Introduction

The capacity of an airport to accommodate a certain type of aircraft depends on its infrastructure. Indeed, the weight of an aircraft can vary from a few hundred kilograms to over 500 tonnes. Consequently, runways, taxiways, and tarmacs can be limiting, and operations at a given site must be subject to prior checks.

It is in this context that the ICAO has defined two types of classifications to establish the compatibility between a type of aircraft and the ground strength of the surfaces of a particular aerodrome. These are the PCN and the ACN, the method of which has been officially used since 1983..

 

PCN

The PCN (Pavement Classification Number) is a set of numbers and letters expressing the maximum load-bearing capacity of a runway, taxiway, or tarmac for unrestricted operations.

It is located in the Aerodrome (AD) section of the AIP and contains the following indicators :

1. PCN

The load index:

• A figure without a unit that will be compared to the ACN

2. Type of surface

• F : Flexible (asphalt or bituminous coating)
• R : Rigide (cement or concrete)

3. Resistance of the subgrade

• Flexible subgrade

The strength of the subgrade is assessed by a bearing ratio coefficient measuring the pressure required to penetrate it. This is the CBR (California Bearing Ratio) expressed as a percentage..

• Rigid Subgrade

The strenght is this time measured in MN/m3 to give a stiffness k:

4. Maximum tyre pressure

• W : No limit
•  X : 1.75 MPa max
• Y : 1.25 MPa max
• Z : 0.5 MPa max

5. Measurement method

• T : technical assessment
• U : by usage

Example

The PCN of runway 06 at Charleroi Airport is read as follows:

 64/F/A/W/T

• 64 : Load index of 64
•  F : Flexible surface
• A : High Strenght
• W : No tyre pressure limit
• T : Result of 64 obtained through technical evaluation

ACN

Each aircraft model has its own technical characteristics. An aircraft has a certain weight, but the distribution of that weight varies depending on its design, particularly that of its landing gear. The pressure exerted on the runway depends not only on the number of wheels but also on the position of the landing gear and the number of wheels per gear..

Each type of aircraft has an ACN (Aircraft Classification Number), published by the manufacturer in the flight manual.

The result is calculated for two configurations :

• Poids maximale
• Poids à vide (OEW)

 As with its PCN counterpart, the ACN is provided for both scenarios:

• Flexible subgrade 
• Rigid Subgrade 

It is given in the same units as the PCN (see previous section on the PCN).

Example of interpretation :

 The published ACN of the Airbus A330-300 is as follows:

UAn Airbus A330-300 with a maximum weight of 2264 kN and a tyre pressure of 1.42 MPa will have an ACN of 68 if used on a flexible surface runway of category B (medium strength)..

Practical case

The aim is therefore to compare the ACN of our aircraft with the PCN of the airport's infrastructure (runway/taxiway/tarmac):

Example

We want to check if we can operate a Boeing 777-200ER at Bordeaux Mérignac Airport (LFBD). If so, with what restrictions??

1. We will operate from the longest runway, which is 05/23. Here is what is published in the AIP of LFBD:

PCN> 64/F/C/W/T

• 64 : Load index of 64
• F : Flexible surface
• C : Low strength
• W : No tyre pressure limit
• T : Result of 64 obtained through technical evaluation

  2. We will need to taxi to the corresponding commercial terminal for tarmacs A, B, C, D, or F:

PCN > 70/R/B/W/U

• 70 : Load index of 70
• R : Rigid Surface
• B : Medium Strength
• W : No tyre pressure limit
• U : Result of 70 obtained through technical evaluation

   3. To achieve this, we will be likely to use the taxiways A, B, C, D, P1 to P6, and S1 to S3 :

PCN> 70/F/B/W/U

• 70 : Load index of 70
• F : Flexible surface
• B : Medium Strength
• W : No tyre pressure limit
• U : Result of 64 obtained through technical evaluation

4. To evaluate this data, let us compare the PCN with the published ACN for a B777-200ER:

• Runway :
• • PCN 64/F/C/W/T
  • ACN max 90

ACN > PCN, restrictive

•  Tarmac:
• • PCN 70/R/B/W/U
  •  ACN max 69

ACN < PCN , not restrictive

•  Taxiways:
• • PCN 70/F/B/W/U
  • ACN max 71

ACN > PCN, restrictive

 5. When the ACN for the maximum weight is greater than the PCN, it is necessary to calculate the maximum limiting weight for each case. 

If operators generally have calculation tools at their disposal, it is always possible to extrapolate it using the following formula :

Wlim   =   Wmin + {(PCN−ACNmin) / (ACNmax−ACNmin)} x (Wmax−Wmin)

• • Wlim > poids maximal limitatif à calculer
  • Wmax > poids maximal
  • Wmin > poids à vide (OEW)
  • ACNmax > ACN pour le poids maximal
  • ACNmin > ACN pour le poids à vide (OEW)
  • PCN > PCN of runway/taxiway/tarmac

Let's apply the formula for the track:

• • Wmax* > 2822 kN = 287 765 kgWmin* > 1425 kN = 145 310 kg
  • ACNmax > 90
  • ACNmin > 32
  • PCN > 64

*1kN = 101,972 kg

Which results in: ​Wlim = 145310 + { (64−32) / (90 − 32) } x (287765-145310) = 223945

The maximum authorised weight for using our B777-200ER on runway 05/23 is 223,945 kg.

Références

-ACN list by aircraft type

-Volume I of ICAO Annex 14 concerning airport design and operations.