Another trick might be using star alignment. A fixed position camera on the top of the plane could sense pitch by tracking which stars are in view. Or similarly

Another trick might be using star alignment. A fixed position camera on the top of the plane could sense pitch by tracking which stars are in view. Or similarly, if you boxed a sensitive GPS receiver so that it could not detect signals from the sides, the satellite IDs which are in view could possibly indicate alignment of the plane as well. The best model would tie in as many observations as possible to maximize the probability that it is right. EDIT: What about fuel sensors in the wing? If the computer knew the volume of the remaining fuel and the altitude was not dropping, it could determine the orientation of the plane if there were sensors inside the tanks to detect which were submerged.