The crew undertook a scheduled flight departing from Paris – Charles de Gaulle (France) bound for Toronto (Canada). The scheduled block time was 11:56 and push-back started at 11:44. The push-back operation from stand A38 was carried out by personnel from two separate handling companies: the headset operator works for Air France Industries and the push-back tractor was driven by two agents employed by Airlines Ground Service (AGS). The driver was a trainee completing his training and accompanied by his instructor. At the end of the push-back, when the aeroplane was nearly on the centreline of the taxiway, the tractor reached its maximum turn angle. The instructor on board the tractor asked the driver to apply the tractor parking brake, alighted the tractor then initiated the operations necessary to disconnect the push-back bar. He actuated the hydraulic extension system of the retractable wheel gear of the tow bar then asked the driver of the tractor to raise the drawbar pin ensuring connection of the bar with the tractor. During this operation, the aeroplane, on which the parking brake was not applied, moved forwards and one of the bar wheels rolled over the right foot of the instructor who became trapped. Under stress, the bar suddenly unhooked from the tractor and hit the instructor?s right leg. One of the bar?s shear pins was found ruptured.The statements collected differ in such a way that the investigation was unable to determine the exact sequence of the accident, in particular: the exact time of the shear pin rupture; whether signals were exchanged between the headset operator and the agents on board the tractor. The fact is that the driver?s instructor alighted and started to remove the tow bar without the aeroplane parking brake being applied and without blocks in position. During disconnection of the bar, a forward movement of the aeroplane led to the immobilisation of the agent, his foot trapped under a tow bar wheel. This agent was then struck by the tow bar after it was released.