1. ACN is calculated based on the design single wheel load (DSWL), which represents the load of a single wheel inflated to 1.25 MPa that would cause equal pavement damage to the aircraft's actual multi-wheel gear. 2. The relationship between ACN and DSWL is that ACN equals 2 times the DSWL. 3. ACN is calculated for the pavement thickness required to withstand 10,000 coverages of the aircraft and depends on both the pavement type (flexible or rigid) and the subgrade category.

1. ACN-PCN System
Bruce Rodway

2. Section Objectives
1. What DSWL means
2. The relationship between ACN and Subgrade
3. The relevance of 10,000 coverages
4. The basis of calculation of ACN

3. ACN Characteristics
ICAO defines ACN as “a number expressing the
relative effect of an aircraft on a pavement for a
specified standard subgrade strength”.
Unlike the selection of a PCN, no judgment is
involved in determining an ACN.

4. ACN Characteristics
The method of calculating the ACN of each
aircraft has been clearly defined by ICAO. Flexible
pavement ACNs do not depend upon pavement
thickness or on the precise CBR of the subgrade.

5. Design Single Wheel Load
ACN is expressed in terms of the DSWL
DSWL is the load on a single wheel, inflated to 1.25 MPa, that
would cause pavement damage equal to that caused by the
aircraft’s actual multi-wheel gear (dual, dual-tandem, tridem
etc) at its actual gear load and its actual inflation pressure.
DamageDSWL=Damagemulti-wheels

6. ACN-DSWL Relationship
ACN = 2 x DSWL
B737-800
DSWL = 27.5 t
ACN=55
Note it will be shown later that the DSWL and hence the ACN is dependent on the
subgrade category

7. Coverages
• ACN is calculated for the pavement thickness
required by 10,000 coverages of the aircraft.
• This thickness is referred to as the “reference
thickness”
Coverages are defined later in the course

8. Subgrade Category
ACN is dependent on the:
• Pavement type
• Subgrade category
Insert ACN table
At an operating weight of 79 tonnes, the ACN of a
B737-800 on a flexible runway ranges from 43 to 55.
The actual value depends on the subgrade category.

9. ACN Examples
Cessna Citation X B737-800 B777-300
MTOW 16.3 t MTOW 79.5 t MTOW 300 t
ACN=12/F/D ACN=55/F/D ACN=100/F/D
All examples are based on a flexible pavement on a subgrade category D.
MTOW = Maximum Take-off Weight

10. Calculation of ACN
Flexible Pavements:
• The CBR procedure detailed in the US Army Corps of
Engineers' 1977 Instruction Report S77-1.
• Utilizes the Boussinesq solutions for displacements
within a uniform elastic material.
Rigid Pavements:
• The US Portland Cement Association’s procedure.
• Utilizes the solution for stresses at the centre of a
loaded elastic plate.

11. Section Summary
1. DSWL - Tyre pressure of 1.25MPa
2. ESWL – Actual tyre pressure
3. ACN = DSWL x 2
4. ACN depends on:
• Pavement type and
• Subgrade category