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The technology of Air Data Test Sets
The continuous improvement of Air Data Instruments called for by the increase of
air traffic is also demanding Air Data Test Sets with increased accuracy and
functionality.
To satisfy the present flight air data instruments requirements an ADTS must
provide the following features:
- Absolute accuracy of testing
- Availability of multiple functions.
- Ease of use
- Reduced testing time
We would like to describe how the DMA-Aero MPS2X and the MPS3X ADTS series of instruments cover all these aspects.
Absolute accuracy of testing
To satisfy this requirement one of the most important and critical tasks is to
select a suitable pressure sensor.
Our experience has shown that the most accurate and reliable transducers for pitot-static measurements are the Vibrating Cylinder-types.
The main sources of errors on these types of sensors are: linearity, temperature and density effects and long term stability.
Density error becomes negligible in all DMA-Aero ADTS, by reducing the
humidity of the air inside the transducers.
The transducers cylindrical shape, which is intrinsically very simple, provides
an exceptional coincidence between the theoretical and experimental models: this
is evident by examining the relative simplicity of the polynomial used to compensate for the linearity error; a complex polynomial to compensate for linearity error
shows a transducers irregular shape.
Once the linearity has been compensated, it does not change for many years. By
properly identifying the transducers, the error due to linearity can be considered not greater than 30 ppm.
The largest error is due to the temperature effects, this is a difficult parameter to control: the temperature can easily be read but its gradient of change dT/dt (derivative of temperature versus time) and dT/dx , dT/dy , dT/dz, (derivative of
temperature versus space) is very difficult to compensate.
D. Marchiori uses three basic approaches to minimize such problems:
· The temperature values are read at a very high sample rate by using an A/D
converter with a large number of bits to calculate the temperature gradient.
· Generate the lowest possible heat inside the ADTS thus minimizing the temperature gradient (all the heat which is not transmitted to the outside, increases the ADTS internal temperature according to the Fourier postulate).
· Design into the ADTS an area that is outside the convective airflow, to ensure
negligible temperature gradients.
Therefore we can say that the best accuracy can be obtained by:
- Selecting the proper transducers.
- Provide an ADTS having very low power consumption, all the D. Marchiori ADTS
have a power consumption lower than 60 watts by means of special design techniques.
With a high reading sample rate giving fast detection of potential errors this
results additionally in a better control capability.
Our long term experience as ADTS manufacturers, supported by the sensor
manufacturer, proves that the vibrating cylinder transducer is the best available
among the many we have tested from those available on the market (better than 50 ppm fs / year).
Repeatability and hysteresis error are not a problem, since they are not larger than 10 ppm (in practice immeasurable with any absolute certainty).
The combination of the above design features allows the MPS2X and MPS3X ADTS to consistently improve the accuracy to give the equivalent of +/- 3 feet error at sea level.
Control accuracy
There are various ways to control pressures: analog or digital . The analog
solution is fast but not so flexible. The digital one is relatively slow but it can
utilize smart mathematical control in a relatively simple way. Earlier controls were
of the analog type. Nowadays computers are fast enough to avoid the problems
related to the speed of response.
There is also an intermediate solution: a digital readout which uses a digital reading to compensate for the low accuracy of the second analog loop.
We consider the last solution is less flexible than the digital one, even if its
accuracy is almost equal.
For example the digital control provides more consistent adaptability to the
volume under control. That means: in presence of a leak on the system under test,
the digital control will stabilize the reading depending on the leak itself
(large leak, bad stabilization), avoiding the well-known problems of analog
control which stabilizes the readings correctly by compensating also very large
leaks; this will mislead the operator by giving the idea that the test is
accomplished correctly, while it is well known that the presence of a large leak
compromises the accuracy of the testing.
In addition, the digital control gives the valuable capability of compensating the
proportional control valves characteristics drift, automatically without any mechanical
adjustment.
Last but not least digital control provides the capability of adding to the
ADTS more functions, simply by loading new software.
