Preheat oven to 375. Pare and slice apples and potatoes. Slice sausage. Fry sausage, save drippings. Combine salt, flour, and sugar. Arrange sausage, apples, and potatoes in layers in casserole. Sprinkle some flour mixture over each layer. Top with layer of sausage. Sprinkle casserole with sausage drippings. Cover tightly. Bake for 1 hour.