To add to my earlier comments, let me quote from Bruce Rodway, a very senior and respected Australian airport engineer, who was writing comments for a draft Australian CASA publication on ACN/PCN. Academically I’m concerned about plagiarism, and since Bruce took the time to write these words, I am going to properly attribute them.
In my experience I have found that, in general, ICAO’s ACN-PCN system is very poorly understood. Many people who make serious efforts to gain an understanding by reading ICAO documents, technical papers and other explanatory documents provided by various authorities often become frustrated, and are mislead and confused both by the explanations that are provided and by the conflicting and incorrect advice that is offered. In particular, the selection of a PCN remains a mystery to many airport operators. It seems that the system itself is hard to explain and hard to understand.
The ACN is a number expressing the relative effect of an aircraft on a pavement for a specified subgrade strength.
The ACN of an aircraft is arbitrarily defined as twice the Derived Single Wheel Load, expressed in thousands of kilograms. So simplistically, an aircraft weighing 10 tonnes with a single tyre on each gear leg, and with 100% of the weight on the two main gear wheels, has 5 tonnes per tyre. The DSWL is 5 tonnes, and the ACN is twice this which is an ACN of 10.
To quote Bruce again:
ACNs must be calculated using a prescribed technical method. In the case of flexible pavements, the method utilises the US Army Corps of Engineers pavement thickness design method, often referred to as method S77-1. An aircraft’s ACN is calculated from its weight, its wheel layout, its tyre pressure, and the ICAO strength category of the subgrade soil beneath the runway it intends to land on. To standardize the ACN calculation and to remove operational frequency from the relative rating scale, the ACN-PCN method specifies that ACN values be calculated for 10,000 coverages. The ACN of each aircraft is clearly defined. Unlike the selection of a PCN, no judgement is involved in determining an ACN.
The actual physical meaning of ACN is often considered to be difficult to grasp. The ACN is defined in terms of an aircraft’s Derived Single Wheel Load (DSWL), which 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 multiwheel gear (dual, dual-tandem, tridem etc) at its actual gear load and its actual tyre inflation pressure. The ACN of an aircraft is arbitrarily defined as twice the DSWL, expressed in thousands of kilograms. For example, if the DSWL of an aircraft has been calculated to be 14,000 kilograms, the aircraft’s ACN is 28. No special significance should be attached to the factor of two. It is just an arbitrary scaling factor, chosen to give a convenient range of ACN numbers. The smallest to the largest current aircraft have ACNs that range from about 2 to 140. Bigger aircraft usually have bigger ACNs, but not always. If the bigger aircraft has more wheels to spread its weight than the smaller aircraft its ACN could be lower. For example, a 116 tonne Boeing 757 has lower ACNs than a 79 tonne Boeing 737.
Each aircraft has an ACN (Aircraft Classification Number). It is a number that expresses the pavement damage caused by an aircraft relative to that caused by other aircraft. In this context, “damage” does not imply any sudden unsafe breaking of the pavement surface. In the case of flexible pavements surfaced with asphalt or sprayed seals it refers to the gradual accumulation of the small, permanent surface deformations that occurs along wheelpaths when aircraft wheels pass along the runway. After 10 to 20 years the surface may become too rough and rutted for use by aircraft and its life is over. Typically the runway is then resurfaced with asphalt to restore surface flatness and its life begins again. In other words “damage” refers to the gradual consumption of the runway’s design life.
Aircraft that have the same ACN do equal damage (ie consume pavement life at the same rate.). However, damage is not simply proportional to ACN. For example, an aircraft with an ACN 10% greater than that of another aircraft does not do only 10% more damage to the pavement. It might do approximately two to four times the damage done by the lighter aircraft. If its ACN is 30% greater than that of the smaller aircraft it could cause more than 10 times the damage. The actual relative damage can be calculated and depends upon the thickness of the pavement and the CBR of the underlying subgrade. In other words it is not possible to say that one aircraft is always X times more damaging than another aircraft. The value of X will be different for each runway.