Airspeed is the speed of the aircraft in relation to the air mass surrounding that aircraft.
Accurate airspeed measurement is obtained by means of a pitot-static system. The system consists of:
- A tube mounted parallel to the longitudinal axis of the aircraft in an area that is free of turbulent air generated by the aircraft, and
- A static source that provides still, or undisturbed, air pressure.
Ram and static pressures may be taken from a single pitotstatic tube or from completely separate sources. A pitot-static tube usually has a baffle plate [Figure 3-21] to reduce turbulence and to prevent rain, ice, and dirt from entering the tube. There may be one or more drain holes in the bottom of the tube to dispose of condensed moisture. A built-in electrical heating element, controlled by a switch inside the aircraft, prevents the formation of ice in the tube.
Reasonable care should be taken with the pitot-static system to ensure continuous, reliable service. The drain holes should be checked periodically to ensure they are not clogged. At the completion of each flight, a cover is placed over the intake end of the tube to prevent foreign objects and moisture from collecting in the tube.
Principles of Operation of Airspeed Indicators
The heart of the airspeed indicator is a diaphragm that is sensitive to pressure changes. Figure 3-21 shows it located inside the indicator case and connected to the ram air source in the pitot tube. The indicator case is sealed airtight and connected to the static pressure source. The differential pressure created by the relative effects of the impact and static pressures on the diaphragm causes it to expand or contract. As the speed of the aircraft increases, the impact pressure increases, causing the diaphragm to expand. Through mechanical linkage, the expansion is displayed as an increase in airspeed. This principle is used in the IAS meter, the TAS meter, and the Machmeter.
There are many reasons for the difference between IAS and TAS. Some of the reasons are the error in the mechanical makeup of the instrument, the error caused by incorrect installation, and the fact that density and pressure of the atmosphere vary from standard conditions.
Indicated Airspeed (IAS)
IAS is the uncorrected reading taken from the face of the indicator. It is the airspeed that the instrument shows on the dial.
Basic Airspeed (BAS)
Basic airspeed (BAS) is the IAS corrected for instrument error. Each airspeed indicator has its own characteristics that cause it to differ from any other airspeed indicator. These differences may be caused by slightly different hairspring tensions, flexibility of the diaphragm, accuracy of the scale markings, or even the effect of temperature on the different metals in the indicator mechanism. The effect of temperature introduces an instrument error due to the variance in the coefficient of expansion of the different metals comprising the working mechanisms. This error can be removed by the installation of a bimetallic compensator within the mechanical linkage. This bimetallic compensator is installed and properly set at the factory, thereby eliminating the temperature error within the instrument. The accuracy of the airspeed indicator is also affected by the length and curvature of the pressure line from the pitot tube. These installation errors must be corrected mathematically. Installation, scale, and instrument errors are all combined under one title called instrument error. Instrument error is factory-determined to be within specified tolerances for various airspeeds. It is considered negligible or is accounted for in technical order tables and graphs.
Calibrated Airspeed (CAS)
Calibrated airspeed (CAS) is basic airspeed corrected for pitot-static error or attitude of the aircraft. The pitot-static system of a moving aircraft has some error. Minor errors are found in the pitot section of the system. The major difficulty is encountered in the static pressure section. As the flight attitude of the aircraft changes, the pressure at the static inlets changes. This is caused by the airstream striking the inlet at an angle. Different types and locations of installations cause different errors. It is immaterial whether the status source is located in the pitot-static head or at some flush mounting on the aircraft. This error is essentially the same for all aircraft of the same model, and a correction can be computed by referring to tables in the appendix of the flight manual.
Equivalent Airspeed (EAS)
Equivalant airspeed (EAS) is CAS corrected for compressibility. Compressibility becomes noticeable when the airspeed is great enough to create an impact pressure that causes the air molecules to be compressed within the impact chamber of the pitot tube. The amount of the compression is directly proportionate to the impact pressure. As the air is compressed, it causes the dynamic pressure to be greater than it should be. Therefore, the correction is a negative value.
The correction for compressibility error can be determined by referring to the performance data section of the aircraft flight manual or by using the F-correction factor on the DR computer.
Density Airspeed (DAS)
Density airspeed (DAS) is calibrated airspeed corrected for PA and TAT. Pitot pressure varies not only with airspeed but also with air density. As the density of the atmosphere decreases with height, pitot pressure for a given airspeed must also decrease with height. Thus, an airspeed indicator operating in a less dense medium than that for which it was calibrated indicates an airspeed lower than true speed. The higher the altitude, the greater the discrepancy. The necessary correction can be found on the DR computer. Using the window on the computer above the area marked FOR AIRSPEED DENSITY ALTITUDE COMPUTATIONS, set the PA against the TAT. Opposite the CAS on the minutes scale, read the DAS on the miles scale. At lower airspeeds and altitudes, DAS may be taken as true airspeed with negligible error. However, at high speeds and altitudes, this is no longer true and compressibility error must be considered. (Compressibility error is explained in the equivalent airspeed section.) When DA is multiplied by the compressibility factor, the result is true airspeed.
True Airspeed (TAS)
TAS is equivalent airspeed that has been corrected for air density error. By correcting EAS for TAT and PA, the navigator compensates for air density error and computes an accurate value of TAS. The TAS increases with altitude when the IAS remains constant. When the TAS remains constant, the IAS decreases with altitude. CAS and EAS can be determined by referring to the performance data section of the aircraft flight manual.