Regulators in Asian hubs like Hong Kong have threatened to retaliate against European Union plans to force airlines to start using takeoff and landing slots frozen during the coronavirus pandemic, a move that could oblige Europe’s carriers to fly empty seats for thousands of miles at a loss. Authorities controlling slots at major Asian airports are ready to slap similar ‘use it or lose it’ conditions on European carriers flying to Asia’s cities—raising the prospect of an industry trade war over the uneven impact of COVID-19. After rare unity during the pandemic, when carriers were being bailed out or trying to stay afloat, industry leaders say the dispute has rekindled fundamental differences across a fragmented sector as the world stages a multi-speed recovery. “Is it a trade war? Certainly the germ of one,” said former Australian aviation negotiator Peter Harbison, chairman emeritus of the Sydney-based CAPA Centre for Aviation consultancy. …