Path　planning　is　an　important　research　field　in　robotics,　and　it　is　also　a　key　technology　for　realizing　automatic　navigation　of　aircraft.　It　is　of　great　significance　both　in　theory　and　in　application.　The　task　of　planning　trajectories　for　an　aircraft　has　received　considerable　attention　in　the　research　literature.　Most　of　the　work　assumes　that　the　threat　to　the　aircraft　is　static;　less　attention　has　been　paid　to　the　problem　of　the　changing　environment.　However,　the　threat　from　the　weather　when　the　UAV　is　flying　in　the　air　is　always　changing.　This　paper　introduces　an　improved　BFS　algorithm,　which　can　find　the　best　safe　path　when　the　threat　is　always　changing.　©　The　Authors,　published　by　EDP　Sciences,　2018.