An aeroplane in flight passes through countless particles of air in the atmosphere. These particles flow in a direction that is exactly opposite to the aeroplane's flight path direction. This is relative airflow, and it's important to remember that the relative airflow changes direction as the aeroplane's flight path changes direction.

With respect to the motion of the aeroplane, these particles form straight lines of flow. Let's have a closer look at what happens to this airflow when it flows around the aerofoil of a swept wing design aeroplane. As the angle of attack increases towards the critical angle of attack, the wingtips of a swept wing aeroplane tend to stall first.

From here, the upper surface airflow produces a wake of turbulent and slower airflow behind the wingtips. The swept wing design means that the wings are angled backwards, so the wingtips are located further aft, towards the tail. The reduced lift, due to the stalled state at the wingtips, causes the center of pressure to shift forward, towards the wing roots.

Forward center of pressure creates an unstable nose pitch up moment. Reduced lift and more drag cause the aeroplane to sink. The separated airflow from the stalled wings immerses the tailplane in the low energy turbulent airflow. The elevator effectiveness is reduced, hence the pilot is unable to recover from the stall.

Common to T tail aeroplane design, this is called a deep stall or super stall, and may cause the aeroplane o become unrecoverable.