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Pilots For Truth > American 77 > Airdata Calibration and Measurement

Posted by: johndoeX Oct 5 2006, 01:50 PM
Although most pilots already know this...

All Airdata systems go through rigorous testing to remove errors at various speed, angles of attack, and position error. The following are highlights to show how an airdata system is calibrated to remove errors when undergoing certification. Full report is linked below.

"The presence of the aircraft in the airstream causes input errors to the measuring
instruments the aircraft disturbs the air that it flies through, thereby also disturbing the airdata
measurements. Figure 1 shows the airflow around an airplane wing."

"Accurate airdata are necessary for many purposes and applications. Obviously, the pilot
cannot safely fly the aircraft without knowing airspeed and pressure altitude. In civil aviation, the
small vertical separation between flight levels assigned by air traffic controllers is based on
accurate knowledge of pressure altitude."

"Static pressure can be measured with a pitot-static tube or a flush-mounted port on the
fuselage. Figure 3 shows a typical subsonic static pressure distribution on an aircraft fuselage
(ref. 2). The measured minus true static pressure, , normalized to compressible dynamic
pressure, , is plotted as a function of fuselage position. Zero static pressure error on the
fuselage exists at locations 2 through 5."

"Even with the selection of the best static port position, some pressure errors will remain, and
these errors must be determined in flight. The difference between the locally measured static
pressure and the ambient static pressure, which is dependent upon angle of attack, airspeed, and
aircraft configuration, is called
position error.
Three calibration types are generally used to determine position error: direct comparison,altimetry, and velocimetry."

"This subsection describes typical maneuvers and methods for most airdata calibrations.
Tower-flyby, trailing static or trailing cone, pacer aircraft, radar tracking, and dynamic maneuvers
are included."

"Lag and attenuation can be estimated or measured experimentally. Criteria can be set for how
quickly pressure can change in the pneumatic system without affecting the airdata. Such
calibration methods as the trailing cone may have very large pneumatic lags and may have to be
used in steady flight."

"Airdata quantities are needed for a multitude of tasks, including flight safety, control,
navigation, weapons delivery, flight test, and flight research. These quantities generally need to
be measured and then calibrated to remove errors. The techniques and procedures have been only
briefly described here; numerous references should be studied if airdata values are to be measured and calibrated."

Conclusion - The Flight Data Recorder doesnt have any errors in terms of air data (altitude, airspeed). Those errors are removed during calibration testing for certification.

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