Consensus on principles, conflict on places reviewing AeroParticipa.pt consultation on Lisbon’s new airport

In 2023–24, Portugal ran AeroParticipa.pt, a year-long online consultation on the location of a new Lisbon airport – a decision stalled for six decades. Using the Consider.it platform and a backcasting methodology, the consultation collected 132,524 opinions from 6,286 participants across 122 staged proposals. This practice review reports what the process produced and where it broke down. Participants reached consensus on values (environmental protection, safety, multimodal connectivity) but rejected all six location options. We call this co-occurrence a deliberative paradox: agreement on principles, conflict on places. The case offers four lessons for planners running participatory consultations on contested infrastructure.