Indicated airspeed (IAS) is the
airspeed of an aircraft as measured by its
pitot-static system
A pitot-static system is a system of pressure-sensitive instruments that is most often used in aviation to determine an aircraft's airspeed, Mach number, altitude, and altitude trend. A pitot-static system generally consists of a pitot tube, a s ...
and displayed by the
airspeed indicator (ASI)
. This is the pilots' primary airspeed reference.
This value is not corrected for installation error,
instrument error, or the actual encountered
air density
The density of air or atmospheric density, denoted '' ρ'', is the mass per unit volume of Earth's atmosphere. Air density, like air pressure, decreases with increasing altitude. It also changes with variation in atmospheric pressure, temperature a ...
, being instead calibrated to always reflect the
adiabatic compressible flow of the
International Standard Atmosphere at sea level
.
It uses the difference between total pressure and static pressure, provided by the system, to either mechanically or electronically measure
dynamic pressure
In fluid dynamics, dynamic pressure (denoted by or and sometimes called velocity pressure) is the quantity defined by:Clancy, L.J., ''Aerodynamics'', Section 3.5
:q = \frac\rho\, u^2
where (in SI units):
* is the dynamic pressure in pascals ( ...
. The dynamic pressure includes terms for both density and airspeed. Since the airspeed indicator cannot know the density, it is by design calibrated to assume the
sea level standard atmospheric density when calculating airspeed. Since the actual density will vary considerably from this assumed value as the aircraft changes altitude, IAS varies considerably from
true airspeed (TAS), the relative velocity between the aircraft and the surrounding air mass.
Calibrated airspeed (CAS) is the IAS corrected for instrument and
position error.
[Clancy, L.J. (1975), ''Aerodynamics'', Section 3.9, Pitman Publishing Limited, London. ]
An aircraft's indicated airspeed in knots is typically abbreviated ''KIAS'' for "
Knots
A knot is a fastening in rope or interwoven lines.
Knot may also refer to:
Places
* Knot, Nancowry, a village in India
Archaeology
* Knot of Isis (tyet), symbol of welfare/life.
* Minoan snake goddess figurines#Sacral knot
Arts, entertainme ...
-Indicated Air Speed" (vs. ''KCAS'' for
calibrated airspeed and ''KTAS'' for
true airspeed).
The IAS is an important value for the pilot because it is the indicated speeds which are specified in the
aircraft flight manual
An aircraft flight manual (AFM) is a paper book or electronic information set containing information required to operate an aircraft of certain type or particular aircraft of that type (each AFM is tailored for a specific aircraft, though aircraft ...
for such important performance values as the
stall speed. These speeds, in true airspeed terms, vary considerably depending upon
density altitude. However, at typical civilian operating speeds, the aircraft's aerodynamic structure responds to dynamic pressure alone, and the aircraft will perform the same when at the same dynamic pressure. Since it is this same dynamic pressure that drives the airspeed indicator, an aircraft will always, for example, stall at the published ''indicated'' airspeed (for the current configuration) regardless of density, altitude or true airspeed.
Furthermore, the IAS is specified in some regulations, and by air traffic control when directing pilots, since the airspeed indicator displays that speed (by definition) and it is the pilot's primary airspeed reference when operating below
transonic
Transonic (or transsonic) flow is air flowing around an object at a speed that generates regions of both subsonic and supersonic airflow around that object. The exact range of speeds depends on the object's critical Mach number, but transoni ...
or
supersonic
Supersonic speed is the speed of an object that exceeds the speed of sound ( Mach 1). For objects traveling in dry air of a temperature of 20 °C (68 °F) at sea level, this speed is approximately . Speeds greater than five times ...
speeds.
Calculation
Indicated airspeed measured by
pitot-tube can be approximately expressed by the following equation delivered from
Bernoulli's equation.
:
NOTE: The above equation applies only to conditions that can be treated as incompressible. Liquids are treated as incompressible under almost all conditions. Gases under certain conditions can be approximated as incompressible. See
Compressibility.
The compression effects can be corrected by use of
Poisson constant. This compensation corresponds to
equivalent airspeed (EAS).
:
where:
*
is indicated airspeed in m/s,
*
is
stagnation or total pressure in pascals,
*
is static pressure in pascals,
*
is fluid density in
, and
*
is the
specific heat capacity ratio (≈1.401 for air
).
IAS vs CAS
The IAS is not the actual speed through the air even when the aircraft is at sea level under International Standard Atmosphere conditions (15 °C, 1013
hPa, 0% humidity). The IAS needs to be corrected for known instrument and position errors to show true airspeed under those specific atmospheric conditions, and this is the
CAS (Calibrated Airspeed). Despite this the pilot's primary airspeed reference, the ASI, shows IAS (by definition). The relationship between CAS and IAS is known and documented for each aircraft type and model.
