It is quite possible for a study to combine engineering cost estimates for some segments of costs with utilization of accounting data for others. Such an analysis was performed some years ago, when a group of economists at the Transportation Center at Northwestern University published a forecast of aircraft prices.52 One important step in arriving at the forecast was the calculation of aircraft operating costs. These were computed for a large number of different types of aircraft and used both actual data and engineering estimates. Crew salaries were calculated with historical data to arrive at an average. Maintenance costs were calculated from a formula supplied by the Air Transport Association. Fuel consumption was obtained from engineering curves produced by aircraft manufacturers and airlines, and reconciled with some actual published data. Other costs (employee benefits, landing fees, liability and property damage insurance) were estimated in similar ways. The resulting curves reflected direct operating costs per aircraft mile for a series of different stage length categories ranging from 0 to 200 miles to 2,500 miles and more. The resulting cost curves generally exhibited a downward slope up to the limit of the stage length that each aircraft could fly nonstop.