The Technology of Air Data Test Sets
The continuous improvement of aircraft installed Air Data Instruments instigated by the increase of world wide air traffic is also demanding air data test systems (ADTS) with increasing accuracy and operating functions.
To satisfy these present flightline air data instruments requirements all ADTS must provide the following features:
- Absolute accuracy of testing
- Availability of all necessary functions.
- Ease of use for the Operator
- Reducing testing time
Manufacturers of air data test systems must respond to these requirements. I would like to describe here how the DMA MPS2X, MPS3X and MPS4X ADTS series cover all of these aspects.
ABSOLUTE ACCURACY OF TESTING
To satisfy this requirement one of the most important and critical tasks is to select appropriate pressure sensors / transducers.
This is the fundamental need of the pressure measurement and must be accurate to satisfy the requirements of the RVSM, (reduced vertical separation minima) international regulations. This demands a stability suitable to ensure the declared measurement accuracy for a period of one year (or more) without any additional calibration, and independent of ambient conditions over the full specified range.
Our experience has shown that there are two ways to obtain this necessary precision:
- The use of Vibrating Cylinder resonant transducers that are intrinsically accurate (probably the most accurate available at present), but gas density dependent.
- The use of high quality silicon diaphragm piezo-resistive transducers with corrections for ambient conditions (mostly temperature) by individual characterization techniques to obtain the required accuracy.
Other solutions, such as other types of resonant transducers or quartz transducers can be used but in fact the characterization installed internally by the manufacturer of the transducer, may be the main problem.
We will now describe the specific method used by DMA in the 2 cases described above:
With the vibrating cylinder, to be independent from the gas density function of temperature and the humidity of air, the air is dehydrated before entering the transducer, and the temperature is constantly measured with a high resolution, multiple times per second, to correct any false readings and to additionally compensate for variations over time.
In this way the density is fully corrected, whereas on the contrary, the method of reading humidity to compensate this effect is considered by us as too imprecise to obtain sufficient accuracy during the pressure variations.
With the piezo-resistive transducers, if a high quality transducer is used, the main problem is then the individual characterization. DMA have developed a mathematical model to calculate the correction functions after tests in environmental chambers with algorithms designed to define the best possible data to achieve the necessary accuracy.
This modelling must be so written to achieve the minimum possible number of bits in accordance with the second law of thermodynamics, avoiding the situation where the functions calculated to improve the pressure readings actually create indeterminate results instead of more accuracy.
After exhaustive testing of many different transducers, mainly to have the highest quality level of long term stability, and discarding the analog techniques of compensation for temperature and linearity errors, DMA have achieved a linearity error less than 30 ppm fs, and similar long term stability.
Hysteresis (mechanical and thermal) is negligible, so the RVSM requirements are fully satisfied by all the ADTS instruments produced by DMA.
To achieve the full potential of the transducers there are another considerations.
The readings are corrected for temperature over a wide range as needed by the ambient operating range. But it is not possible to correct exactly for the gradient of temperature, in time and space, due to the fact that the power absorbed by the ADTS is transformed into heat generation. The best way to avoid the indeterminate variations caused by the dT/dt, dT/dx, dT/dy, dT/dz derivatives, is to reduce the power absorption. So the instrument design is concentrated on minimal heat being generated, and, consequently, it is possible to have additional features incorporated such as internal batteries without incurring large weight and dimension penalties.
All the DMA air data testers have a digital control based system.
Compared to the previous solutions, i.e. digital readout and analog control, the digital control provides best 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 for very large leaks; this will mislead the operator by giving the impression that the test is carried out correctly, while it is universally known that the presence of a large leak severely compromises the accuracy of the testing.
In addition, the digital control gives the capability of compensating the proportional control valves, which can show over time and usage, some shift, automatically without any mechanical adjustment.
Lastly, but by no means least, the digital control provides the capability of adding to the ADTS extra capabilities and more functions, by the simple expedient of loading new software into the instrument, for example during the annual calibration exercise.
