PCN and CDF Overload Methods

1
PCN and CDF Overload Methods
2
ICAO ACN Overload Criteria
Chapter 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

1. For flexible pavements occasional movements by
   aircraft with ACN not exceeding 10 per cent above
   the reported PCN should not adversely affect the
   pavement.
2. 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.
3. If the pavement structure is unknown the 5 per
   cent limitation should apply and
4. The annual number of overload movements should
   not exceed approximately 5 per cent of the total
   annual aircraft movements.

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Relative ACN Values
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Relative ACN Values
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Exact and Approximate Rigid ACN
Figure 1-4 from ICAO Pavement Design Manual, k
units are MN/m3
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?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.

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Using Cumulative Damage Factor (CDF) for Overload
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Computational Procedure

• 1. Compute CDF for current airplane mix
• 2. Check structural condition for current mix,
• 3. Compute CDF for overload aircraft
• 4. Compare CDF with and without overload aircraft
• 5. Evaluate increase in theoretical damage (?CDF)
  from overload operations

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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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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.

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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

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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

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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.

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Numerical Example - 5

• 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

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Numerical Example - 6
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Numerical Example - 7
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Numerical Example - 8

• 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 gt 1.0,
  at least 240, maybe as many as 1,000.
• Also remember tire pressure.

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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?