AIR SPEED INDICATOR (ASI)
This is the aircraft's speedometer and indicates the aircraft's air speed, norm
ally measured in knots (1 knot is a unit of speed = 1 nautical mile per hour) rather than MPH. In no wind conditions while at lower altitudes the speed over the ground will be the same as the airspeed. i.e. if the ASI indicates 100kts the aircraft will take 1 hour to cover 100 nautical miles. If however the aircraft is flying into a head wind of say 10kts the ASI will still indicate 100kts but the aircraft will only cover 90 (100-10 = 90) nautical miles in one hour. With a 10kts tail wind the ASI will still indicate 100kts but the aircraft will cover 110 nautical miles in one hour.
Due to a condition known as density altitude ( air pressure is less at altitude) the ASI under reads and the higher the aircraft goes the greater the inaccuracy. Therefore if the aircraft is flying above say 3000 feet the pilot must make allowances for this inaccuracy. This allowance can be made by an adjustment on the ASI or by calculating the error and making the necessary allowance.