At the initial development phase of stall (or during extreme maneuvers), airflow over the wing becomes unsteady; the separation line over the wing (or over any other lifting surface) keeps fluctuating. This causes the aircraft to shudder and is a warning to the pilot. The aircraft structure is not affected and is not necessarily at its maximum loading.

5.2.1 Flutter

This is the vibration of the structure – primarily the wing but also any other compo­nent depending on its stiffness. At transonic speed, the load on the aircraft is high while the shock-boundary layer interaction could result in an unsteady flow causing vibration over the wing, for example. The interaction between aerodynamic forces and structural stiffness is the source of flutter. A weak structure enters into flutter; in fact, if it is too weak, flutter could happen at any speed because the deformation would initate the unsteady flow. If it is in resonance, then it could be catastrophic – such failures have occurred. Flutter is an aeroelastic phenomenon.

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