The basic means of controlling the aircraft is by making use of the position of the engine. If the engines are mounted under the center of gravity, as is the case in most passenger jets, then increasing the thrust will raise the nose, while decreasing the thrust will lower it. This control method may call for control inputs that go against the pilot's instinct: when the aircraft is in a dive, adding thrust will raise the nose and vice versa. Additionally, asymmetrical thrust may be used for directional control: if the left engine is idled and power is increased on the right side this will result in a yaw to the left, and vice versa. If throttle settings allow the throttles to be shifted without affecting the total amount of power, then yaw control can be combined with pitch control. If the plane is yawing, then the wing on the outside of this yaw movement will go faster than the inner wing. This creates higher lift on the faster wing, resulting in a rolling movement, which helps to make a turn. Controlling speed is very difficult with engine control only, and will most likely result in a fast landing. A fast landing would be required anyway if the flaps cannot be extended due to loss of hydraulics. Only jet aircraft with an engine mounted on the vertical tail in addition to wing-mounted power plants will be able to control the speed to a higher degree, as this engine is on the fuselage centerline and above the center of gravity. Aircraft that have two or four engines mounted on the sides of the empennage will only have limited benefit from asymmetrical thrust. Because this type of aircraft control is difficult for humans to achieve, some researchers have attempted to integrate this control ability into the computers of fly-by-wire aircraft. Early attempts to add the ability to real aircraft were not very successful, the software having been based on experiments conducted in flight simulators where jet engines are usually modeled as "perfect" devices with exactly the same thrust on each engine, a linear relationship between throttle setting and thrust, and instantaneous response to input. Later, computer models were updated to account for these factors, and aircraft have been successfully flown with this software installed. However, it remains a rarity on commercial aircraft.