The basic airspeed indicator on a Cessna 152 or an
F-15E both read Indicated Airspeed (IAS). This device measures the difference
between STATIC pressure (usually from a sensor not in the airstream) and IMPACT
pressure (called the stagnation pressure received from an aircraft's
PITOT TUBE -- which is in the airstream). When the aircraft is not moving, the
pressures are equal (and the airspeed is zero). On takeoff, the on-rushing air
will result in a greater pressure in the PITOT TUBE and this difference in
pressure from the static sensor can be used to calculate the airspeed (in miles
per hour (MPH) or nautical miles per hours (KNOTS)) at which the aircraft is
moving through the air.
The velocity is given by the square root of (2 x [stagnation
pressure - static pressure] / air density).
Colour arcs are applied to the ASI
which are specific to that type of aircraft. The white arc denotes the airspeeds
at which it is safe to deploy the flaps. The maximum speed is called Vfe.
The bottom end of the white arc is set at the stall speed Vs0 when the
aircraft is configured at maximum take-off weight usually with one or two stages
of flaps deployed. The precise definition of VsO will be defined in the pilot
operating manual of the aircraft.
The green arc marks the airspeeds
at which it is safe to input sudden and full control movements. The top end of
the green arc is defined as Va. This is also the maximum speed at which
turbulence may be flown through. The bottom end of the green arc Vs1 is
usually the lowest speed at which the aircraft can fly without flaps deployed.
It is important to remember that any aircraft will stall at any speed if the
angle of attack is exceeded.
The yellow arc denotes the
airspeeds at which care must be taken not to overstress the aircraft by the use
of abrupt and sudden control inputs.
The red line denotes VNE,
the velocity to never exceed in the aircraft.
basic aircraft speeds
Indicated airspeed (IAS) reflects true airspeed (TAS) only when
ICAO standard atmospheric conditions prevail i.e., temperature 15°C, and
pressure of 29.92 in. Hg at sea level. Calibrated airspeed (CAS) corrects the
indicated airspeed for errors primarily resulting from the position of the
static source and, to a much lesser degree, from pitot tube locations. The major
errors are mainly due to differences in airflow over the static port at varying
angles of attack. The errors usually are greatest in the low and high-speed
ranges and smallest in normal operating speeds. Calibrated airspeed tables
correct the whole range of indicated airspeed for these installation errors and
can be found in the aircraft flight manual.
The flight computer calculates the TAS by converting the IAS under actual
conditions to a standard temperature and pressure. This conversion is necessary
because the pitot-static system operates accurately only at the standard
conditions mentioned above.
By using a flight computer, the pilot can calculate the TAS
by applying the actual outside air temperature to the pressure altitude. Some
airspeed indicators incorporate a TAS computer enabling the pilot to read TAS directly
from the outermost scale on the face of the indicator.
groundspeed is another important airspeed to pilots. Groundspeed is the
aircraft's actual speed across the earth. It equals the TAS plus or minus the
wind factor. For example, if your TAS is 500 MPH and you have a direct (180
degrees from your heading) tail-wind of 100 MPH, your groundspeed is 600 MPH.
Groundspeed can be measured by onboard Inertial Navigation Systems (INS) or by
Global Positioning Satellite (GPS) receivers. One "old-fashion" method is to
record the time it takes to fly between two known points. Then divide this time
by the distance. For example, if the distance is 18 miles, and it took an
aircrew in an F-15E 2 minutes to fly between the points, then their groundspeed
18 miles / 2 minutes = 9 miles per minute
This can be
converted to miles per hour by multiplying by 60 (60 minutes in an hour)
9 miles per minute X 60 minutes per hour = 540 Miles per hour
Calibrated Airspeed (CAS)
Although aircraft designers attempt to keep airspeed errors
to a minimum, it is not possible to achieve complete accuracy throughout the
complete range of the instrument. Two types of errors can be introduced. a.
Installation error caused by the static ports sensing erroneous pressure. This
is due to the unpredictability of the effects of the slipstream around the
aircraft at various speeds and attitudes. b. The pitot tube does not always
present the same frontal appearance to the atmosphere at varying attitudes. The
pilot should consult the Pilot Operating Handbook (POH) for the table applicable
to the aircraft being flown.
True Airspeed (TAS)
As altitude increases , air density decreases. The impact
pressure at the port of the pitot tube is less at higher altitudes. The airplane
is actually travelling through the air faster than indicated on the ASI.
Consequently as altitude increase, Indicated Airspeed decreases.
A mathematical correction factor must be applied to Indicated
Airspeed (or Calibrated Airspeed) to arrive at a correct True Airspeed (TAS).
This calculation can be made with he E6B Flight computer, or an approximate
correction can be made by adding 2 percent per 1,000 feet of altitude to the IAS.
EXAMPLE: Given IAS is 140kt and ALT is 6,000 feet.
2% x 6 = 12% (.12)
140 x 0.12 = 16.8
140 + 16.8 = 156.8 kt. (TAS)
Some airspeed indicators have built-in adjustment scales that
allows the pilot to adjust the instrument for temperature and pressure. Both the
IAS and TAS can be read from such an airspeed indicator.
The Pilot Operating Handbook normally lists various airspeeds
for differing situations and conditions. The definition of the usual V speeds is
shown below. is an abbreviation for Velocity.
Design manoeuvring speed
Maximum flap extend speed
Maximum landing gear extend speed
Maximum landing gear operating speed
Never exceed speed
Maximum structural cruising speed
The power-off stalling speed or minimum flight speed in landing
The power-off stalling speed (clean) with flaps and landing gear retracted
Best angle of climb speed
Best rate of climb speed/Best Glide Speed