The industry has also recognized the growing impact of the volatility in air traffic management and the need for review of the current practices of cooperation between airlines and ATM authorities. In this paper the authors propose an approach to evaluate reliability of the planned schedule based on some systematization of the reliability measures. To illustrate the concept an example of application of proposed methodology is presented. It involves the analysis of sensitivity of expected daily operational margin from scheduled operations with relation to the timely completion of the aircraft overnight maintenance.