In estimating the frequency of an aircraft crashing into a facility, it has been found convenient to break the problem down into two broad categories. One category estimates the aircraft crash frequency due to air traffic from nearby airports, the so-called near-airport environment. The other category estimates the aircraft crash frequency onto facilities due to air traffic from airways, jet routes, and other traffic flying outside the near-airport environment The total aircraft crash frequency is the summation of the crash frequencies from each airport near the facility under evaluation and from all airways, jet routes, and other traffic near the facility of interest. This paper will examine the problems associated with the determining the aircraft crash frequencies onto facilities outside the near-airport environment. This paper will further concentrate on the estimating the risk of aircraft crashes to ground facilities due to high altitude air carrier and air taxi traffic. High altitude air carrier and air taxi traffic will be defined as all air carrier and air taxi flights above 18,000 feet Mean Sea Level (MSL).

A computer based system for performing probabilistic safety assessments (PSAs) of aircraft crashes to ground structures is under development. The system called ACRA (aircraft crash risk assessment) employs a GIS (geographical information system) for locating, mapping, and characterizing ground structures; and a multiparameter data base system that supports the analytical PRA (probabilistic risk assessment) model for determining PSAs for aircraft crashes. The Salt Lake City International Airport (SLC) is being employed as the base case for study and application of ACRA and evaluation of the projected safety assessment.