IAS and V speeds
The aircraft's pilot manual usually gives critical
V speeds as IAS, those speeds indicated by the airspeed indicator. This is because the aircraft behaves similarly at the same IAS no matter what the TAS is: E.g. A pilot landing at a
hot and high airfield will use the same IAS to fly the aircraft at the correct approach and landing speeds as he would when landing at a cold sea level airfield even though the TAS must differ considerably between the two landings.
Whereas IAS can be reliably used for monitoring critical speeds well below the speed of sound this is not so at higher speeds. An example: Because (1) the compressibility of air changes considerably approaching the speed of sound, and (2) the speed of sound varies considerably with temperature and therefore altitude; the maximum speed at which an aircraft structure is safe, the
never exceed speed (abbreviated ''V''
NE), is specified at several differing altitudes in faster aircraft's operating manuals, as shown in the sample table below.
Ref: ''Pilot's Notes for
Tempest V Sabre IIA Engine'' -
Air Ministry
The Air Ministry was a department of the Government of the United Kingdom with the responsibility of managing the affairs of the Royal Air Force, that existed from 1918 to 1964. It was under the political authority of the Secretary of Stat ...
A.P.2458C-PN
IAS and navigation
For navigation, it is necessary to convert IAS to
TAS and/or
ground speed (GS) using the following method:
* correct IAS to
calibrated airspeed (CAS) using an aircraft-specific correction table;
* correct CAS to
true airspeed (TAS) by using
Outside Air Temperature (OAT), Pressure-altitude and CAS on an
E6B
The E6B flight computer is a form of circular slide rule used in aviation and one of the very few analog calculating devices in widespread use in the 21st century.
They are mostly used in flight training, because these flight computers have ...
flight computer or equivalent functionality on most
GPSs;
* convert TAS to
ground speed (GS) by allowing for the effect of
wind
Wind is the natural movement of air or other gases relative to a planet's surface. Winds occur on a range of scales, from thunderstorm flows lasting tens of minutes, to local breezes generated by heating of land surfaces and lasting a few ...
.
With the advent of
Doppler radar
A Doppler radar is a specialized radar that uses the Doppler effect to produce velocity data about objects at a distance. It does this by bouncing a microwave signal off a desired target and analyzing how the object's motion has altered the fr ...
navigation and, more recently,
GPS receivers, with other advanced navigation equipment that allows pilots to read
ground speed directly, the TAS calculation in-flight is becoming unnecessary for the purposes of navigation estimations.
TAS is the primary method to determine aircraft's cruise performance in manufacturer's specs
, speed comparisons and pilot reports.
Other airspeeds
From IAS, the following speeds can also be calculated:
* convert CAS to
equivalent airspeed (EAS) by allowing for compressibility effects (not necessary at slow speed or low altitude); EAS is used by aircraft engineers and some very high-altitude flying aircraft such as the U-2 and the SR-71;
* convert EAS to
true airspeed (TAS) by allowing for differences in
density altitude.
On large jet aircraft the IAS is by far the most important speed indicator. Most aircraft speed limitations are based on IAS, as IAS closely reflects dynamic pressure. TAS is usually displayed as well, but purely for advisory information and generally not in a prominent location.
Modern jet airliners also include
ground speed (GS) and
Machmeter. Ground speed shows the actual speed that the aircraft uses compared to the ground. This is usually connected to a
GPS or similar system. Ground speed is just a pilot aid to estimate if the flight is on time, behind or ahead of schedule. It is not used for takeoff and landing purposes, since the imperative speed for a flying aircraft always is the speed against the wind.
The Machmeter is, on subsonic aircraft, a warning indicator. Subsonic aircraft must not fly faster than a specific percentage of the speed of sound. Usually passenger airliners do not fly faster than around 85% of speed of sound, or Mach 0.85.
Supersonic
Supersonic speed is the speed of an object that exceeds the speed of sound ( Mach 1). For objects traveling in dry air of a temperature of 20 °C (68 °F) at sea level, this speed is approximately . Speeds greater than five times ...
aircraft, like the
Concorde
The Aérospatiale/BAC Concorde () is a retired Franco-British supersonic airliner jointly developed and manufactured by Sud Aviation (later Aérospatiale) and the British Aircraft Corporation (BAC).
Studies started in 1954, and France an ...
and military fighters, use the Machmeter as the main speed instrument with the exception of take-offs and landings.
Some aircraft also have a
taxi speed indicator for use on the ground. Since the IAS often starts at around (on jet airliners), pilots may need extra help while taxiing the aircraft on the ground. Its range is around .
See also
*
Acronyms and abbreviations in avionics
*
ICAO recommendations on use of the International System of Units
*
Air speed
*
Calibrated airspeed
*
Equivalent airspeed
*
Flight instruments
*
Global Positioning System
The Global Positioning System (GPS), originally Navstar GPS, is a satellite-based radionavigation system owned by the United States government and operated by the United States Space Force. It is one of the global navigation satellite ...
*
True airspeed
References
Bibliography
* Gracey, William (1980)
"Measurement of Aircraft Speed and Altitude" (11 MB), NASA Reference Publication 1046.
{{DEFAULTSORT:Indicated Airspeed
Airspeed