tank mobility can be viewed in sharp contrast to that of wheeled armoured fighting vehicles. The majority of blitzkrieg operations were conducted at the pedestrian pace of , and that was only achieved on the roads of France. The tank's power plant supplies kinetic energy to move the tank, and electric power via a generator to components such as the turret rotation motors and the tank's electronic systems. The tank power plant has evolved from predominantly petrol and adapted large-displacement aeronautical or automotive engines during World Wars I and II, through diesel engines to advanced multi-fuel diesel engines, and powerful (per unit weight) but fuel-hungry gas turbines in the T-80 and M1 Abrams. Operational mobility Strategic mobility Strategic mobility is the ability of the tanks of an armed force to arrive in a timely, cost effective, and synchronized fashion. For good strategic mobility transportability by air is important, which means that weight and volume must be kept within the designated transport aircraft capabilities. Nations often stockpile enough tanks to respond to any threat without having to make more tanks as many sophisticated designs can only be produced at a relatively low rate. The US for instance keeps 6,000 MBTs in storage. In the absence of combat engineers, most tanks are limited to fording small rivers. The typical fording depth for MBTs is approximately , being limited by the height of the engine air intake and driver's position. Modern tanks such as the Russian T-90 and the German Leopard 1 and Leopard 2 tanks can ford to a depth of when properly prepared and equipped with a snorkel to supply air for the crew and engine. Tank crews usually have a negative reaction towards deep fording but it adds considerable scope for surprise and tactical flexibility in water crossing operations by opening new and unexpected avenues of attack. Amphibious tanks are specially designed or adapted for water operations, such as by including snorkels and skirts, but they are rare in modern armies, being replaced by purpose-built amphibious assault vehicles or armoured personnel carriers in amphibious assaults. Advances such as the EFA mobile bridge and armoured vehicle-launched scissors bridges have also reduced the impediment to tank advance that rivers posed in World War II. Crew Most modern tanks most often have four crew members, or three if an auto-loader is installed. These are the: Commander – The commander is responsible for commanding