The Missing Interface: Why Complex Projects Fail Where Nobody Is Looking

Complex projects are assumed to be coordinated when their documents are aligned. This article argues that the assumption fails in a specific and costly way. Projects often fail at their human interfaces, the boundaries between people, teams and organisations where shared understanding either exists or does not, while the management systems mainly measure the technical and documentary layer. Drawing on publicly documented cases, including the opening of Heathrow Terminal 5, the Mars Climate Orbiter and Ariane 5 flight 501, the article names this gap the Illusion of Coordination and proposes Human Interface Management (HIM) as the missing counterpart to Technical Interface Management (TIM). It introduces N² + H, a model that adds human layers to the N² matrix of systems engineering: beneath each cell of the matrix sit the owners on each side and the state of the relationship between them, a property called Interface Depth. It also introduces Interface Health, an assessment of five human factors: clarity of responsibility, communication quality, decision speed, trust and delivery capability. The two disciplines together are called Integrated Interface Management (IIM), which the article places in the sociotechnical systems tradition. This is the first public article in a larger body of work.