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AP4ATCO - Speed Indication in Aircraft

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Indicated Airspeed

An aneroid instrument, the airspeed indicator measures the dynamic pressure of the outside air entering a pitot tube. At sea level, and an atmospheric pressure of 1013.2 mb, and with no wind effect, the airspeed indicated is the true speed of the aircraft relative to the surface. As the aircraft climbs, the air density decreases and the indicated speed will be less than the True Air Speed (TAS). However, when it comes to controlling the aircraft, because the flight characteristics of the aircraft also alter with reduction in atmospheric density, the indicated airspeed is of greater importance than the true airspeed. This is why control speeds (e.g. V1, V2, etc) are given as KIAS (Knots Indicated Airspeed). The ASI uses the aircraft pitot-static system to compare pitot and static pressure and thus determine forward speed.

Airspeed Indicator (ASI)

Airspeed is usually measured (and indicated) in knots (nautical miles per hour) although other units of measurement are sometimes encountered.

On older aircraft, airspeed is usually indicated to the pilot on a graduated scale over which a pointer moves (left hand picture below.

In modern aircraft, it is usually indicated on a speed tape which forms part of the Electronic Flight Instrument System display (left hand side of right-hand picture below).

Conventional ASI
Conventional ASI
Speed tape on left hand side of EADI
Speed tape on left hand side of EADI

In a simple ASI, pitot pressure is fed into a barometic capsule which is located in a sealed container (usually the rear of the instrument) fed with static pressure. One end of the capsule is fixed; the other is connected to the instrument pointer by a suitable mechancial system.

Air Speed Indicator
Schema of a simple ASI

Mach Number

The ratio between the the true air speed (TAS) and the local speed of sound (LSS). This ratio, which equals one when the TAS is equal to the LSS, is known as the Mach Number (M) and is very important in aircraft operating at high speed.

Mach Meter

The machmeter uses the aircraft pitot-static system to generate M and usually portrays this on a simple needle and dial instrument, such as that shown below.


Alternatively, the machmeter may be combined with the Air Speed Indicator (ASI), in which case it is often referred to as a Combined Speed Indicator (CSI).

High speed aircraft, including airliners and business jets, have limiting mach numbers which must not be deliberately exceeded. If the aircraft is deliberately or accidentally allowed to exceed its limiting mach, shock waves are likely to form on the aerofoils and can result in buffet or mach tuck.

Some aircraft use a constant mach number (rather than constant speed) technique for cruise operations. Constant mach technique may be used to separate aircraft on the same track and at the same altitude whilst in a non radar environment.

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