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Presentation Transcript

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 ManualPart 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)

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.

AC 150/5335-5B …PCN

Gross Weight data may transition.

- The Master Record is required to be updated periodically.
- PCN is now mandatory and Gross Weight data will possibly be phased out with time.

PCN data request now part of all airport inspections

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 SpreadsheetFlexible 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.

PAVEMENT TO BE EVALUATED IN COMFAA

COMFAA Support SpreadsheetFlexible Pavement Output- 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.

Click “CBR” – Enter 9.0 in the dialog box.

Enter the evaluation thickness = 33.8 in.

Main Window - COMFAA InputPCN based on using aircraft ACN can be reported as 54.

COMFAA generates an aircraft ACN table. CBR 9 indicates B subgrade designation.

COMFAA Output – Details (I)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 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 SpreadsheetFlexible Charts

Thickness Comparison

- Compare thickness and gross weights.
- When CDF thickness (yellow) is less than evaluation thickness (red), excess PCN is available.

COMFAA Support SpreadsheetFlexible 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.

CASE 1

Traffic Data

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

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.

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.

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.

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.

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/BPCN = (60/R/B)

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 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 – Case 1 Results

Copy and Paste Data into Support Spreadsheet

COMFAA Support SpreadsheetCase 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 SpreadsheetRigid 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 SpreadsheetRigid 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 SpreadsheetRigid Charts – Case 1

Form 5010

- Enter PCN Number.
- PCN can be reported as:96/R/B/W/T

Case 1 – PCN ComparisonPCN calculated assuming each airplane is treated as the design airplane

A380

B747

B777

B757

COMFAA 3.0 – Options

- Batch
- 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

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”

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