Can px4 detect free fall and start flying?


I am just wondering if px4 has functionality that it can detect free fall and automatically arm motors and starts flying instantly.
Or at least there is any message which tells the drone is free falling?


Yes it can detect Freefall, that is all I can tell you, since I saw this in a Log when I had a misconfiguration of my ESC (Motors stopped completely in Mid Air due to Sync Issues). But I think you can configure Emergency Devices like Parachutes and others to trigger on this. Most LIkely it would also show up in QGC Messages.