The Rotating Self-Nulling Probe System developed as a part of the National Aeronautics and Space Administration’s Airframe Structural Integrity Program has been shown in the past to provide a quick and reliable method to detect fatigue cracks hidden under airframe rivets [1–3]. The previous research has concentrated on detecting cracks in the top layer of an airframe lap-splice joint, obtaining a 90% probability of detection for 0.032” long top layer fatigue cracks buried under airframe rivets [2,3]. Although this was a major accomplishment, the question of detectability of fatigue cracks in the lower layers of the joint has been left open. In this work the ability of the system to detect second and third layer fatigue cracks will be explored.