American Fairchild Republic A-10 Thunderbolt II and the Soviet-built Sukhoi Su-25 ground attack aircraft, as well as the Mil Mi-24 Hind ground-attack helicopter, the pilot sits in a titanium enclosure known as the "bathtub" for its shape. In addition, the windscreens of larger aircraft are generally made of impact-resistant, laminated materials, even on civilian craft, to prevent damage from bird strikes or other debris. Armoured fighting vehicles The most heavily armoured vehicles today are the main battle tanks, which are the spearhead of the ground forces, and are designed to withstand anti-tank guided missiles, kinetic energy penetrators, high-explosive anti-tank weapons, NBC threats and in some tanks even steep-trajectory shells. The Israeli Merkava tanks were designed in a way that each tank component functions as added back-up armour to protect the crew. Outer armour is modular and enables quickly replacing damaged parts. Layout For efficiency, the heaviest armour on an armoured fighting vehicle (AFV) is placed on its front. Tank tactics require the vehicle to always face the likely direction of enemy fire as much as possible, even in defence or withdrawal operations. Sloping and curving armour can both increase its protection. Given a fixed thickness of armour plate, a projectile striking at an angle must penetrate more armour than one impacting perpendicularly. An angled surface also increases the chance of deflecting a projectile. This can be seen on v-hull designs, which direct the force of an Improvised explosive device or landmine away from the crew compartment, increasing crew survivability. Spall liners Beginning during the Cold War, many AFVs have spall liners inside of the armour, designed to protect crew and equipment inside from fragmentation (spalling) released from the impact of enemy shells, especially high-explosive squash head warheads. Spall liners are made of Kevlar, Dyneema, Spectra Shield, or similar materials. Appliqué Appliqué armour, or add-on armour, consists of extra plates mounted onto the hull or turret of an AFV. The plates can be made of any material and are designed to be retrofitted to an AFV to withstand weapons that can penetrate the original armour of the vehicle. An advantage of appliqué armour is the possibility to tailor a vehicle's protection level to a specific threat scenario. Improvised Vehicle armour is sometimes improvised in the midst of an armed conflict by vehicle crews or individual units. In World War II, British, Canadian and Polish tank crews welded spare strips of tank