At cyclic loads (and every flight of an airliner has a cyclic load), the fuselage of a sealed aircraft is slightly inflated by internal pressure at altitude and returns to its original dimensions upon landing. This leads to metal fatigue. At the same time, microcracks appeared in the places of riveting, and the corners of square portholes, as well as similar zones around hatches, served as places of stress concentration. Therefore, they are oval.
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