1. COMFAA 3.0 Beta

2. Acknowledgments Gordon Hayhoe, Rodney Joel and Jeff Rapol, FAA.
Ken DeBord and Mike Roginski, Boeing Commercial Airplane Co.

3. Outline Brief review of ACN/PCN system and ICAO definitions.
FAA guidance on PCN calculation.
Draft AC 150/5335-5B and computer programs (COMFAA 3.0 and new support program).
Flexible example using COMFAA 3.0.
Rigid example using COMFAA 3.0 for a large hub airport.
Using COMFAA 3.0 additional features.

4. The ACN/PCN System - General
Aircraft Classification Number (ACN) is specified as a standard by ICAO in Annex 14 to the Convention on International Civil Aviation.
Aircraft manufacturers are required to publish properly computed ACN values for all of their aircraft.
Pavement Classification Number (PCN) procedures are given in the ICAO Aerodrome Design Manual, Part 3, Pavements.
The PCN procedures in the manual are for guidance only and a great deal of latitude is provided.
Airport operators are responsible for determining and publishing PCN values for runways.

5. ICAO Documents
Annex 14 to the Convention on International Civil Aviation
ICAO Aerodrome Design Manual Part 3 – Pavements

6. ACN/PCN System - Definitions
ACN – A number expressing the relative effect of an aircraft on a pavement for a specified standard subgrade strength.
PCN - A number expressing the bearing strength of a pavement for unrestricted operations.
Therefore, if a particular aircraft at a given weight has an ACN less than, or equal to, the PCN of a particular pavement (ACN  PCN), then no restrictions need to be placed on operation of that aircraft on that pavement.
Special provisions for overload evaluation.

7. ACN Computation
ACN is computed as the ratio of a computed (derived) single-wheel load to a reference single-wheel load.
Flexible: Based on the US Army Corps of Engineers ESWL CBR method of design using alpha factors adopted by ICAO October Thickness is computed for 10,000 coverages.
Rigid: PCA Westergaard interior stress method of design. Thickness is computed for 10,000 coverages.
These are fixed, standard procedures. Other design procedures or traffic levels cannot be substituted.

8. Subgrade Strength for ACN Computation
Flexible: The CBR of the subgrade soil.
Rigid: The k-value at the top of the support, including all subbase layers.
It is not the same as the k-value of the subgrade soil.

9. PCN Reporting Format
PCN values are reported in a coded format using 5 parts separated by “/”
Sample 39/F/B/X/T
Information includes:
Numerical PCN Value (39 in this example)
Pavement Type (F = Flexible, R = Rigid)
Subgrade Strength Category (A, B, C or D)
Allowable Tire Pressure (X  1.5 MPa = 218 psi)
PCN Evaluation Method (U = Using, T = Technical)

10. Proposed Change to ICAO PCN Tire Pressure Limits (Flexible Only)
Tire Pressure Category
Current ICAO Designations and Limits
Proposed ICAO Designations and Limits W
High: no pressure limit
Unlimited X
Medium: pressure limited to 1.50 MPa (217.6 psi)
High: pressure limited to 1.75 MPa (253.8 psi) Y
Low: pressure limited to
1.00 MPa (145.0 psi)
Medium: pressure limited to 1.25 MPa (181.3 psi) Z
Very low: pressure limited to 0.50 MPA (72.5 psi)
0.50 MPA (72.5 psi)

11. PCN – Using Aircraft Method
Find the ACN of all of the aircraft regularly using the pavement and pick the largest ACN to be the PCN of the pavement.
But see page 3-27 of the ICAO manual:
“Support of a particularly heavy load, but only rarely, does not necessarily establish a capability to support equivalent loads on a regular repetitive basis.”
Where is the line between “regular” and overload operation?

12. PCN – Technical Method
The ICAO manual covers in detail a very broad range of methods, including:
Any rational design procedure developed specifically for airport pavements but applied in reverse for pavement evaluation.
Pavement surface deflection measured under the load from a representative aircraft.
Non-destructive test results with backcalculation.
Allows for design and evaluation procedures not in use when the manual was written.

