Presented to: 2010 FAA Worldwide Airport Technology Transfer Conference By: Gordon Hayhoe, FAA...

Presented to: 2010 FAA Worldwide Airport Technology Transfer Conference

By: Gordon Hayhoe, FAA AJP-6312, WJHTC

Date: April 20, 2010

Federal AviationAdministration

PCN and CDF Overload Methods

February 20, 2008 2 2Federal AviationAdministration

Overload for FAA TT ConferenceApril 20, 2010

ICAO ACN Overload CriteriaChapter 2 of the ICAO Pavement Design Manual

Guidance on Overload Operations

For those operations in which magnitude of overload and/or the frequency of use do not justify a detailed analysis the following criteria are suggested:

a. For flexible pavements occasional movements by aircraft with ACN not exceeding 10 per cent above the reported PCN should not adversely affect the pavement.

b. For rigid pavements, in which a rigid pavement layer provides a primary element of the structure, occasional movements by aircraft with ACN not exceeding 5 per cent above the reported PCN should not adversely affect the pavement.

c. If the pavement structure is unknown the 5 per cent limitation should apply; and

d. The annual number of overload movements should not exceed approximately 5 per cent of the total annual aircraft movements.



Relative ACN Values

2

1

2

1

2

Pavements FlexibleFor

tt

ACNACN

2

1

2

1

2

Pavements RigidFor

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ACNACN



Relative ACN Values

2aircraft of ACN1aircraft of ACN

2

1

ACNACN

coverages 10,000at 2aircraft for cknessDesign thicoverages 10,000at 1aircraft for cknessDesign thi

2

1

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Exact and Approximate Rigid ACN

0

10

20

30

40

50

60

70

80

0 100 200 300 400 500 600

Reference Slab Thickness, h, mm

Der

ived

Sin

gle

Whe

el L

oad,

DSW

L, to

nnes

k = 150 80 2040

Dark lines areDSWL = Constant * h2

2Constant1000

2 tDSWLACN

Figure 1-4 from ICAO Pavement Design Manual, k units are MN/m3



ACN Computation• Pavement thickness for the overload

aircraft is not the same as for the design aircraft (used to compute pavement PCN).

• ACN Traffic for the overload aircraft is the same as for the design aircraft (10,000 coverages).

• But by definition the traffic for the overload aircraft is much less than for the design aircraft.



Using Cumulative Damage Factor (CDF) for Overload

IIT

IF

II

CDFCDF

NNCDF

Rule) sMiner'(by

NI = number of coverages applied to a pavement by aircraft I NIF = number of coverages to failure for aircraft I

CDFT = Total CDF for all aircraft in the traffic mix



Computational Procedure

1. Compute CDF for current airplane mix2. Check structural condition for current mix, 3. Compute CDF for overload aircraft4. Compare CDF with and without overload

aircraft5. Evaluate increase in theoretical damage

(CDF) from overload operations



How to Determine CDF Limiting Criteria?

• ACNR is the ratio of ACNs for the overload aircraft to the design aircraft.

• St is the sensitivity of change in pavement life to change in pavement thickness.

• See the paper for details

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SACNSCDF tRt



Numerical Example - 1• Run COMFAA in Flexible ACN Mode.

• 737-500 at 134,000 lbs, B subgrade, ACN = 37.4 (t1 = 27.84).

• Adjust load until ACN = 1.1 x 37.4 (ACN = 0.1).

• 737-500 at 144,500 lbs, ACN = 41.1, (t2 = 29.19). Represents an overload aircraft at the ACN limit.



Numerical Example - 2• Run FAARFIELD with:

– 4-in P-401– 8-in P-109– Balance P-154– 737-500 at 134,000 lbs and 1,200 annual departures

• Run Design on the subbase:– 10.9-in P-154– CDF = 1.0



Numerical Example - 3• Increase the load to 144,500 lbs at the same

traffic and run Life– CDF = 2.14 over 20 years– Total departures to CDF of 1.0 = 11,208 in 9.3 years

CDF corresponding to ACN = 0.1 is CDF = 1.14

• Increasing annual departures by 1,368 also gives CDF = 1.14



Numerical Example - 4

• Therefore a CDF can represent either an increase in load or an increase in traffic, or a combination of the two.

• But a ACN is affected only by an increase in load. Traffic is fixed and the structure is a dependent variable.




• Add a true overload aircraft to the 737-500 at 134,000 lbs.– 787 at 486,000 lbs at 12 annual departures

• Run Life:– CDF = 1.05 CDF = 0.05– CDF for the 737 = 1.0– CDF for the 787 = 0.65




• 737 ACN = 37.4.• 787 ACN = 80.1, does not meet the ACN criterion.• Probably need to apply a minimum total number of

departures for overload aircraft with CDF >= 1.0, at least 240, maybe as many as 1,000.

• Also remember tire pressure.

787 Annual Deps.

Total CDF

737 CDF

787 CDF

12 1.05 1.00 0.65 24 1.35 1.00 1.31 36 2.00 1.00 1.96 48 2.65 1.00 2.62



ACN-PCN versus CDF

• Is it possible to compare the ACN-PCN procedure with the CDF procedure and to derive compatible overload criteria?

• Is the equation derived in the paper valid?

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Presented to: 2010 FAA Worldwide Airport Technology Transfer Conference By: Gordon Hayhoe, FAA...

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