ACN rigid pavement conversion chart.
A number expressing the relative effect of an aircraft on pavement for a specified standard subgrade category (ICAO). Pavement refers to runways or operating areas. Using this method, it is possible to express the effect of individual aircraft on different pavements by a single unique number, which varies according to pavement type and subgrade strength, without specifying a particular pavement thickness. The ACN is twice the derived single-wheel load expressed in thousands of kilograms, with single-wheel tire pressure standardized at 1.25 megapascals (= .09 ton/ft2
). Additionally, the derived single-wheel load is a function of the subgrade strength. The ACN is defined for only four subgrade categories (high, medium, low, and ultralow). ACN values for selected aircraft have been calculated by the International Civil Aviation Organization (ICAO) using two computer programs, one for rigid pavements and the other for flexible pavements. Manufacturers are required to calculate ACNs for new aircraft as they come into service and publish the results in flight manuals. The tables give ACN values for two weights, one at the maximum total weight authorized and the other at the operating weight when empty. If an aircraft is operating at an intermediate weight, the ACN value can be calculated by a linear variation between the limits. Extrapolation is not permissible. ACN is calculated with respect to the center of gravity position, which yields the critical loading on the critical gear. Normally, the aftmost center of gravity, or CG position, appropriate to the maximum gross apron (ramp) mass, or ramp weight
, is used to calculate the ACN. In exceptional cases, the forwardmost CG position may make the nose-landing gear loading more critical. See also ramp weight