1. ICAO-UPDATE, ACN/PCN Federal Aviation Administration
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Federal Aviation Administration
Airport Pavement Working Group Meeting
ICAO-UPDATE, ACN/PCN
Presented by
Cyril FABRE / Head of Airfield Pavement

2. Contents The ACN calculation method: As it is
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Contents
The ACN calculation method: As it is
The ACN calculation method based on MLEA techniques
Advantages
Ways for implementation
Impact on PCN
Further work under ICAO-AOSWG-PSG task

3. THE AIRCRAFT CLASSIFICATION NUMBER(80’s)
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THE AIRCRAFT CLASSIFICATION NUMBER(80’s)
ACN: A number expressing the relative effect of an aircraft on a pavement for a specified standard subgrade strength.(ICAO, Annex14)
Numerically, the ACN is two times the derived single wheel load (DSWL)
The DSWL is obtained by equating the thickness given by the mathematical model for an aircraft LG to the thickness for a single wheel inflated at a standard tire pressure of 1.25 MPa.
“Same thickness requirement  Same applied stress to the pavement”
For Flexible pavement, the mathematical model widely in use at this time (80’s) was the Boussinesq solution for stresses and displacements in a homogeneous isotropic half-space under surface loading. The “CBR Design model” was then selected for ACN computation to allow the maximum correlation to world-wide pavement design methodology.

4. THE AIRCRAFT CLASSIFICATION NUMBER- NEW
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THE AIRCRAFT CLASSIFICATION NUMBER- NEW
ICAO-PSG Task OBJECTIVE:
To align the new ACN procedure with the current recommended practice for pavement design and analysis method, the multi-layered linear elastic analysis (MLEA).
Take advantage of the latest advanced methodology in pavement thickness design by keeping the current ACN-PCN structure unchanged (number, pavement type, subgrade code…).
Each aircraft would still be assigned a number that would express the structural loading effect of the aircraft on a pavement for a specified pavement type and standard subgrade category.

5. Advantages of the MLEA and Hypothesis
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Advantages of the MLEA and Hypothesis
Mechanical approach allowing de facto the elimination of the Alpha-factors since the multi-wheel effects are already considered in the MLEA system.
The current one-leg approach would be replaced by the entire aircraft MLG arrangement as the input parameter for the ACN calculation. The gear interactions for complex LG arrangement would be then assessed and incorporated in the new ACN procedure.
The adoption of the MLEA for ACN calculation would require a number of hypotheses such as the critical failure mechanism, the typical flexible pavement structure in a multi-layered system, the standard SWL condition, the number of coverages etc.

6. HOW TO ACHIEVE A NEW ACN? 1st Solution
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HOW TO ACHIEVE A NEW ACN? 1st Solution
Two contemplated procedures still based on the ESWL concept:
Both procedures eliminate any reference to pavement thickness as an ACN parameter.
Keep the same procedure as today by replacing the CBR design procedure by the MLEA procedure
The use of the MLEA procedure implies:
To define a standard tire pressure for the DSWL (1.5MPa recommended)
To define the number of coverages ( or more to represent modern traffic level) to be applied on both the aircraft in question and the DSWL which require the same pavement thickness than the aircraft MLG to reach a CDF of one.
To define a single agreed computer program, MLEA based.
To retain the subgrade failure mode criteria
To define a fixed surface and stabilized base course, then adjust subbase course thickness until CDF is equal to one.

7. HOW TO ACHIEVE A NEW ACN? 2nd Solution
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HOW TO ACHIEVE A NEW ACN? 2nd Solution
Create a standard (reference) flexible structure for each subgrade category.
For each reference structure, compute the CDFmax for a given number of coverages Ni (subgrade failure mode criteria) of the aircraft in question.
Find the ESWL (inflated at a standard tire pressure) which gives the same CDFmax for Ni coverages on the reference structure.
The number of coverages Ni can be fixed arbitrarily since equal damage calculation gives identical ESWL due to the linearity between the CDF and the applied level of traffic.

8. Solution -1 versus Solution-2
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Solution -1 versus Solution-2
Solution-1 uses a pavement thickness adjustment procedure for a given number of coverages as per current ICAO procedure for calculating ACNs with the introduction of the MLEA system.
Solution-2 does not adjust pavement thickness to determine the DSWL but compute the DSWL which produces equal damage on a fixed reference structure as per the actual aircraft and for the same applied level of traffic.
Both solutions could be achieved on the same computer program, but solution-2 should open a long debate on the reference structure definition whereas solution-1 keep same reasoning compared to the current procedure and changes would not be as substantial as they might otherwise appear to those who are unfamiliar with airfield pavement.

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Which impact on PCNs
The FAA-AC 150/5335-5B gives a clear and complete guidance for PCN determination and publication which remains “ICAO compliant”.
The FAA guidance still based on the CBR method even CDF concept is introduced.
As mentioned by Gordon Hayhoe ( , January 25, 2012) , similar procedure can be implemented on any other program using the CDF concept (e.g. FAARFIELD, ALIZE), see next slide...
Those procedures are based on aircraft ACNs, thus a change in ACN number could have a direct impact on pavement PCN which would have been determined with former ACN method.
As a consequence, new PCN guidance will have to be addressed further to handle the change in ACN so that the entire ACN/PCN system could work under MLEA method.

10. PCN determination with FAARFIELD (G. Hayhoe)
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PCN determination with FAARFIELD (G. Hayhoe)
Procedure similar to what is done with COMFAA, but using FAARFIELD (MLEA)
Enter the pavement structure into FAARFIELD (FF),
Enter the traffic mix into FF, then run “LIFE”,
Select the aircraft (critical aircraft-CA) with the highest CDF contribution and record the total CDF (for all aircraft in the traffic mix),
Remove all aircraft in the list except the CA,
Increase the number of departures of the CA until its CDF is equal to the total CDF recorded in step3,
Adjust the CA gross weight until the CDF equals to 1  MAGW,
Calculate the ACN of the CA at its MAGW by one of the method described before.

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Further Work
During the last ICAO-AOSWG mtg. (Oct.2011), the need to improve the way of calculating the ACNs based on MLEA techniques was widely recognized and the group encouraged to detail a new procedure,
French software “ALIZE” is currently being modified to address these new procedures. Similar “customized” modification on FAARFIELD would be greatly appreciated.
Concept of “reference structure” must be seriously addressed since each structure will be a fixed parameter of the method if solution-2 is adopted.
A set of aircraft, pavement structure (with different materials) will have to be tested on both methods with different computer program, MLEA based (ALIZE, FAARFIELD…)
A single procedure with a single computer program will have to be selected and adopted by the group considering the pros and cons of each proposed procedure

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Conclusion
A change of ACN calculation method must be seen as a part of the ACN-PCN system improvement. New PCN guidance based on MLEA method should also accompany the new ACN calculation method
This first step gives only the basis of a new approach and will have to be further detailed, which would take time before a global consensus and ICAO approval.
Similar approach on rigid pavement, based on FEM could be also contemplated if clear benefits are identified.
Guidance on overload operations, MLEA based would also need to be addressed

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