@pilotquiz: What you’re seeing on this A321 is a transonic shock wave made visible by light refraction. In cruise, the aircraft may be flying around Mach 0.78–0.82, but the airflow over the upper surface of the wing accelerates locally beyond Mach 1. That creates a small supersonic region above the wing. Further downstream, the pressure recovery forces the flow back to subsonic speed through a shock wave. Across that very thin region, velocity drops sharply while pressure, temperature, and density rise abruptly. The “line” you see is not the aircraft breaking the sound barrier, it’s the sharp density change around the shock bending sunlight slightly, creating a shadow/refraction effect on the wing. Its position can move with turbulence, angle of attack, Mach number, and local wing loading, which is why it may appear to shimmer or “walk” along the wing. Feel free to follow me to see more aviation content! 🚀 #aviation #pilot #airplane #studentpilot

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Thursday 14 May 2026 16:00:20 GMT