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To further increase the areal density to 2 TB/in2, the head-disk spacing needs to be reduced to sub 1 nm by using a thermal flying-height control technique that can adjust slider pole tip protrusion dynamically. At such low spacing, the slider pole tip protrusion-disk contact is inevitable. This contact will lead to slider vibration and head-stack assembly (HSA) going offtrack. This paper focuses on HSA offtrack dynamics induced by slider protrusion-disk contact, which was observed to have significantly different characteristics with traditional HSA dynamics excited by a voice-coil motor (VCM). With VCM excitation, the most dominant modes are from the entire HSA structure. However, with slider protrusion touch down (SPTD) excitation, head gimbal assembly (HGA) local modes are dominant. Both numerical analysis and experimental verification were used to explore the major factors influencing this characteristic. It is found that the gain of HGA sway is very sensitive to bending location when slider is excited by SPTD. Moreover, if the HGA local modes are close to HSA modes, they are easily coupled, which makes the offtrack vibration increase significantly. These findings can be used as references for optimizing the design of the HGA and HSA in order to improve the offtrack dynamics of HSA.