Field maintenance actions reported in a deployed air defense system were reviewed to determine and quantify (1) the nature of operator and maintainer influences on system malfunctions, and (2) the nature of man-machine problems being encountered by system personnel in actual diagnosis and repair activities. Maintenance events described in 213 problem reports from the field were categorized according to three probable sources of malfunction: primarily human, combination human and equipment, and primarily equipment. Human involvement in system malfunctions was clearly established in 25% of the events reported. Additional information contained in 21 documents analyzing system maintenance was examined. The nature of human influence on system malfunction is described by examples of “unfortunate” field maintenance, “ordinary” field maintenance, and troubleshooting deficiencies. The nature of reported man-machine problems highlighted the fact that human engineering participation in establishing maintainability design requirements can serve to make equipment that will be suitable to the presence and actions of human users.