Originally Posted by
Concours77
PineappleFrenzy.
Wound down sure. Absent a shaft brake or lock, what does windmilling do in such a circumstance?
Windmilling simply adds more opportunity for parts to shake loose. It also stresses the airframe.
Given the lack of vibration depicted in video of the SW1380 incident, I'd say the engine wasn't windmilling at all. The first two videos below show what it's like inside an airplane with a windmilling but unbalanced engine.
As seen in previous catastrophic engine failure events, often the initial shaking caused by an unbalanced high speed rotating mass is so strong that the engine simply sheds its exterior components (see the third video), if it isn't ripped from the pylon altogether. Even when blades remain within the containment ring, and even if friction prevents the engine from windmilling, it certainly seems possible for a piece of the cowling to catch the airflow at just the right angle to lead it directly into the side of the plane and damage a window as the engine winds down.
AirAsia X D7237:
United 1175:
Exterior shot of United 1175: