 Original fatigue calculations had not taken into account the high number of g-force loading cycles that the German F-104 fleet was experiencing, and many airframes were returned to the depot for wing replacement or outright retirement.Towards the end of "Luftwaffe" service, some aircraft were modified to carry a flight data recorder or "black box" which could give an indication of the probable cause of an accident.Erich Hartmann, the world's top-scoring fighter ace, commanded one of Germany's first post-war jet fighter-equipped squadrons and deemed the F-104 to be an unsafe aircraft with poor handling characteristics for aerial combat.