The most common engineering economics failure is the invisible cost of maintenance. Teams invest in building capabilities, account for the infrastructure cost, and do not model the ongoing engineering overhead — the time spent on incidents, the time spent on updates, the cognitive load of understanding the system in a high-turnover environment. FM3 (Unbounded Resource Growth) in the maintenance dimension: every capability added is a permanent maintenance obligation that does not appear in the investment proposal.
The second failure is cost of delay in reverse: prioritising work by engineering preference or backlog age rather than economic urgency. A feature with a ten-thousand-euro monthly cost of delay that has been in the backlog for six months has cost sixty thousand euros of delay. A feature with a one-thousand-euro monthly cost of delay that ships this sprint costs one thousand euros of delay. The backlog ordering by engineering preference has cost the organisation fifty-nine thousand euros.
FM6 (Hotspotting) at the economic level: infrastructure costs that are concentrated in a small component — a single database, a single service, a single data transfer path — make the cost curve non-linear. When the hot component reaches its capacity ceiling, the cost to continue scaling jumps discontinuously. Engineering organisations that do not model their cost hot spots are surprised by cliff-edge cost events that could have been predicted from the cost model.
Concept: Engineering Economics Thread: T12 (Tradeoffs) ← technical property → economic consequence → investment decision Core Idea: Engineering decisions are economic decisions; cost of delay, total cost of ownership, and scaling cost curves are the vocabulary that makes technical investments legible to non-technical stakeholders. Tradeoff: AT2 — latency (higher infrastructure cost per unit) vs throughput (lower cost per unit at scale) Failure Mode: FM3 — unbounded resource growth; maintenance overhead of capabilities not modelled in investment proposals Signal: When a board or executive asks “is this worth it?” and engineering cannot answer with numbers — the investment was not framed economically; stop and produce the three-number structure before proceeding Maps to: Reference Book, Frameworks 4, 9