13. FAA Guidance on PCN Calculation
The FAA is responsible for certifying all commercial airports in the U.S. and is the organization generally responsible for complying with international agreements on aviation.
Well defined procedures are therefore required for determining and publishing PCN values for runways at all commercial airports in the U.S.

14. AC 150/5335-5A (2006) Standardized Method of Reporting Airport Pavement Strength – PCN
Complete rewrite of AC 150/
Standardized the procedures for computing and reporting PCN values for inclusion in the 5010 database.
AC 150/5335-5A is based in large part on the procedures described in Boeing Report D “Precise Methods for Estimating Pavement Classification Number,” 1998.
D is, in turn, based largely on recommendations contained in the ICAO Aerodrome Design Manual.

15. Draft Advisory Circular 150/5335-5B
AC 150/5335-5A, PCN, to be replaced by AC 150/5335-5B.
Draft AC was posted for comment October 2009.
Comment period has ended.
Copy of draft AC is included on the CD.

16. AC 150/5335-5B STANDARDIZED METHOD OF REPORTING AIRPORT PAVEMENT STRENGTH - PCN
The Pavement Classification Number (PCN) field has been added to FAA Form 5010 and data collection is underway.
During each airport inspection, the airport owner will be asked to provide runway PCN information.
WHY?
With release of AC 150/5320-6E, the “design aircraft” concept has been replaced.
This means the Aircraft Gross Weight fields on the 5010 will no longer be used to describe load carrying capacity of runways.

17. PCN data request now part of all airport inspections
AC 150/5335-5B …PCN
Gross Weight data may transition.
PCN data request now part of all airport inspections
The Master Record is required to be updated periodically.
PCN is now mandatory and Gross Weight data will possibly be phased out with time.

18. Draft AC 150/5335-5B - Changes from AC 150/5335-5A
The procedure for selecting the critical aircraft has been substantially revised.
The procedure for computing equivalent departures has been replaced by a new procedure based on cumulative damage factor (CDF).
Except for obtaining the structure and aircraft properties, the procedure has been completely automated in a revised version of the computer program COMFAA (COMFAA 3.0).
A spreadsheet application has been developed to facilitate determining the evaluation thickness.

19. Draft AC 150/5335-5B
The design procedures recommended in the new AC are:
CBR ESWL with the new alpha factors for flexible pavements.
Edge stress Westergaard as implemented in AC 150/5320-6C and -6D.
The PCA center stress method can also be selected in COMFAA 3.0.
These were selected for backward compatibility with established methods and compatibility with the ACN computation procedure.

20. New PCN Methodology
The current methodology (-5A) finds the critical aircraft and then finds the ACN of that aircraft at the maximum allowable gross weight. That ACN is then the PCN.
The new methodology is the same except that the ACN at maximum allowable gross weight is calculated for all of the aircraft in the mix. The CDF procedure is used for equivalent coverages.
The largest ACN value is then selected as the PCN.
There is a need to eliminate “occasional or overload” aircraft from the mix.

21. ACN-PCN – Technical Evaluation
7 Basic Steps to Determine Pavement Classification Number in AC 150/5335-6B:
Identify pavement features and properties.
Determine traffic mixture.
Convert traffic to equivalent traffic of “critical” aircraft.
Determine allowable operating weight of critical airplane.
Determine ACN of critical airplane at allowable weight.
Repeat with each airplane the critical airplane.
Report PCN.

22. AC 150/5335-5B Computer Programs
COMFAA 3.0 Program
Support Spreadsheet for COMFAA (Excel)
Input Support:
Flexible Layer Equivalency Worksheet
Rigid Pavement k-Value Worksheet
Output Support:
Output Data Parsing
Rigid and Flexible Chart Creation
FAA Form 5010 Preparation

23. COMFAA Support Spreadsheet
Flexible Layer Equivalency
The equivalent pavement has three layers:
5 in. P-401,
8 in. P-209,
P-154.
The spreadsheet determines the maximum thickness for the equivalent pavement based on the user-defined layer equivalency factors.

24. COMFAA Support Spreadsheet Flexible Pavement Input
ENTER (or confirm) layer equivalency factors.
Refer to Table A2-1.
ENTER all existing pavement layers starting at the surface of the pavement.
ENTER the subgrade CBR value.

