Reading skills are becoming an increasingly essential requirement for success in professional examinations, or for those working with complex systems, where safety is an issue. The lack of even basic skills could cause difficulties for workers who are experienced in their own field of work, but, because of their illiteracy, cannot deal with the written work that relates to theoretical professional knowledge. This is a problem that concerns French crane-drivers. They have to pass an exam that includes a reading-comprehension test of written technical graphics or tables representing crane load transport limits. This exam is compulsory and takes place at the end of a period of training : illiterate trainees often fail the test, even though they are experienced crane-drivers. In order to help them, a multimedia teaching aid tool was specifically designed for the comprehension and manipulation of tables and graphics of load transport limits. This paper presents three main stages in the design and conception of learning situations. First of all, a cognitive work analysis was carried out, involving 10 crane-drivers. The results clearly show that crane-drivers have an implicit intuitive knowledge of the weight-distance function of the crane, and that they are able to express this function explicitly in the context of an analogical format of representation, keeping chosen modalities of crane driving activity. Second, on the basis of these results, we designed a multimedia simulator of working crane. The aim of this simulator was to assist in the reading, understanding, and manipulation of tables and graphics of transport load limits. This teaching tool is based on an explicit and concrete connexion between the internal representation of the crane-drivers and an external symbolic form of representation of the weight-distance function. Finally, we tested experimentally the effects of a learning session with the simulator on the comprehension of tables and graphics during two training periods in two different training centres. Two experiments were conducted, involving 70 illiterate crane-drivers. The results show that this teaching simulator strongly increases the comprehension of tables and graphics.