the free stream. The fin height may need to be increased to restore its required effectiveness in certain flight conditions. The Panavia Tornado had a tall fin for directional stability at high angles of incidence. Trim and control in yaw The rudder is the directional control surface and is usually hinged to the fin or vertical stabilizer. Moving it allows the pilot to control yaw about the vertical axis, i.e., change the horizontal direction in which the nose is pointing. Maximum rudder deflection is usually controlled by a rudder travel limiter. The largest achievable angle of a rudder at a particular flight condition is called its blowdown limit. It represents a balance between the aerodynamic forces on the rudder and the mechanical forces from the actuating mechanism. Multi-engined aircraft, especially those with wing-mounted engines, have large powerful rudders. They are required to provide sufficient control after an engine failure on take-off at maximum weight and cross wind limit and cross-wind capability on normal take-off and landing. For taxiing and during the beginning of the take-off, aircraft are steered by a combination of rudder input as well as turning the nosewheel or tailwheel. At slow speeds the nosewheel or tailwheel has the most control authority, but as the speed increases the aerodynamic effects of the rudder increases, thereby making the rudder more and more important for yaw control. In some aircraft (mainly small aircraft) both of these mechanisms are controlled by the rudder pedals so there is no difference to the pilot. In other aircraft there is a special tiller controlling the wheel steering and the pedals control the rudder, and a limited amount of wheel steering (usually 5 degrees of nosewheel steering). For these aircraft the pilots stop using the tiller after lining up with the runway prior to take-off, and begin using it after landing before turning off the runway, to prevent over correcting with the sensitive tiller at high speeds. The pedals may also be used for small corrections while taxiing in a straight line, or leading in or out of a turn, before applying the tiller, to keep the turn smooth. With the controls in the neutral position, a plane may still gently yaw to one side. This is corrected through the setting of a trim surface, often a separate trim tab mounted on the rudder but sometimes the rudder itself, to counteract the yaw and ensure