25. COMFAA Support Spreadsheet Flexible Pavement Output
PAVEMENT TO BE EVALUATED IN COMFAA
The spreadsheet determines and consolidates COMFAA software input values and recommends three of five necessary PCN codes.
The spreadsheet updates the graphical representation of the existing and equivalent pavement.

26. Using the COMFAA 3.0 Program

27. COMFAA Input

28. Aircraft Window – COMFAA Input

29. Main Window - COMFAA Input
Click “PCN Flexible Batch”
Click “CBR” – Enter 9.0 in the dialog box.
Enter the evaluation thickness = 33.8 in.

30. COMFAA Output Click “Details” to view the detailed output.
Message “Flexible Computation Finished”

31. COMFAA Detailed Output Screen
Summary Aircraft Table with -6D Thickness Requirements

32. COMFAA Output – Details (I)
COMFAA generates an aircraft ACN table. CBR 9 indicates B subgrade designation.
PCN based on using aircraft ACN can be reported as 54.

33. COMFAA Output – Details (II)
COMFAA generates a table based on the CDF method. For each aircraft, allowable gross weight and corresponding PCN are identified.
CBR 9 indicates B subgrade designation.

34. COMFAA Output – Details (III)
CDF method identifies (3) aircraft that contribute substantial structural damage based on pavement structure: , , and A300-B4.
PCN based on technical CDF method can be reported as the highest PCN of these aircraft = 73.

35. COMFAA Detailed Output Screen
Copy and paste data into COMFAA support spreadsheet.

36. COMFAA Support Spreadsheet
Data Parse
Copy/Paste output data into Cell B5
Click “Create Flexible Pavement Charts”
Airplanes are ordered by PCN number, with the aircraft at top giving the highest PCN when treated as critical.

37. COMFAA Support Spreadsheet Flexible Charts
Thickness Comparison
Compare thickness and gross weights.
When CDF thickness (yellow) is less than evaluation thickness (red), excess PCN is available.

38. COMFAA Support Spreadsheet Flexible Charts
PCN Comparison
PCN needed for using traffic is 54.
PCN based on CDF analysis can be reported as high as 73.

39. Technical Method Example – Case 1
International hub airport.
Rigid pavement.
Large number of narrow-body, dual-wheel-gear aircraft.
Example courtesy of Rodney Joel.

40. AIRCRAFT
ACN gear
Operating Weight, lb
Gear Load. lbs.
Wheel Load, lbs.
Avg Annual Departures
Total Coverages 20 yrs
DC-9-30/40 D
109000
51775
25888 8 44 B
122000
57950
28975
301
1701
A std
124500
59138
29569
654
3593
A std
142500
67688
33844
13,002
70663 A
151000
71725
35863
15,280
79583
MD-80/83/88
161000
76475
38238
739
4322
MD-90
168500
80038
40019
213
1283
B basic
185200
87970
43985
111
760 B
188200
89395
44698
18,133
96709 B 2D
271000
128725
32181
10,079
51555
DC-8-60/70
358000
170050
42513 79
472
A300-b4 std
365750
173731
43433
831
4579 B
413000
196175
49044
2,521
14006
DC-10
458000
217550
54388
115
622
B787-8
478000
227050
56763 32
169
A std
509000
241775
60444 88
936
A std
608000
243200
60800
179
1884
MD-11
633000
245288
61322
240 B
877000
208288
52072
754
4358
A380 (2D)
469300
117325 59
309
B base 3D
537000
255075
1,095
5153
CASE 1
Traffic Data
This is the original traffic data provided by Denver. Based upon the design report, several dual wheel gear aircraft were consolidated into the B model. This artificially creates a high annual depature for this particular airplane. We also speculate that the maximum gross weight for the B is considerably higher than many of the models used to generate the annual departure level. This increases the conservatism of this particular design and may force the B737 to become the design airplane rather than some of the other models with relatively high departure levels.

