WR963 relies on pneumatics for some of her systems. Apart from the tyres, pressurised gas is also required in the main gear oleos (suspension) and also the brakes.
Unlike air, which contains water vapour and oxygen, nitrogen is dry and inert, reducing corrosion and the risk of fire or explosion.
Although nitrogen is readily available as 80% of the air we breath, it has to be purified and pressurised for us to use it. The bottles containing it are big and heavy, hence the trolley to move them around.
The bottles are connected to a control panel under the cover on the end of the trolley enabling us to control the pressure supplied via a long hose which is connected to the valves on whichever aircraft system requires a top-up.