All of these features allow the operator to perform the required testing with very high accuracy, confidence and repeatability.
The MPS2X and MPS3X ADTS instruments are all digital designs, with a speed of 20 to 40 readings/second (depending of the model).
Operational flexibility and effectiveness
The functions available today in modern aircraft are made possible by the latest
air data computer technology, and these in turn call for air data testers
with additional functions to provide a better test of the Pitot/Static systems.
All ADTS series MPS2X and MPS3X can be easily controlled to perform all the
functions required to accomplish a calibration or a leak check of the Pitot/static
systems.
All the test functions to simulate the aircraft flying characteristics, are
entered via the color-coded keypad on the ADTS, or in the Hand Held Remote
Control Unit (HHRCU) in a simple and intuitive way. The tester is easy to understand
even for the first time user, since the air data parameters required to accomplish all the
functions are clearly shown.
The keyboards are arranged to allow easy and logical operation; the data of all
the functions which are to be performed are simultaneously available in the display
always at the same positions.
All indications and controls are located exactly where the operator expects them to be.
Commanded values can be entered either by digitizing directly the numbers or by
increasing or decreasing the values by a selectable amount using UP and DOWN arrows.
In addition the MPS2X and MPS3X ADTS also provide the following features:
· MACH Mode where the operator can directly enter the target Mach Number.
· EPR Mode to carry out gas turbine EPR system tests.
· T.A.S generation in place of I.A.S.
· Considerable time saving. This allows the full requirements of FAR 43 Appendix E testing; it is possible to program the stabilization time, the reading time, for the leak test, as well as the waiting time before taking the readings.
· Automatic or Semiautomatic Leak Test.
· center-line Correction, to compensate for height difference between the ADTS and the UUT.
· Encoding Altimeter or Encoder Test with automatic checking of all the tests point
sequences.
· Automatic Test of Radio Altimeter for the complete EGPWS (Enhanced Ground
Proximity Warning System) test.
· Self-contained Battery pack to allow 3 hours of full operation without external power.
· ARINC 429 bus.
· Full ADTS Control via an external PC.
· ULTRA LOW SPEED Function to generate very low airspeed values (5 to 200 kts with 0.1 knots resolution over 20 Knots).
· Full Automatic Calibration by using a DMA-Aero PAMBX Transfer Standard and specific Software.
· Rugged LCD with backlit display HHRCU, for single-person operation.
· Programmable safety limits to ensure safe aircraft testing.
· In case of accidental power loss, the battery back-up provides a fully operational capability for several hours giving the possibility to complete the full test without wasting time to vent to ambient.
Ease of operation
With modern Air Data testers increasing complexity, there is a call for simpler use of the instruments: combining customers and DMA experience, we have designed a user interface that allows the operator to learn the basic use of the ADTS in a few minutes.
Additionally all the hardware and software protections allow the first time user
to operate the ADTS safely and be confident that no damage will be caused to either the UUT or ADTS.
Testing time
Length of testing is mainly dependant on the airplane limitations, such as the
altitude rate of change, which cannot be altered.
Additional time can be saved by reducing the manual operation and the eventual
Trouble-shooting procedures. We feel the main factors are:
· Capability of programming the test, to store it in the ADTS and
· execute it automatically just by pressing a key
· To considerably reduce the leak testing time we incorporate the important Multiple
Isolators, integrated in the ADTS box to enable up to four separate pitot/static lines to be connected to the aircraft simultaneously. This provides the capability to switch between the aircraft pitot/static systems electrically controlled by the Hand Held Remote Control Unit. This feature makes leaks testing very simple, detecting the exact location of system leaks for immediate rectification without wasting time to vent to ambient, disconnect and reconnect hoses to the aircraft.
While the use of a dedicated external manual, or automatic, valving box will help to
accomplish such leak isolation, it will be less effective because it compromises the one-man operation opportunity and makes the hose switching difficult giving the possibility of more operator mistakes.
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