Preventing aircraft from getting too close to each other is an essential element of safety of the air transportation industry, which becomes ever more important as the air traffic increases. The problem consists in enforcing a minimum distance threshold between flying aircraft, which naturally results in a bilevel formulation with a lower-level subproblem for each pair of aircraft. We propose two single-level reformulations, present a cut generation algorithm which directly solves the bilevel formulation and discuss comparative computational results.