airspeed indicator

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).

The arcs

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 15C, 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 is:

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. Find TAS.
 

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.

V Speeds

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.

VA
VFE
VLE
VLO
VNE
VNO
VR
VS0
VS1
VX
VY

Design manoeuvring speed
Maximum flap extend speed
Maximum landing gear extend speed
Maximum landing gear operating speed
Never exceed speed
Maximum structural cruising speed
Rotation speed
The power-off stalling speed or minimum flight speed in landing configuration
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