Airspeed indicator: Difference between revisions
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[[Atmospheric icing|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. |
[[Atmospheric icing|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. |
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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 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. |
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== Use == |
== Use == |
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Revision as of 03:22, 20 February 2008
 | This article includes a list of references, related reading, or external links, but its sources remain unclear because it lacks inline citations. (February 2008) |
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.
Markings
Light aircraft
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 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 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.
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 causing structural damage.
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.
Operation
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 dynamic 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, alternative placement 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.
Use
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.
During instrument flight, the airspeed indicator is used in addition to the Artificial horizon as an instrument of reference for pitch control during climbs, descents and turns.
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 NBDs, VORs or GPS.
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.
Types of airspeed measurements
At high Density Altitude, the airspeed indicator will show a lower indicated airspeed than the aircraft's true airspeed (TAS), but the same indicated airspeed limits (IAS) 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 10,000 feet (3,000 m) 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.
See also
Sources
- 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.
This article incorporates text from this source, which is in the public domain: Airplane Flying Handbook
This article incorporates text from this source, which is in the public domain: Instrument Flying Handbook
This article incorporates text from this source, which is in the public domain: Pilot's Handbook of Aeronautical Knowledge