COMFAAPresented to: 34th Annual Airport Conference By: David R. Brill, P.E., Ph.D. Date: March 2, 2011
COMFAA 3.0 Beta
Ken DeBord and Mike Roginski, Boeing Commercial Airplane Co.
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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.
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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.
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ICAO Aerodrome Design Manual
COMFAA 3.0 Beta
March 2, 2011
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.
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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.
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Flexible: The CBR of the subgrade soil.
Rigid: The k-value at the top of the support, including all subbase layers.
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COMFAA 3.0 Beta
March 2, 2011
PCN Reporting Format
PCN values are reported in a coded format using 5 parts separated by “/”
Sample 39/F/B/X/T
Information includes:
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)
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Proposed Change to ICAO PCN Tire Pressure Limits (Flexible Only)
Tire Pressure Category
W
Y
Z
Low: pressure limited to 0.50 MPA (72.5 psi)
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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?
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COMFAA 3.0 Beta
March 2, 2011
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.
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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.
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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.
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Draft AC was posted for comment October 2009.
Comment period has ended.
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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.
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The Master Record is required to be updated periodically.
PCN is now mandatory and Gross Weight data will possibly be phased out with time.
Gross Weight data may transition.
PCN data request now part of all airport inspections
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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).
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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.
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COMFAA 3.0 Beta
March 2, 2011
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.
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COMFAA 3.0 Beta
March 2, 2011
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.
Determine allowable operating weight of critical airplane.
Determine ACN of critical airplane at allowable weight.
Repeat with each airplane the critical airplane.
Report PCN.
Input Support:
FAA Form 5010 Preparation
5 in. P-401,
8 in. P-209,
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Refer to Table A2-1.
ENTER all existing pavement layers starting at the surface of the pavement.
ENTER the subgrade CBR value.
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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.
PAVEMENT TO BE EVALUATED IN COMFAA
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Enter the evaluation thickness = 33.8 in.
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Message “Flexible Computation Finished”
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COMFAA Output – Details (I)
PCN based on using aircraft ACN can be reported as 54.
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CBR 9 indicates B subgrade designation.
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CDF method identifies (3) aircraft that contribute substantial structural damage based on pavement structure: 727-200, 747-400, and A300-B4.
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Click “Create Flexible Pavement Charts”
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Compare thickness and gross weights.
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PCN needed for using traffic is 54.
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International hub airport.
Example courtesy of Rodney Joel.
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3D
537000
255075
42513
1,095
5153
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8″ CTB
Effective k at top of base = 323 psi/in
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.
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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.
AIRCRAFT
PCA Design Thickness (in)
3D
537000
1,095
8.85
9.46
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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.
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CASE 1
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.
AIRPLANE
3D
920500
23190
115.2
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(from COMFAA)
Assuming the B737-700 as the design airplane per the procedures in 5335-5A
Calculated PCN 59.8/R/B
Weight Restrictions
2D
1235000
65.8
B787-8
2D
478000
68.3
MD-11
2D
633000
69.6
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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.
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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.
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Subgrade Category B
Copy and Paste Data into Support Spreadsheet
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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.
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Compare thickness and gross weights.
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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.
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96/R/B/W/T
Case 1 – PCN Comparison
PCN calculated assuming each airplane is treated as the design airplane
A380
B747
B777
B757
Automatically invoked by PCN Flex and Rigid Batch function.
Metric
Equivalent coverages computed with the PCA interior stress design method.
PCA GW
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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 3.0 Beta
March 2, 2011
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Existing
Layer
Thickness
1.4
1.5
1.6
1.8
1.9
2.0
1.2
1.3
1.4
1.3
1.4
1.5
1.6
1.7
1.8
1.6
1.7
1.8
1.0
1.1
1.2
1.2
1.3
1.4
1.3
1.4
1.5
Evaluation CBR = 9.0
Recommended PCN Codes: F/B/X
Rigid Pavement Thickness
1. 6D thickness at traffic mix
GW
25.7
25.7
25.5
27.5
28.8
23.1
GW
33.2
31.4
31.9
29.5
29.3
27.5
mix
370,000
330,000
600,000
370,000
820,000
185,000
767-200
ER
DC8-63
777-200
A300-B4
747-400
727-200
Rigid Pavement Thickness
1. 6D thickness at traffic mix
GW
11.1
10.8
11.1
10.6
10.5
10.6
15.0
15.0
15.0
15.0
14.9
14.9
877,000
509,000
608,000
1,235,000
478,000
633,000
B747-400
A330-200
std
A340-300
std
A380-Wing
B787-8
(Preliminar
y)
MD11
Subgrade code= B323.0, t= 17.0
0
100
200
300
400
500
600
700
800
GW
63.0
61.2
61.4
65.8
68.3
69.6
max. GW
PCN or ACN
Subgrade code= B at CBR= 9.0, t= 33.8
0
500
1,000
1,500
2,000
2,500
3,000
3,500
GW
45.2
48.8
51.3
52.4
53.9
48.2
max. GW
FLEXIBLE PAVEMENT MAXIMUM TIRE PRESSURE METHOD USED TO DETERMINE PCN
RIGID PAVEMENT
FAA Form
5010 Data
96
2D OR 3D (dual tandem OR triple tandem wheel gear)
W/B (tandem gear under wing AND tandem gear under body)
A Subgrade Category (CBR 15)
B Subgrade Category (CBR 10)
C Subgrade Category (CBR 6)
D Subgrade Category (CBR 3)
A Subgrade Category (k 552 pci)
B Subgrade Category (k 295 pci)
C Subgrade Category (k 147 pci)
D Subgrade Category (k 74 pci)
Using Aircraft