A quasiexperimental study involved 18 male students enrolled in an aircraft systems course at the University of Illinois. The control group of 10 students studied 39 schematic diagrams of aircraft systems. The treatment group of eight students studied the same schematic diagrams plus conceptual diagrams of the systems. Otherwise, the instruction for the two groups was the same. The treatment group achieved significantly higher scores than the control group on tests to assess the students' knowledge of the structural, functional, and behavioral aspects of the systems. Further analysis of the scores indicated that the conceptual diagrams had little effect on treatment group members' understanding of system structure, but had a significant impact on their behavioral understanding. A card-sorting activity to assess the participants' ability to construct expert-like models showed that the treatment group members were better able to place concept cards in the correct location on knowledge structure maps. Conclusions were as follows: (1) functional flow diagrams increased students' understanding of structural, functional, and behavioral aspects of technical systems; and (2) functional flow diagrams enhanced students' conceptual models of technical systems. (Contains 46 references.) (CML)