2 things you can check. in the aircraft.cfg file under the [fltsim.x] entries make sure sim= matches the file name of the .air file, for example if the .air file is 747.air then the entry should be "sim=747" without the .air extension. second, in the model folder open the model.cfg file and make sure there is no .mdl extension, for example it should say normal=747, not normal=747.mdl. the entry after normal= is the model file name minus the .mdl extension