Fuel and crew salaries are two of an airline's largest expenses. Using the number of passengers on a flight as the cost driver, how would you classify the behavior of these two costs?

If an airline were to consider crew salaries as variable, what would be a reasonable cost driver? A standard cost metric used by airlines is "cost per available seat mile"?This metric is calculated by dividing the total operating costs by the available seat miles flown? How would you classify this metric's cost behavior?