Airspeed indicator
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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.
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[edit] Markings
[edit] Light aircraft
A redline mark indicates VNE, or velocity (never exceed). This is the absolute maximum 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 minimum steady flight 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 blue line near to the bottom of green arc for Vmc, the speed below which full rudder is insufficient to keep the aircraft from yawing at full power with the critical engine inoperative and a blue line for VYSE, the speed for best rate of climb with the critical engine inoperative.
The white range is the normal range of operating speeds for the aircraft with the flaps down. The green range is the normal range of operating speeds for the aircraft with the flaps up. 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.
One of the most important V Speeds that is not marked on airspeed indicators for light aircraft is VA (maneuvering speed, the speed at which full and abrupt control movement can be applied without the possibility of causing structural damage, and, separately, the maximum speed at which the aircraft can be flown in turbulent conditions).
[edit] 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 VA and VNE like piston-engined aircraft, but instead have a maximum operating IAS, VMO and maximum Mach number, MMO. The maximum permissible speed is indicated by a needle which 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.
[edit] Operation
Along with the altimeter and vertical speed indicator, the airspeed indicator is a member of the pitot-static group 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 port(s) and total pressure due to "ram air", captured through a pitot tube. This rise 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, without any disturbance from the passage 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.
Pitot tubes face forward, in the direction of flight. Icing is a problem for pitot tubes when visible moisture is present in the atmosphere, as when flying through clouds or precipitation. Electrically heated pitots are used to prevent clogging with ice.
The airspeed indicator is rendered inoperative by blockage in the static system. To prevent this, most aircraft intented for use in instrument meteorological conditions are equipped with an alternate source of static air. This is usually less accurate, but is still workable.
[edit] Use
The primary use of the airspeed indicator is to provide guidance during climb, descent and landing, so that an appropriately slow airspeed is maintained while still operating safely above stall speed and without entering slow flight. During approach and landing, the aircraft is typically operated at airspeeds specified by air traffic control or the aircraft's operating handbook depending on conditions and the phase of the approach. The airspeed indicator is also important for ensuring that structural speeds are not exceeded, beyond which the airframe may be stressed and damaged.
During instrument flight, the airspeed indicator is the primary instrument of reference for pitch control during climbs and descents, and a secondary instrument of reference for pitch control during cruise and turns.
The airspeed indicator is also used in dead reckoning, where time, speed, and bearing are used for navigation.
[edit] Alternatives
The "lift reserve indicator", or LRI, has been proposed but poorly received as an alternative to the airspeed indicator for use during critical stages of flight. The LRI shows the margin of speed above stall speed. Since indicated stall speed varies with conditions (particularly gross weight), the LRI is simpler to use.
Some aircraft are equipped with a ground speed display, which is calculated by radionavigation equipment.
[edit] Types of airspeed measurements
At high Density Altitude, the airspeed indicator will show a lower speed than the aircraft's true airspeed (TAS), but aerodynamically, the same indicated airspeeds (IAS) apply. 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 locations for the static ports which accurately sense static pressure at all speeds and angles of attack.
Because Bernoulli's principle states that total pressure is constant along a streamline, there is usually little or no position error due to sensing pitot tube pressure.
Position error is typically small (a few percent), and, for small planes, the IAS will be lower than CAS at slow speeds and higher than CAS at high speeds. 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 10,000 feet and 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.
