Complex technical systems assist air traffic controllers in management of air traffic in the airspace. Their very important task is to provide the controller with visualization of the traffic situation, thereby enabling situational awareness and making right decisions. In practice, errors sometimes occur, despite the efforts to improve the reliability of visualization systems. The purpose of this paper is to present a method to determine the impact of error types on the air traffic safety. The assessment of the threat level is influenced by subjective factors and cannot be expressed in a precise way. Therefore, the fuzzy reasoning theory has been used in the paper. The developed fuzzy model has been used to obtain a tool enabling the experimental simulation of the impact of various factors on traffic safety assessment. As the results of experiments indicate, the most important determinants of safety are: the time when air traffic controller remains unaware of the breakdown and the total time he/she does not have full knowledge of the traffic situation. It can be therefore stated that the key role for the proper operation of the air traffic visualization system and thus the restoration of full situational awareness is played by self-diagnostic systems that can restore the visualization system’s correct functioning without even the controller being aware of the error occurrence. Their role in assuring safety might be even greater than redundancy which is commonly used.