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The airspeed indicator or airspeed gauge is an instrument used in an aircraft to display the craft's airspeed, typically in knots, to the pilot. In its simplest form, an ASI measures the difference in pressure between that which is generally around the craft and the increased pressure caused by propulsion. The needle tracks pressure differential but the dial is marked off as airspeed.
The airspeed indicator is used by the pilot during all phases of flight, from take-off, climb, cruise, descent and landing in order to maintain airspeeds specific to the aircraft type and operating conditions as specified in the Operating Manual.
On light aircraft 
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 colored 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.
A redline mark indicates VNE, 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 VNO (maximum structural cruise speed) to VNE. A green band runs from VS1 to VNO. VS1 is the stall speed with flaps and landing gear retracted. A white band runs from VSO to VFE. VSO is the stall speed with flaps extended, and VFE 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 Vmc, the minimum indicated airspeed at which the aircraft can be controlled with the critical engine inoperative and a blue line for VYSE, the speed for best rate of climb with the critical engine inoperative.
On large aircraft 
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.
Jet aircraft do not have VNO and VNE like piston-engined aircraft, but instead have a maximum operating IAS, VMO and maximum Mach number, MMO. To observe both limits, the pilot of a jet airplane needs both an airspeed indicator and a Machmeter, each with appropriate red lines. In some general aviation jet airplanes, the Machmeter is combined into a single instrument that contains a pair of concentric indicators, one for the indicated airspeed and the other for indicated Mach number.
An alternative single instrument is the "maximum allowable airspeed indicator." It has a movable pointer that indicates the never-exceed speed, which changes with altitude to avoid the onset of transonic shock waves on the wing. The pointer is usually red-and-white striped, and thus known as a "barber pole". As the aircraft climbs to high altitude, such that MMO rather than VMO becomes the limiting speed, the barber pole moves to lower IAS values.
Modern aircraft employing glass cockpit instrument systems employ two airspeed indicators: an electronic indicator on the primary flight data panel and a traditional mechanical instrument for use if the electronic panels fail. The airspeed is typically presented in the form of a "tape strip" that moves up and down, with the current airspeed in the middle. The same color scheme is used as on a mechanical airspeed indicator to represent the V speeds.
Along with the altimeter and vertical speed indicator, the airspeed indicator is a member of the pitot-static system of aviation instruments, so named because they operate by measuring pressure in the pitot and static circuits.
Airspeed indicators work by measuring the difference between static pressure, captured through one or more static ports; and stagnation pressure due to "ram air", captured through a pitot tube. This difference in pressure due to ram air is called impact pressure.
The static ports are located on the exterior of the aircraft, at a location chosen to detect the prevailing atmospheric pressure as accurately as possible, that is, with minimum disturbance from the presence of the aircraft. Some aircraft have static ports on both sides of the fuselage or empennage, in order to more accurately measure static pressure during slips and skids. Aerodynamic slips and skids cause either or both static ports and pitot tube(s) to present themselves to the relative wind in other than basic forward motion. Thus, the alternative placements on some aircraft.
Icing is a problem for pitot tubes when the air temperature is below freezing and visible moisture is present in the atmosphere, as when flying through cloud or precipitation. Electrically heated pitot tubes are used to prevent ice forming over the tube.
The airspeed indicator and altimeter will be rendered inoperative by blockage in the static system. To avoid this problem, most aircraft intended for use in instrument meteorological conditions are equipped with an alternate source of static pressure. In unpressurised aircraft, the alternate static source is usually achieved by opening the static pressure system to the air in the cabin. This is less accurate, but is still workable. In pressurised aircraft, the alternate static source is a second set of static ports on the skin of the aircraft, but at a different location to the primary source.
The Lift Reserve Indicator (LRI) has been proposed as an alternative or backup to the Airspeed Indicator (ASI) during critical stages of flight. This is an elegant device but is rarely found in light aircraft or even transport jets. The conventional Airspeed Indicator is less sensitive and less accurate as airspeed diminishes, thus providing less reliable information to the pilot as the aircraft slows towards the stall. The actual stall speed of an aircraft also varies with flight conditions, particularly changes in gross weight and wing loading during maneuvers. The ASI does not show the pilot directly how the stall is being approached during these maneuvers, whereas the LRI does.
The LRI shows the pilot directly the Potential of Wing Lift (POWL) above the stall at all times and at any airspeed, so it is more descriptive and easier for the pilot to use. The LRI uses dynamic differential pressure and Angle of Attack to operate. It is very fast acting and extremely accurate at low airspeeds, thus providing more reliable information to the pilot as airspeed diminishes and becomes critical.
