Aviation system planning forecasts, unlike other aviation forecasts, require information on the geographic distribution of activity to make possible evaluation of the trade-offs between nearby facilities in the attraction of activity, such as originating air passengers or based general aviation aircraft. To obtain forecasts of this geographic distribution, it Is necessary to develop base demographic data and forecasts for defined geo-, graphic areas within the study area. Using. these demographic forecasts, bottom-up forecasts of aviation activity can be generared, often controlled by a top-down control total. These activity forecasts are then assigned to airports on the basis of accessibjllty and service level considerations, using either manual or computerized methods. Demographic forecasts for small geographic areas are difficult to obtain and often must be generated from current data and larger area forecasts. The exceptions to this are the urban transportation planning data sources for most urban areas. The federally mandated urban transportation planning process includes the forecasting of variables that affect urban travel at a Transportation Analysis Zone level. Aviation forecasts wer: e recently completed as part of the Continuing Florida Aviation System Planning Process and the Ohio Aviation System Plan Update, using the techniques of top-down forecasts controlling bottom-up distributions, with the bottom-up data based, for the urban areas, on urban transportation planning data and, for the rural areas, on current data and regional forecasts.

Aviation forecasts are done for several reasons by aircraft manufacturers, airlines, investment analysts, the Federal Aviation Administration (FAA), airport owners, and state and local governments. Each of these forecasters brings to the task an agenda of needs, data sources, and expectations. The forecasts differ accordingly. For instance, the forecast being used to determine the viability of a bond issue is inherently more conservative than the master plan forecast being used to determine land acquisition needs, as well it should be because the consequences of erring on the high side are much greater. Aviation system planning also has a particular set of needs to forecast and special data requirements to meet those needs. Aviation system planning forecasts are special cases because it is not sufficient to simply develop a forecast of future demand. It is also necessary to determine where this demand will come from in order to formally include competition between facilities in meeting the demand. Not all system plan forecasts have appropriately addressed these special needs. This paper is a discussion of some of the unique requirements, processes, and data needs of aviation system plan forecasting, illustrated by recent examples.