COMFAA 3.0 Beta

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

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.

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.

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

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.

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 2007. 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.

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.

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)

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)

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?

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.

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.

AC 150/5335-5A (2006) Standardized Method of Reporting Airport Pavement Strength – PCN Complete rewrite of AC 150/5335-5. 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 D6-82203 “Precise Methods for Estimating Pavement Classification Number,” 1998. D6-82203 is, in turn, based largely on recommendations contained in the ICAO Aerodrome Design Manual.

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.

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.

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.

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.

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.

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.

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.

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

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.

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.

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.

Using the COMFAA 3.0 Program

COMFAA Input

Aircraft Window – COMFAA Input

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

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

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

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.

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.

COMFAA Output – Details (III) CDF method identifies (3) aircraft that contribute substantial structural damage based on pavement structure: 727-200, 747-400, and A300-B4. PCN based on technical CDF method can be reported as the highest PCN of these aircraft = 73.

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

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.

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.

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.

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

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 B717-200 122000 57950 28975 301 1701 A318-100 std 124500 59138 29569 654 3593 A319-100 std 142500 67688 33844 13,002 70663 A320-100 151000 71725 35863 15,280 79583 MD-80/83/88 161000 76475 38238 739 4322 MD-90 168500 80038 40019 213 1283 B727-200 basic 185200 87970 43985 111 760 B737-700 188200 89395 44698 18,133 96709 B757-300 2D 271000 128725 32181 10,079 51555 DC-8-60/70 358000 170050 42513 79 472 A300-b4 std 365750 173731 43433 831 4579 B767-300 413000 196175 49044 2,521 14006 DC-10 458000 217550 54388 115 622 B787-8 478000 227050 56763 32 169 A330-200 std 509000 241775 60444 88 936 A340-300 std 608000 243200 60800 179 1884 MD-11 633000 245288 61322 240 B747-400 877000 208288 52072 754 4358 A380 (2D) 1235000 469300 117325 59 309 B777-200 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 B737-700 model. This artificially creates a high annual depature for this particular airplane. We also speculate that the maximum gross weight for the B737-700 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.

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 17.00. 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.

-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 B717-200 122000 301 9.66 8.65 A318-100 std 124500 654 9.23 8.18 A319-100 std 142500 13,002 12.17 10.14 A320-100 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 B727-200 basic 185200 111 10.92 9.81 B737-700 188200 18,133 15.47 12.71 B757-300 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 B767-300 413000 2,521 11.62 11.26 DC-10 458000 115 9.52 9.21 B787-8 478000 32 10.57 10.1 A330-200 std 509000 88 10.84 10.28 A340-300 std 608000 179 10.99 10.45 MD-11 633000 44 10.65 B747-400 877000 754 11.17 11.07 A380 (2D) 1235000 59 10.60 10.13 B777-200 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 B737-700 appears to be the “design” airplane based on 5335-5A procedures. B737-700 is comprised of several D gear airplanes. Based on the procedure in 5320-6D, the B737-700 at 18,133 EAD, requires the largest pavement section and therefore becomes the “design” airplane. The PCNwg questioned whether the B737-700 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.

CASE 1 – Current Procedure Following 5335-5A procedures the B737-700 would become the “Critical Aircraft” due to its individual pavement thickness requirement. This is true for either the 5320-6D 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.

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 A318-100 std 181000 345027 46.1 B717-200 158900 386168 50.1 A319-100 std 189250 168314 52.1 A320-100 189500 125753 55.2 B737-700 201400 45167 59.8 MD-80/83/88 184000 92089 60.1 MD-90 189100 74317 61.4 B727-200 basic 222200 48463 65.7 B757-300 2D 385300 130968 68.7 A300-b4 std 481000 32997 84.9 DC-8-60/70 457500 36135 85.6 B767-300 553000 20131 87.3 A330-200 std 672000 9296 89.3 A340-300 std 806000 10907 90.1 A380 (2D) 1568000 20322 92.0 B747-400 1148000 15972 92.8 DC-10 653600 13572 93.0 B787-8 608500 11612 96.4 MD-11 820500 9462 100.6 B777-200 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 B737-700 is comprised of several D gear airplanes, some with lower operating weights. By applying the procedures outlined in 5335-5A, the PCN of the pavement would be based upon the ACN of the B737-700. EAD’s are calculated using the gear factors and the LOG-LOG equation for departure equivalents.

CASE 1 ACN values for each airplane (from COMFAA) Gear type for ACN Operating Weight Airplane ACN/R/B A318-100 std D 124500 29.5 DC-9-30/40 109000 31.3 B717-200 122000 36.5 A319-100 std 142500 37.2 B757-300 2D 271000 41.4 A320-100 151000 42.2 B777-200 baseline 3D 537000 47.4 MD-80/83/88 161000 51.3 B727-200 basic 185200 52.7 MD-90 168500 53.5 B737-700 188200 55.2 DC-10 458000 56.4 A300-b4 std 365750 57.3 B767-300 413000 57.4 DC-8-60/70 358000 60 A330-200 std 509000 61.2 A340-300 std 608000 61.4 B747-400 877000 63 A380 (2D) 1235000 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 B737-700 as the design airplane per the procedures in 5335-5A 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 5335-5A, the B737-700 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

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.

COMFAA 3.0 – Case 1

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.

COMFAA 3.0 – Aircraft Window

COMFAA 3.0 – Case 1 Results Subgrade Category B

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

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.

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.

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.

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

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

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.

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

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

More Information Contact Dr. Gordon Hayhoe Gordon.Hayhoe@faa.gov FAA Airport Technology R&D Group www.airporttech.tc.faa.gov Click on “Downloads” then “Pavements”

Questions?