The LRI uses a three zone, red-white-green display. During flight, the green zone is well above the stall where flight controls are firm, angle of attack is low, and the unused POWL is high. The white zone is near the stall where flight controls soften, angle of attack is high, and the unused POWL is diminished. The top of the red zone defines the beginning of the stall. The severity of stall increases as the needle travels deeper into the red. During the takeoff, the LRI uses dynamic pressure to operate and will not lift the needle above the red zone until enough airspeed energy is available to fly.
The pilot adjusts the instrument to indicate the edge of the red-white zone during minimum airspeed practice at altitude, indicating the aircraft has zero POWL beyond that point. Since the wing will stall at the same angle of attack at any airspeed, once properly adjusted the LRI will indicate the red-white edge anytime the stall is approached. This includes landing stalls, climbing stalls, and accelerated stalls. After adjustment, the black line in the center of the white indicates maximum angle of climb and maximum angle of descent with enough reserve lift for the landing flare. With practice, the pilot can use the LRI to determine the exact moment for liftoff with minimum ground roll and maximum angle of climb combined.
The LRI has been well received by STOL pilots and pilots of experimental or home-built aircraft. The LRI is very useful for short field landings, short field takeoffs, and slow speed maneuvers such as steep turns, steep climbs, and steep descents, and also allows pilots of fast or "slippery" aircraft to land with little or no float very reliably. Since the LRI is so useful at the critical lower end of the flight envelope, most pilots will use the LRI as a complement to the ASI, using the LRI for slow speed work and the ASI for cruising and navigational work.
Types of airspeed measurements 
||This section may require cleanup to meet Wikipedia's quality standards. (July 2009)|
Memory aid: "ICE-T" (iced tea), or Indicated->Calibrated->Equivalent->True.
Another memory aid: This is a Pretty Cool Drink, giving the errors compensated for between the speeds Position/Pressure, Compression, and Density.
At increased altitude (more accurately, density altitude), for the same given indicated airspeed (IAS) the aircraft's true airspeed (TAS) will be higher, but the same indicated airspeed limits apply. Likewise, most efficient cruise speed, total drag, available lift, stall speed, and other aerodynamic information depend on calibrated, not true airspeed. Most aircraft exhibit a small difference between the airspeed actually shown on the instrument (indicated airspeed, or IAS) and the speed the instrument should theoretically show (calibrated airspeed or CAS). This difference, called position error, is mainly due to inaccurate sensing of static pressure. It is usually not possible to find a position for the static ports which, at all angles of attack, accurately senses the atmospheric pressure at the altitude at which the aircraft is flying.
Bernoulli's principle states that total pressure is constant along a streamline. Pitot pressure is equal to total pressure so pitot pressure is constant all around the aircraft and does not suffer position error. (However, pitot pressure can suffer alignment error if the pitot tube is not aligned directly into the oncoming airflow.)
The position of static ports must be selected carefully by an aircraft designer because position error must be small at all speeds within the operating range of the aircraft. A calibration chart specific to the type of aircraft is usually provided.
At high speeds and altitudes, calibrated airspeed must be further corrected for compressibility error to give equivalent airspeed (EAS). Compressibility error arises because the impact pressure will cause the air to compress in the pitot tube. The calibration equation (see calibrated airspeed) accounts for compressibility, but only at standard sea level pressure. At other altitudes compressibility error correction may be obtained from a chart. In practice compressibility error is negligible below about 3,000 m / 10,000 feet and 100 m/s / 200 knots CAS.
The true airspeed can be calculated as a function of equivalent airspeed and local air density, (or temperature and pressure altitude which determine density). Some airspeed indicators incorporate a slide rule mechanism to perform this calculation. Otherwise, it can be performed with a calculator such as the E6B handheld circular slide rule. For a quick approximation of TAS add 2% per 300m / 1000 feet of altitude to IAS (or CAS). e.g. IAS = 52 m/s /100 Knots. At 3000 m / 10,000' Above Sea Level, TAS is 62 m/s / 120 Knots.
See also 
- Airplane Flying Handbook. U.S. Government Printing Office, Washington D.C.: U.S. Federal Aviation Administration. 2004. FAA-8083-3A.
- Instrument Flying Handbook. U.S. Government Printing Office, Washington D.C.: U.S. Federal Aviation Administration. 2005-11-25. FAA-H-8083-15.
- Pilot's Handbook of Aeronautical Knowledge. U.S. Government Printing Office, Washington D.C.: U.S. Federal Aviation Administration. 2003. FAA AC 61-23C.
Installing and flying the Lift Reserve Indicator, article and photos by Sam Buchanan http://home.hiwaay.net/~sbuc/journal/liftreserve.htm