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Airdata Calibration and Measurement

rob balsamo
post Oct 5 2006, 02:50 PM
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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."

http://dtrs.dfrc.nasa.gov/archive/00000221/01/104316.pdf

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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tnemelckram
post Dec 22 2009, 06:19 PM
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Hi Rob!

The calibration Manual was interesting. Here's a few quotes from it that illustrate your poinyt about the primacy of pressure altitude and thus its proper calibration and accuracy:

QUOTE
Airdata are vital to successfully complete an aircraft's mission and are derived from the air
surrounding the aircraft. References 14 supply pertinent information regarding airdata
measurement and calibration. These airdata encompass indicated and true airspeed, pressure
altitude, ambient air temperature, angles of attack and sideslip, Mach number, and rate of climb.
Typically, pitot and static pressures are sensed and converted (by mechanical means in the
instruments themselves) into indications on the altimeter, vertical speed indicator, airspeed
indicator, and Machmeter. Similarly, measured local flow angles establish angles of attack and
sideslip, and the outside air temperature is measured and indicated in the cockpit. (Instruments
that can perform the conversion, such as airspeed indicators, altimeters, and Machmeters, do not correct for errors in the input values.) These measured parameters are commonly input to the airdata computer, which, using appropriate algorithms and correction factors (or calibrations, as discussed later), can provide other parameters, such as true airspeed, required by the aircraft's avionics or flight control system.


So pressure altitude and altimeter are one of about five of the big things to calibrate.


QUOTE
Three calibration types are generally used to determine position error: direct comparison,
altimetry, and velocimetry. The direct-comparison calibration type involves measuring the true
static pressure from a known source. The result is then compared with the static pressure of the
airplane being calibrated. Direct comparisons are completed using the trailing cone and pacer
methods described in later sections of this memorandum. The altimetry type adds one level of
complexity by first determining the true pressure altitude. This altitude is then converted to static pressure.


So again pressure altitude is mentioned as a huge concern.


QUOTE
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.


Pressure altitude is important because it's what is used to keep the planes from running into each other, and reliable enough for a purpose that by nature does not permit error.


QUOTE
During unsteady flight, pneumatic lag and attenuation may affect pressure measurements. Pressure variations propagate as waves through the pneumatic tubing to the pressure transducer.The wave propagation is damped by frictional attenuation along the walls of the tubing and fluid viscosity. This damping produces a magnitude attenuation and a phase lag. After the wave reaches the transducer, it is reflected back up the tube. Depending upon frequency distribution of the incoming wave energy and tubing length, the reflected wave may cancel or reinforce incoming pressure wave. If the waves cancel each other out, further spectral attenuation occurs. If the waves reinforce each other, the power of the incoming wave is amplified and resonance occurs.


As I read this, altimiter lag usually occurs during up and down flight at high altitude. Hence they use the words "pressure variations" - up, down, up, down - which does not seem to include Hani's pretty steady descent on the smooth circular path claimed by the government.
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