Preheat the air fryer to 370 degrees F (190 degrees C). In a small bowl, stir together the flour, baking powder, sugar, cream of tartar, and salt. Add the butter and, using your fingers, mix until it resembles coarse crumbs.
Pour the milk into the flour mixtures and stir until it becomes a dough. Gently knead the dough just until it holds together and is smooth. Using your hands, shape into a square or circle and flatten to 1-inch thick. Cut the dough evenly into 4, carefully place them in a parchment-lined air fryer, and, optionally, brush with milk. Cook for 6-10 minutes or until golden brown.
Meanwhile, in a medium pan over medium-high heat, add the sausage and cook for about 3-5 minutes, until no longer pink. Stir in the flour and cook for 1-2 minutes. Gradually pour in the milk, stirring continuously to avoid lumps. Once the mixture begins to thicken, reduce the heat to low and stir in the salt and pepper.
Slice the biscuits in half and serve topped with the sausage gravy.