41. Effective k at top of base = 323 psi/in
CASE 1
Existing Pavement Data
Effective k at top of base = 323 psi/in
17″ PCC, R = 775 psi
8″ CTB
k-value = 160 psi/in
Pavement design parameters as reported. Note that the original design called for 16.3 inches and was rounded up to
The original design based on procedures in AC 150/5320-6D called for 16.3 inches of PCC. This value was rounded to 17.0 inches.

42. -6D Edge Load Thickness (in) PCA Design Thickness (in)
AIRCRAFT
ACN gear
Operating Weight (lb)
Annual Departures
-6D Edge Load Thickness (in)
PCA Design Thickness (in)
DC-9-30/40 D
109000 8
8.04
7.24 B
122000
301
9.66
8.65
A std
124500
654
9.23
8.18
A std
142500
13,002
12.17
10.14 A
151000
15,280
12.98
10.73
MD-80/83/88
161000
739
11.67
10.46
MD-90
168500
213
11.49
10.27
B basic
185200
111
10.92
9.81 B
188200
18,133
15.47
12.71 B 2D
271000
10,079
10.85
10.31
DC-8-60/70
358000 79
10.05
9.72
A300-b4 std
365750
831
10.78
10.62 B
413000
2,521
11.62
11.26
DC-10
458000
115
9.52
9.21
B787-8
478000 32
10.57
10.1
A std
509000 88
10.84
10.28
A std
608000
179
10.99
10.45
MD-11
633000 44
10.65 B
877000
754
11.17
11.07
A380 (2D) 59
10.60
10.13
B base 3D
537000
1,095
8.85
9.46
CASE 1
Pavement thickness requirements based on actual annual departures:
FAA AC 150/5320-6D
PCA method
B appears to be the “design” airplane based on A procedures.
B is comprised of several D gear airplanes.
Based on the procedure in D, the B at 18,133 EAD, requires the largest pavement section and therefore becomes the “design” airplane. The PCNwg questioned whether the B should be the design airplane, however, none of the 2D gear airplanes generate much pavement thickness requirement. Even if the artificial EAD for the B737 are reduced, it still generate a considerable thickness requirement. We have to reduce the EAD to below 1,100 for the A320 to take over as the design airplane (~13 inches). There was considerable discussion about whether a 3D gear airplane should be evaluated for design rather than a D gear airplane. If the PCA design method is employed (center slab rather than edge load) then the 2D gear airplanes tend to have greater impact on the design. However, in this case the B737 remained as the critical airplane followed by the A320.

43. CASE 1 – Current Procedure
Following A procedures the B would become the “Critical Aircraft” due to its individual pavement thickness requirement.
This is true for either the D procedure or the PCA center slab procedure.
It was speculated that the B737 was artificially elevated as the design airplane due to high departure levels caused by lumping several D-gear airplanes into one.

44. Allowable Operating Weight
CASE 1
AIRPLANE
ACN gear
Allowable Operating Weight
Equiv.
Annual Departures
5335-5A PCN/R/B
DC-9-30/40 D
148600
739535
45.4
A std
181000
345027
46.1 B
158900
386168
50.1
A std
189250
168314
52.1 A
189500
125753
55.2 B
201400
45167
59.8
MD-80/83/88
184000
92089
60.1
MD-90
189100
74317
61.4
B basic
222200
48463
65.7 B 2D
385300
130968
68.7
A300-b4 std
481000
32997
84.9
DC-8-60/70
457500
36135
85.6 B
553000
20131
87.3
A std
672000
9296
89.3
A std
806000
10907
90.1
A380 (2D)
20322
92.0 B
15972
92.8
DC-10
653600
13572
93.0
B787-8
608500
11612
96.4
MD-11
820500
9462
100.6
B base 3D
920500
23190
115.2
PCN values calculated by following the procedures in AC 150/5335-5A and assuming that each airplane is the “design airplane.”
In this mix, the B is comprised of several D gear airplanes, some with lower operating weights.
By applying the procedures outlined in A, the PCN of the pavement would be based upon the ACN of the B EAD’s are calculated using the gear factors and the LOG-LOG equation for departure equivalents.

