<P> The airspeed indicator is also used in dead reckoning, where time, speed, and bearing are used for navigation in the absence of aids such as NDBs, VORs or GPS . </P> <P> Airspeed indicators in many Light and Recreational aircraft can only show the pilot Indicated Airspeed (IAS). For True Airspeed (TAS) other components would have to be added by the manufacturer . Airspeed Indicator markings use a set of standardized coloured bands and lines on the face of the instrument . The white range is the normal range of operating speeds for the aircraft with the flaps extended as for landing or takeoff . The green range is the normal range of operating speeds for the aircraft without flaps extended . The yellow range is the range in which the aircraft may be operated in smooth air, and then only with caution to avoid abrupt control movement . </P> <P> A redline mark indicates V, or velocity (never exceed). This is the maximum demonstrated safe airspeed that the aircraft must not exceed under any circumstances . The red line is preceded by a yellow band which is the caution area, which runs from V (maximum structural cruise speed) to V. A green band runs from V to V. V is the stall speed with flaps and landing gear retracted . A white band runs from V to V. V is the stall speed with flaps extended, and V is the highest speed at which flaps can be extended . Airspeed indicators in multi-engine aircraft show a short radial red line near to the bottom of green arc for V, the minimum indicated airspeed at which the aircraft can be controlled with the critical engine inoperative and a blue line for V, the speed for best rate of climb with the critical engine inoperative . </P> <P> The airspeed indicator is especially important for monitoring V - Speeds while operating an aircraft . However, in large aircraft, V - speeds can vary considerably depending on airfield elevation, temperature and aircraft weight . For this reason the coloured ranges found on the ASIs of light aircraft are not used - instead the instrument has a number of moveable pointers known as bugs which may be preset by the pilot to indicate appropriate V - speeds for the current conditions . </P>

What does the lower limit of the green arc on an airspeed indicator represent