trail edge of the control surfaces called "trim tabs". Trim is used to reduce the amount of pressure on the control forces needed to maintain a steady course. On wheeled types, Brakes are used to slow and stop the plane on the ground, and sometimes for turns on the ground. A craft may have two pilots' seats with dual controls, allowing two pilots to take turns. This is often used for training or for longer flights. The control system may allow full or partial automation of flight, such as an autopilot, a wing leveler, or a flight management system. An unmanned aircraft has no pilot but is controlled remotely or via means such as gyroscopes or other forms of autonomous control. Cockpit instrumentation On manned fixed-wing aircraft, instruments provide information to the pilots, including flight, engines, navigation, communications, and other aircraft systems that may be installed. The six basic instruments, sometimes referred to as the "six pack", are as follows: The airspeed indicator (ASI) shows the speed at which the plane is moving through the surrounding air. The attitude indicator (AI), sometimes called the artificial horizon, indicates the exact orientation of the aircraft about its pitch and roll axes. The altimeter indicates the altitude or height of the plane above mean sea level (AMSL). The vertical speed indicator (VSI), or variometer, shows the rate at which the plane is climbing or descending. The heading indicator (HI), sometimes called the directional gyro (DG), shows the magnetic compass direction that the plane's fuselage is pointing towards. The actual direction the plane is flying towards is affected by the wind conditions. The turn coordinator (TC), or turn and bank indicator, helps the pilot to control the plane in a coordinated attitude while turning. Other cockpit instruments might include: A two-way radio, to enable communications with other planes and with air traffic control. Airplanes built before World War II may not have been equipped with a radio but they are practically essential now in case of emergency. A horizontal situation indicator (HSI), to indicate the position and movement of the plane as seen from above with respect to the ground, including course/heading and other information. Instruments showing the status of each of the plane's engines (operating speed, thrust, temperature, and other variables). Combined display systems such as primary flight displays or navigation aids. Information displays such as onboard weather radar displays. A radio direction