Strafing is the military practice of attacking ground targets from low-flying aircraft using aircraft-mounted automatic weapons ranging from machine guns (5mm to 20mm) to autocannons or rotary cannons (typically 20mm to 37mm). This means that, although ground attack using automatic weapons fire is very often accompanied with bombing or rocket fire, the term "strafing" does not specifically include the last two. The term "strafing" can cover either fixed guns, or aimable (flexible) guns. Fixed guns firing directly ahead tend to be more predominant on fixed wing aircraft, while helicopters tend to use aimable weapons which can be fired in many different directions independent of the direction the aircraft is pointing in (in most cases, flexible guns on a fixed wing aircraft are for defense purposes only, although they can sometimes be used to fire on ground targets to limited effect). Some fixed wing aircraft, like fighter-bombers, are capable of flying either air-combat missions or ground attack missions (P-47 Thunderbolt), while others are dedicated ground-attack types (Il-2 Shturmovik). In cases where an aircraft is capable of both types of combat, when it is assigned to a ground attack role, and thus expected to be using the guns mostly for strafing, the fixed weapons are often mounted so that the convergence point is lower and at a greater range than would be used for air combat. This is helpful because it allows the pilot to aim at a target without having to dive towards the ground as steeply, decreasing the risk of collision with the ground and increasing the amount of firing time available before having to pull up, and it also increases the range from the target, helping avoid anti-aircraft fire and potential damage from exploding targets. Because of the low altitude and relatively low airspeed required for accurate strafing, it is very risky for the pilot, who is exposed not only to the risk of flight-into-terrain and obstacles such as power lines, but also to anti-aircraft weapons, including surface-to-air missiles (both vehicle mounted and hand-held), anti-aircraft artillery and small caliber weapons fire (such as machine guns and small arms). Planes purposely designed for ground attack may include additional armour around and underneath the cockpit and other vulnerable areas such as engines to protect the pilot and key flight components, while aircraft designed mostly for air combat tend to have most of their armor placed to protect directly ahead or to the rear, where fire from other aircraft is most likely, leaving them more vulnerable to fire from directly below or to the sides, where much ground fire often comes from.