A final element that has not been mentioned yet is the chap who mans the headset during pushback. It used to be a Mech/Tech/Eng. It is now an appropriatly trained individual who, depending on the operator/ground handling agent, may have years or only hours of experience.
On of their tasks prior to push back is to do a walk round to ensure all doors and hatches are secure.
I'm wondering if all the hire and fire brigade will want their head on a plate too.