An armoured vehicle-launched bridge
) is a combat support vehicle, sometimes regarded as a subtype of combat engineering vehicle
, designed to assist militaries in rapidly deploying tanks
and other armoured fighting vehicles
across rivers. The AVLB, usually built from a converted tank chassis (and hence a tracked vehicle), carries a folding metal bridge atop its chassis, often in 3 sections. The AVLB's job is to accompany armoured or infantry divisions to water crossings. When a river too deep for non-deep-wading vehicles is reached, and no bridge is conveniently located (or sufficiently sturdy, a substantial concern when moving 60-ton tanks), the bridge-layer unfolds and launches its bridge, a process that takes only minutes. Once the span has been emplaced, the AVLB decouples from its bridge, and moves aside to allow traffic to pass. Once all of the vehicles have been moved, it reattaches to the bridge from the other side and retracts the span. A similar procedure can be employed to allow crossings of small chasms or similar obstructions. AVLBs can carry bridges of 60 feet (19 meters) or greater in length. By using a tank chassis, the bridge-layer is able to cover the same terrain as main battle tanks
, and the provision of armour allows them to operate even in the face of enemy fire. However, this is not a universal attribute: some exceptionally sturdy 6x6 or 8x8 truck chassis have lent themselves to bridge-layer applications.
The roots of the modern AVLB can be found in World War I, at the dawn of tank warfare. Having developed tanks, British and French were confronted with the problem of mounting tank advances in the face of the trenches that dominated the WWI battlefields. Early engagements, such as at Cambrai demonstrated the tank's utility, but also highlighted its vulnerability to battlefield geography -- many early tanks found themselves ignominiously stuck in the trenches, having insufficiently long tracks to cross them (as at right). To counter this disadvantage, tanks (especially the common British Mark series) began to go into battle with fascines hanging over their bows, sometimes as simple as a bundle of heavy sticks. By dropping these into the trenches, they were able to create a wedge over which the tank could drive. Later, some tanks began to carry rails on their decks -- the first AVLBs.
World War II & subsequent use
It was in the World War II
era that that the importance of armoured bridge-layers, as well as combat engineering vehicles
and armoured recovery vehicles
, became fully clear. With the advent of Blitzkrieg
warfare, whole divisions had to advance along with tanks, which were suddenly far out-pacing the speed of infantry soldiers. Besides leading to the advent of self-propelled artillery
, mobile anti-aircraft
and armoured personnel carriers
, it became clear that functions like vehicle repair, mine-clearing, and the like would have to be carried out by armoured vehicles advancing along with tanks. Moreover, these forces would have to be able to cross all forms of terrain without losing speed, and without having to concentrate their thrusts over certain bridges (and the rising weight of armoured vehicles meant that fewer and fewer bridges could support these massed crossings). The only feasible solution to the dilemma posed by the mobility of all-mechanised armed forces was a dedicated platform that could improvise river and obstacle crossings at short notice and in otherwise inconvenient locations. Tracked and armoured, it was capable of operating right alongside combat units, crossing rough terrain and advancing in the face of light fire. To maximize on common parts and ease maintenance complications, they were usually based on existing tank chassis.
One of the earliest series-produced examples is the Brückenleger IV, a German AVLB, which entered Wehrmacht service in 1940; 20 were built. Problems of excessive weight limited the vehicle's effectiveness, and eventually all 20 were converted back to tanks, but the trend was underway. The Allies developed similar equipment, mostly based on the ubiquitous Churchill and Sherman medium tanks of the British and U.S. armies, respectively. In some early designs, bridge-layers could emplace bridges, but not retract them. Other AVLBs were integral to the bridge themselves, wading to the middle of a river and extending simple ramps in both directions (such as the Churchill Ark); following vehicles would drive directly over the bridge-layer.
Having proven their utility in the war, and being especially useful for amphibious operations, many major tank operators (at least, those with rivers or similar obstacles to cross) employ some number of bridgelayers, usually converted from existing tank designs (sometimes using old tank designs no longer useful for front-line combat service, as in the M60A1 AVLB. A number of designs, including the French PTA 2, now use 6x6- or 8x8-wheel trucks as their base instead of tracked chassis.
Notable AVLBs in service