop airfield separation standards and uses existing or newly developed lateral and vertical deviation models to estimate the risk of collision. The study helped define recommended risk acceptability criteria and it was validated using MOS cases approved by the FAA.