Formal methods are considered to be an important technique towards achieving the levels of assurance needed for high integrity systems. Formal specification is the essential part of the formal development process. The use of formal specification techniques on critical systems has shown significant growth in the last few years. In particular, there are a number of successful applications of using formal specification techniques in the aviation industry. Safety critical systems, such as aviation systems controlled by software, often have hard real time requirements. Producing the correct result at the right time is the fundamental goal of such systems. Formally specifying the system functions and the timing requirements is the crucial step towards achieving such a goal. Furthermore, aviation systems often need to be modified or upgraded on a regular basis, i.e. functionality and timing constraints may be altered. Therefore, the formal specification of such systems needs to be easily maintained and modified. We are interested in applying formal object modelling techniques to specify scheduling requirements of the multi parallel processes of an aircraft operational flight program (OFP). Our aim is not only to formalise the scheduling requirements for a particular aircraft, but more importantly to demonstrate an incremental and extendible modelling approach such that our model can be readily reused to specify other aircraft OFP scheduling requirements.