The three traditional objectives of computer security are confidentiality, integrity and availability [8]. Availability can be defined as the prevention of denial of service. Confidentiality and integrity have been addressed in several theoretical works whereas the concept of availability has not been much investigated by the scientific community. This paper is an attempt to define through a case study the concept of availability. We first define a set of availability constraints that avionic data bus protocols should enforce. Then, we consider the ARINC 629 Basic Protocol (BP) [2] which was implemented on the Boeing 777, and we check whether that ARINC 629 BP respects our availability constraints or not.