The failure of hardware or software in a critical system can lead to loss of lives. Design errors are a major source of the defects that can be introduced during the system devel- opment. A complementary approach like formal methods is considered as an alternative approach to identify the possible defects in the software development process using rigorous mathematical reasoning. The increasing system complexity and failure rate invoke the area of verification and validation of avionic systems. This paper describes a stepwise formal development of an aircraft landing system using Event-B. The formal models include the complex behaviour, tempo- ral behaviour and sequence of operations of a landing gear system. The refinement based incremental development al- lows to verify and to validate the required safety properties. This case study is considered as a benchmark for techniques and tools dedicated to the verification of behavioural prop- erties of systems.