The
true
airspeed
(
TAS
; also
KTAS
, for
knots true airspeed
) of an
aircraft
is the
speed
of the aircraft relative to the
air mass
through which it is flying. The true airspeed is important information for accurate navigation of an aircraft. Traditionally it is measured using an analogue
TAS indicator
, but as the
Global Positioning System
has become available for civilian use, the importance of such air-measuring instruments has decreased. Since
indicated
, as opposed to
true
, airspeed is a better indicator of margin above the
stall
, true airspeed is not used for controlling the aircraft; for these purposes the
indicated airspeed
? IAS or KIAS (knots indicated airspeed) ? is used. However, since indicated airspeed only shows true speed through the air at standard sea level pressure and temperature, a TAS meter is necessary for navigation purposes at cruising altitude in less dense air. The IAS meter reads very nearly the TAS at lower altitude and at lower speed. On jet airliners the TAS meter is usually hidden at speeds below 200 knots (370?km/h). Neither provides for accurate
speed over the ground
, since surface winds or winds aloft are not taken into account.
An analog true airspeed indicator for an airplane. The pilot sets the
pressure altitude
and
air temperature
in the top window using the knob; the needle indicates true airspeed in the lower left window. Here the speed is displayed both in
knots
(kn) and
miles per hour
(mph).
TAS is the appropriate speed to use when calculating the range of an airplane. It is the speed normally listed on the flight plan, also used in flight planning, before considering the effects of wind.
Airspeed sensing errors
edit
The
airspeed indicator
(ASI), driven by ram air into a
pitot tube
and still air into a barometric static port, shows what is called
indicated airspeed
(IAS). The differential pressure is affected by
air density
. The ratio between the two measurements is temperature-dependent and pressure-dependent, according to the
ideal gas law
.
At sea level in the
International Standard Atmosphere
(ISA) and at low speeds where air compressibility is negligible (i.e., assuming a constant air density), IAS corresponds to TAS. When the air density or temperature around the aircraft differs from standard sea level conditions, IAS will no longer correspond to TAS, thus it will no longer reflect aircraft performance. The ASI will indicate less than TAS when the air density decreases due to a change in altitude or air temperature. For this reason, TAS cannot be measured directly. In flight, it can be calculated either by using an
E6B
flight calculator or its equivalent.
For low speeds, the data required are
static air temperature
, pressure altitude and IAS (or
CAS
for more precision). Above approximately 100 knots (190?km/h), the compressibility error rises significantly and TAS must be calculated by the Mach speed. Mach incorporates the above data including the compressibility factor. Modern aircraft instrumentation use an
air data computer
to perform this calculation in real time and display the TAS reading directly on the
electronic flight instrument system
.
Since temperature variations are of a smaller influence, the ASI error can be estimated as indicating about 2% less than TAS per 1,000 feet (300?m) of altitude above sea level. For example, an aircraft flying at 15,000 feet (4,600?m) in the international standard atmosphere with an IAS of 100 knots (190?km/h), is actually flying at 126 knots (233?km/h) TAS.
Use in navigation calculations
edit
To maintain a desired
ground track
while flying in the moving airmass, the pilot of an aircraft must use knowledge of wind speed, wind direction, and true air speed to determine the required heading. See also
wind triangle
.
Calculating true airspeed
edit
At low speeds and altitudes, IAS and CAS are close to
equivalent airspeed
(EAS).
TAS can be calculated as a function of EAS and air density:
where
-
is true airspeed,
-
is equivalent airspeed,
-
is the air density at sea level in the
International Standard Atmosphere
(15?°C and 1013.25 hectopascals, corresponding to a density of 1.225?kg/m
3
),
-
is the density of the air in which the aircraft is flying.
TAS can be calculated as a function of
Mach number
and static air temperature:
where
-
is the speed of sound at standard sea level (661.47 knots (1,225.04?km/h; 340.29?m/s)),
-
is Mach number,
-
is static air temperature in
kelvins
,
-
is the temperature at standard sea level (288.15?K).
For manual calculation of TAS in knots, where Mach number and static air temperature are known, the expression may be simplified to
(remembering temperature is in kelvins).
Combining the above with the expression for Mach number gives an expression for TAS as a function of
impact pressure
, static pressure and static air temperature (valid for subsonic flow):
where:
-
is impact pressure,
-
is static pressure.
Electronic flight instrument systems
(EFIS) contain an
air data computer
with inputs of impact pressure, static pressure and
total air temperature
. In order to compute TAS, the air data computer must convert total air temperature to static air temperature. This is also a function of Mach number:
where
-
total air temperature.
In simple aircraft, without an air data computer or
machmeter
, true airspeed can be calculated as a function of
calibrated airspeed
and local air density (or static air temperature and pressure altitude, which determine density). Some airspeed indicators incorporate a
slide rule
mechanism to perform this calculation. Otherwise, it can be performed using
this applet
or a device such as the
E6B
(a handheld circular
slide rule
).