Avionics are highly critical systems that require extensive testing to comply with international safety standards. Cockpit display systems (CDS) are a mandatory part of modern cockpits of both manned and unmanned aircraft. The information from various avionics components is displayed on CDS using a variety of flight instruments. An important part of testing avionics systems is to evaluate whether the displayed information on the CDS is correct or not. A common industrial practice is to manually test CDS, which is time-consuming, labor-intensive, and error-prone. In this paper, we propose a model-based approach to automate the CDS testing of aircraft. The proposed approach tests the CDS at two levels: (i) at the system level to verify that the CDS are working correctly and (ii) at system integration level of CDS when these are integrated with various avionics components. As a part of our approach, we develop a UML profile to model various elements of the CDS. The models are then used to support the automated testing process. We evaluate our approach on two industrial case studies, the first case study represents a primary flight display (PFD) of an aircraft and the second one is the CDS of the ground control station (GCS-CDS) of an unmanned aerial vehicle. The evaluation results show that three potential faults are identified in the PFD and four major faults are found in the GCS-CDS.