Developing safety critical systems is one of the most challenging problems in Engineering nowadays. There is usually a pressure to reduce the total time of the development, what makes it dicult to deliver systems with an acceptable low level of defects. In the recent years, there has been an increase in air trac, what demands a modernization in the current air trac systems, which are very dependent on the human controller. Air trac systems are considered safety critical and real time systems. The objective of the present work is to establish a modeling and formal verication method for critical systems, applicable to the air trac domain. By adopting modeling and formal verication techniques, it is expected to ensure the systems' correctness compared with the initially specied requirements and the error detection in the initial phases of the project. Guidelines are provided for applying the method by means of a case study, based in three modules of and ATC system in a low abstraction level, for the validation of the operation of software modules. For the formal verication, it is used the NuSMV tool and the properties to be checked are described in the computational tree logic (CTL) to ensure that the system satises requirements of liveness and safety types.