  The failure of hardware or software in a critical system can lead to loss of
lives. The design errors can be main source of the failures that can be
introduced during system development process. Formal techniques are an
alternative approach to verify the correctness of critical systems, overcoming
limitations of the traditional validation techniques such as simulation and
testing. The increasing complexity and failure rate brings new challenges in
the area of verification and validation of avionic systems. Since the
reliability of the software cannot be quantified, the \textit{correct by
construction} approach can implement a reliable system. Refinement plays a
major role to build a large system incrementally from an abstract specification
to a concrete system. This paper contributes as a stepwise formal development
of the landing system of an aircraft. The formal models include the complex
behaviour, temporal behaviour and sequence of operations of the landing gear
system. The models are formalized in Event-B modelling language, which supports
stepwise refinement. This case study is considered as a benchmark for
techniques and tools dedicated to the verification of behavioural properties of
systems. The report is the full version of a paper published for the ABZ 2014
Case Study. is
