Current strategic decisions involving air traffic flows are limited to the allocation of airport slots. In this work, a first strategic market-based mechanism for the allocation of en route resources, i.e., sector capacity, is proposed. Identifying en route capacity shortages at this phase can lead to strategic actions to reduce flight delays on the day of operations. Tradable mobility permits previously developed for roadway transportation are analyzed and adapted to the air transport case. A trading mechanism for the strategic alleviation of air traffic congestion that uses time-place specific permits is proposed, providing alternative implementations. We call this new approach Tradable Flight Permit System (TFPS). An example of its use is illustrated, showing how it can lead to cost reductions for the airlines competing for a single sector access permit. We also highlight the improvements it can provide compared to current practice and a variety of alternative methods.