This article is part of an experience feedback project for the improvement of flight safety. The work primarily consists in the development of a description language of pilot's activity, in order to allow the modeling of piloting missions. This language aims 10 compare a reference activity (the envisaged mission) with the real activity (the actual mission). To do so, a calculus operator is created winch can express any deviation between two language descriptions. In order to interpret this result and with the aim of a systematic flight analysis, we establish a simple deviation typology between two piloting activities. As a result we obtain an unequivocal correspondence between a simple deviation and its typology interpretation.