Modern aircrafts are heavily relied on flight management systems to automate a wide variety of in-flight tasks, including producing flight plans, reducing the workload on the pilot, or allow the airplane to hook up the autopilot. Vulnerabilities in such systems could allow an attacker to manipulate critical data that are important during a flight. In this talk, we will present the result of our research on the security of a famous flight management system and how we managed to detect a weakness in its security mechanism using reverse engineering. Then we will discuss possible risk scenarios regarding manipulation of mentioned critical data. During this research, we have found a method to modify the navigation data on a flight management computer and also identified some risk scenarios that we think could cause some problems. We hope this may lead to future research and make the aviation industry more secure.