45. CASE 1 ACN values for each airplane (from COMFAA)
Gear type for ACN
Operating Weight
Airplane ACN/R/B
A std D
124500
29.5
DC-9-30/40
109000
31.3 B
122000
36.5
A std
142500
37.2 B 2D
271000
41.4 A
151000
42.2
B baseline 3D
537000
47.4
MD-80/83/88
161000
51.3
B basic
185200
52.7
MD-90
168500
53.5 B
188200
55.2
DC-10
458000
56.4
A300-b4 std
365750
57.3 B
413000
57.4
DC-8-60/70
358000 60
A std
509000
61.2
A std
608000
61.4 B
877000 63
A380 (2D)
65.8
B787-8
478000
68.3
MD-11
633000
69.6
CASE 1
Airplanes can operate without restriction
ACN values for each airplane (from COMFAA)
Assuming the B as the design airplane per the procedures in A
Calculated PCN 59.8/R/B PCN = (60/R/B)
The current issue is defined by this slide. If we follow the current guidance in A, the B should be used to generate the PCN. The calculate value of ~60/R/B will require that several of the airplanes in the traffic mixture which generated the pavement, would require restricted operations (reduced weight). This identifies a disconnect between the design procedure and the PCN process. Keep in mind that the pavement section was rounded up and additional 0.7 inches and should represent a stronger than necessary pavement section for the given traffic.
Weight Restrictions

46. Case 1 – New CDF PCN Procedure
The procedure for finding equivalent coverages in AC 150/5335-5A is based on gear equivalency factors and the ratio of wheel loads.
An alternative procedure based on cumulative damage factors (CDF) gave more consistent, and rational, results.
The new procedure for finding equivalent coverages has been incorporated in the new AC 150/5335-5B.

47. COMFAA 3.0 – Case 1

48. COMFAA Support Spreadsheet
Rigid Pavement k-Value
Each subbase layer contributes to an improved subgrade support k-value.
ENTER all existing pavement layers.
ENTER the flexural strength of the concrete and the subgrade support k-value.
The spreadsheet updates the graphical representation of the existing and equivalent pavement.

49. COMFAA 3.0 – Aircraft Window

50. COMFAA 3.0 – Case 1 Results
Subgrade Category B

51. COMFAA 3.0 – Case 1 Results
Copy and Paste Data into Support Spreadsheet

52. COMFAA Support Spreadsheet Case 1
Data Parse
Copy/Paste output data into Cell B5
Click “Create Rigid Pavement Charts”
Airplanes are ordered by PCN number, with the aircraft at top giving the highest PCN when treated as critical.
Next, click Rigid Chart tab.

53. COMFAA Support Spreadsheet Rigid Charts – Case 1
Thickness Comparison
Compare thickness and gross weights.
When CDF thickness (yellow) is less than evaluation thickness (red), excess PCN is available.

54. COMFAA Support Spreadsheet Rigid Charts – Case 1
PCN Comparison
6 most demanding aircraft in mix.
PCN needed for using traffic is 70.
PCN based on CDF analysis can be reported as high as 96.
Next, click on Form 5010 tab.

55. COMFAA Support Spreadsheet Rigid Charts – Case 1
Form 5010
Enter PCN Number.
PCN can be reported as: 96/R/B/W/T

56. Case 1 – PCN Comparison PCN calculated assuming each airplane is treated as the design airplane
B747
B777
B757

57. COMFAA 3.0 – Options Batch Metric PCA Thick PCA GW
Runs all airplanes in succession.
Automatically invoked by PCN Flex and Rigid Batch function.
Metric
PCA Thick
Equivalent coverages computed with the PCA interior stress design method.
PCA GW
Maximum gross weight computed with the PCA interior stress design method.

58. COMFAA 3.0 – Computational Modes
ACN – Computes ACN at indicated gross weight & strength.
PCN – Computes PCN using CDF-based procedure.
Thickness – Computes by AC 150/5320-6D procedure.
MGW – Computes maximum gross weight for airplane.
Life – Computes coverages to failure for indicated thickness.
Interior Stress
Edge Stress

59. COMFAA 2.0 Will Continue to be Supported – Simpler and Old Alphas

60. More Information Contact Dr. Gordon Hayhoe
FAA Airport Technology R&D Group
Click on “Downloads” then “Pavements”

61. Questions?