A stage-length adjusted CASM is derived from a formula involving
- One constant (the average CASM at cruising altitude)
- One variable, consisting of departure-related constants (e.g., ground crew, extra time, labor, and fuel associated with climbing to and descending from cruise), divided by the number of miles in the stage.
The first element would appear as a horizontal line on the stage-length CASM chart. The second would appear as an asymptotic inverse-function curve, with a trend to infinity as stage-length approaches zero, and a trend to zero as stage-length approaches infinity. When you combine the two together, you get the asymptotic inverse-function curve, shifted up by the value obtained from the first element.
The big problem is figuring out the two values based on numbers that are not known outside small circles within the airlines (if they're even known within those circles). They have to be divined from lots of data taken from lots of sources. They're not precise when determined this way.
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