This decreases the lift the wing produces at the tip to improve  the stall characteristics of the wing. Also called washout. Two-stroke engine.  A simple form of reciprocating engine  that completes its operating cycle in two strokes of its  piston, one down and one up. Two-stroke-cycle engines are  inefficient in their use of fuel, but their simplicity makes  them popular for powering light-sport aircraft and ultralight  vehicles where light weight and low cost are paramount. Ultralight.  A single person only vehicle as defined by 14  CFR section 103.1. Uncontrolled airspace.  Class G airspace that has not been  designated as Class A, B, C, D, or E. It is airspace in which  air traffic control has no authority or responsibility to control  air traffic; however, pilots should remember there are VFR  minimums which apply to this airspace. Unicom.  A nongovernment air/ground radio communication  station which may provide airport information at public use  airports where there is no tower or automated flight service  station. Unusable fuel.  Fuel that cannot be consumed by the engine.  This fuel is considered part of the empty weight of the  aircraft. Useful load.  The weight of the pilot, copilot, passengers,  baggage, usable fuel, and drainable oil. It is the basic empty  weight subtracted from the maximum allowable gross weight.  This term applies to general aviation aircraft only. Unstabilized approach.  The final approach of an aircraft that  has not achieved a stable rate of descent or controlled flight  track by a predetermined altitude, usually 500 feet AGL. Unusual attitude.  An unintentional, unanticipated, or  extreme aircraft attitude. Useful load.  The weight of the pilot, copilot, passengers,  baggage, usable fuel, and drainable oil. It is the basic empty  weight subtracted from the maximum allowable gross weight.  This term applies to general aviation aircraft only. User-defined waypoints.  Waypoint location and other data  which may be input by the user, this is the only GPS database  information that may be altered (edited) by the user. Upwind Leg.  A flight path parallel to the landing runway in  the direction of landing. Vapor lock.  A problem that mostly affects gasoline-fuelled  internal combustion engines. It occurs when the liquid fuel  changes state from liquid to gas while still in the fuel delivery  system. This disrupts the operation of the fuel pump, causing  loss of feed pressure to the carburetor or fuel injection system,  resulting in transient loss of power or complete stalling.  Restarting the engine from this state may be difficult. The fuel  can vaporise due to being heated by the engine, by the local  climate, or due to a lower boiling point at high altitude. VASI.  See visual approach slope indicator. Vehicle.  Manmade means of transportation; an ultralight  aircraft (not a light-sport aircraft). Venturi.  A specially shaped restriction in a tube designed  to speed up the flow of fluid passing through in accordance  with Bernoulli’s principle. Venturis are used in carburetors  and in many types of fluid control devices to produce a  pressure drop proportional to the speed of the fluid passing  through them. Venturi Effect.  The effect of Bernoulli’s principle, which  states that the pressure of a fluid decreases as it is speeded up  without losing or gaining any energy from the outside. Verified.  Confirmation of information or configuration  status. Vertical axis (yaw).  An imaginary line passing vertically  through the center of gravity of an aircraft. The vertical axis  is called the z-axis or the yaw axis. Vertical speed indicator (VSI).  An instrument that uses  static pressure to display a rate of climb or descent in feet  per minute. The VSI can also sometimes be called a vertical  velocity indicator (VVI). Vertigo.  A type of spatial disorientation caused by the  physical senses sending conflicting signals to the brain.  Vertigo is especially hazardous when flying under conditions  of poor visibility and may cause pilot incapacitation, but may  be minimized by confidence in the indication of the flight  instruments. VFR.  See visual flight rules. VFR Terminal Area Charts . Charts designated to depict  Class B airspace in greater detail and greater scale than  sectional charts.  G-18