Note: a duplicate article is at airspeed indicator
The Airspeed Indicator is an instrument pressent on nearly all aircraft panels. Together with the altimeter, it is the most important instrument in the panel. The altimeter, airspeed indicator, and vertical speed indicator are the members of the pitot-static group of instruments, so named because they operate by measuring pressure in the pitot and static circuits.
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2 Uses 3 Alternatives 4 Types of airspeed measurements 5 Markings |
Airspeed indicators work by measuring the difference between static pressure, captured through one or more static port(s) and dynamic pressure, captured through a pitot tube. 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.
The pitot tube accumulates "ram air", that is, air forced against the opening of the tube by the passage of the aircraft. 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 meterological conditions are equipped with an alternate source of static air. This is usually less accurate, but is still workable.
The most important use of the airspeed indicator is to provide guidance during slow flight, particularly landings, so that as low an airspeed as possible is maintained while still operating safely above stall speed. During approach and landing, the aircraft is typically operated at specific airspeeds depending on conditions and the phase of the approach (downwind, base, final, for visual approaches).
During instrument flight, the airspeed indicator is primary for pitch control during climbs and descents, and secondary for pitch control during cruise and turns. A skilled pilot can fly the aircraft using only the altimeter and airspeed indicator, and a navigation source, during instrument conditions.
The airspeed indicator is also used in dead reckoning, where time, speed, and bearing are used for navigation.
There are some advocats of the "lift reserve indicator", or LRI, as a better alternative to the airspeed indicator, at least for slow 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.
Most aircraft have an inherent difference between calibrated (theoretical) airspeed (CAS) and the airspeed actually shown on the instrument (indicated airspeed, or IAS). This error is due to interference between the structure of the aircraft and the pitot tube. The placement of the pitot tube is chosen to minimize this error. It 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 is usually provided but rarely used.
The true airspeed (TAS), can be calculated as a function of temperature and pressure altitude. Some airspeed indicators incorporate a slide rule mechanism to perform this calculation. Otherwise, it can be performed with and E6-b type device.Operation
Uses
Alternatives
Types of airspeed measurements
