The airspeed indicator needs input from both the pitot and static sources to be able to display airspeed. If either fails, the indicator will misread or fail to read completely.
Indicators can be marked in mph, kph or knots. Some may have both mph and knots, in which case be sure which scale you are reading.
Markings
The airspeed indicator has coloured arcs to alert you to airspeed limits, and you should be aware of their significance.
The white arc
The low-speed end of the arc indicates the power-off stall speed in the landing configuration (VS0), and the high-speed end indicates the maximum allowable speed with flaps fully extended (VFE).
The green arc
The low-speed end of the arc indicates the power-off stall speed in a specified configuration, usually gear and flaps up (VS1), and the high-speed end indicates maximum structural cruising speed (VNO) which should be exceeded only in smooth air.
The yellow arc
This is the caution range and pilots should avoid operating at airspeeds in this range except when in smooth air.
The red line
This is the aircraft’s never-exceed speed (VNE). Destructive aerodynamic forces may result from flight at or above the red line.
Speeds not marked
Every airplane has a design manoeuvring speed (VA), which is the optimum speed in turbulence at maximum gross weight. Manoeuvring speed is reduced as weight is reduced. Flight at manoeuvring speed ensures that the airplane will stall before damaging aerodynamic loads are imposed on the wing structure.
Best rate-of-climb speed (VY) will give the greatest gain in altitude in a given time, better visibility over the nose, and better engine cooling. If getting to altitude quickly is your goal, use VY.
Best angle-of-climb speed (VX) is the speed which gives the most altitude in a given distance. You should fly at VX to clear obstructions and only to clear obstructions. Flight at VX requires a steep pitch attitude, so engine cooling airflow and visibility are both reduced. Climb at VX only as long as necessary to clear the obstruction, then accelerate to VY.
Although you should be aware of the aircraft’s ideal V speeds, you should keep in mind that these speeds are affected by the weight of the aircraft, the air temperature, humidity, crosswinds and other factors.