## 1 Introduction  
### Genesis of APCI Idea  
The safety of aircraft operations within the ground maneuvering area depends on many factors. From the airport infrastructure point of view, the technical condition of the airport's functional elements (AFE) is of key importance. We are talking not only about artificial surfaces (Runways, Taxivays, or Aprons) on which aircrafts move, but also about natural surfaces (Side Shoulders, Runway End Safety Areas) providing protection in the case of an aircraft leaving the AFE. Proper management of the technical condition of pavement should be complex and include both ongoing maintenance and planning future renovations. Monitoring of the airport functional elements should be based on reliable information about the technical condition of the pavement obtained in a systemic manner. It means that each decision should be the result of collected data deriving from periodic reviews and tests. Only in this way it is possible to rationally plan repairs and renovations of all airport pavements, regardless of their structure. The experience gained by various institutions such as Cambridge Systematics, New Jersey Department of Transportation, or The Louisiana Department of Transportation and Development, shows that proper management of airport infrastructure [1], as well as road infrastructure [2; 3; 4; 5; 6], must rely on a detailed and up-to-date information on the technical condition of pavement. Early diagnostics of the airport pavement's technical condition allows to avoid most of the costs incurred in the operation process, which is related to the pavement life cycle. Proper recognition of the AFE pavement technical condition at a given moment, and more importantly its anticipating in the future, is the basis for taking appropriate actions much earlier than when basing only on the assessment of the pavement in terms of visual inspection. Figure 1 shows the pavement life cycle, taking into account its technical condition. Early response to changes in the pavement technical condition allows for a significant reduction in both financial and social costs [1]. The first period of pavement life is a slow degradation process, in which the costs of repairing any deterioration are low and the complete restoration of the element does not entail serious consequences. The dynamics of pavement degradation accelerates over the years reaching such a level that the repair costs increase dramatically year by year. During this period of time, the observed deteriorations are so large that complete pavement restoration entails costs that are incomparable to previous periods. In the short life of the surface, repair costs can increase up to five times. Proper management of the pavement surfaces of airport functional elements, in particular anticipating their technical condition in the future, requires the use of appropriate tools (databases containing field test results, mathematical models for evaluation, pavement's condition prediction models, etc.) based on reliable and up-to-date information on their current technical condition. This also applies to flight protection elements such as natural airfield pavements in the immediate vicinity of the AFE. A comprehensive assessment of the technical condition of AFE's natural pavements based on the APCI (Airfield Pavement Condition Index) is presented in the article. The APCI index is based on the results of field tests, the scope of which includes the test of the turf layer strength to a depth of 0.3 m and the test of the load bearing capacity (define by CBR) of the natural pavement to a depth of 0.85 m below the ground level. The APCI method connects two different methods into one parameter that allows clearly evaluate condition of natural pavement. APCI index describe pavement as a layer package unlike before mention methods. A further part of the work presents the assessment procedure containing the mathematical model of the indicator and its exemplary use based on real test results performed at one of the active airport facilities.