What was found
IN FLIGHT, APPROX 10 MINUTES AFTER TAKEOFF, AT CRUISE STRONG FUMES OF ELECTRICAL/PLASTIC BURNING SMELLED IN THE COCKPIT. TURNED ECS OFF, OPENED WINDOW. NO OTHER INDICATIONS OF ELECTRICAL FIRE. REMAINED APPROX 28.4V, 30A WITH ECS OFF (L SL ON FOR BIRD MITIGATION). FUMES DISSIPATED WHILE ECS OFF. RETURNED TO BASE, LANDED AS SOON AS PRACTICABLE.