All of the features mentioned permit the performance of the aircraft checking with very high accuracy, confidence and repeatability.
OPERATIONAL FLEXIBILITY AND EFFECTIVENESS
All ADTS series MPSXY can be easily controlled to perform all the functions
required to accomplish a calibration or a leak check of the Pitot/Static systems.
Due to the high readings rate (from 20 to 100 times per second depending on the different models) all the test functions to simulate the aircraft flying characteristics, are entered via the color-arranged keypad on the ADTS, or on the Hand Held Remote Control Unit (HHRCU) in a simple and intuitive way. The test is simple and straightforward even for the first time user, since the air data required to accomplish all the functions are clearly shown either in English or Metric values.
The keyboards are color-coded and arranged to allow easy and logical operation.
All indications and controls are located exactly where the operator expects them to be.
Commanded values can be entered either by digitizing directly the values or by increasing or decreasing the values of a fixed step using UP and DOWN arrows.
In addition the MPSXY ADTS also provide the additional, following features:
- MACH Mode where the operator directly enters the target Mach Number.
- EPR Mode to carry out engine EPR system tests.
- TAS generation.
- Up to 30 programmable different test profiles, each one with 26 different test points, can be stored and executed to provide automatic testing capability for a considerable time saving. This allows the full automatic testing of FAR 43 Appendix E ; it is possible to program the stabilization time, the reading time for the leak test as well as the waiting time before recording the readings.
- Automatic or Semiautomatic Leak Test.
- Centre-line Correction, to compensate for height difference between the ADTS and the UUT (or cockpit or flight deck height above ground).
- Encoding Altimeter or Encoder Test with automatic check of all tests points sequence.
- Automatic Test of Radio Altimeter for the complete EGPWS test
- Self-contained Battery pack to allow up to 4 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 (5 to 200 kts) values.
- Full Automatic Calibration by using a DMA PAMBX and a specific SW.
- Rugged LCD with back lit display HHRCU, for one-person only operation.
- Programmable safety limits to ensure safe aircraft testing.
- Different type of remote control units, also with touch-screen color display and USB flash disk to store the test results and examine directly with the remote unit or by an external PC.
- With the MPSRE terminal it is also possible to store more and more test profiles with a large number of features like quantity of different instruments under test, tolerances for every profile step, leak test with programmable stabilization time, reading time, tolerances etc.
- For special needs there is an ATEX remote control unit with touch screen color display.
EASE OF OPERATION
The design of MPSXY has been particularly developed to ensure an easy and intuitive operation.
Training required for the operation of MPSXY is very limited; all the HW and SW protections will allow the first time user to operate safely and with confidence.
Length of time for testing is mainly determined by the airplane limitations, mainly the Altitude Rate of Change, which cannot be exceeded.
The ADTS can save time by stabilizing test altitudes and airspeed values in a very short time.
One very interesting feature is the Multiple Isolator, integrated in the ADTS box to enable up to four separate pitot/static outputs to be connected to the aircraft simultaneously. This provides the capability to switch between the aircraft pitot/static systems directly controlled by the Hand Held Remote Control Unit.
This feature makes leak checks very simple, detecting the exact location of system leaks for immediate rectification without wasting time to vent to ambient, disconnect and reconnect hoses to aircraft.
Otherwise the use of a dedicated external box to accomplish such function compromises the one-man operation capability and makes the hoses connections more difficult.
Additionally; the leaks can be controlled in separate modes:
- classic mode, but with multiple isolator to find the particular line that is leaking.
- static measure mode and pitot control mode or vice versa allows the leak check for the two lines simultaneously thus reducing further measurement time.
These described modes of leak testing have another advantage in addition to saving time, it is the test reliability, not comparable with multiple lines in parallel used in standard two connections (static and pitot) leak test, and without the mix measure/Ps - control/Pt and vice versa mode.
OTHER ADTS PRODUCTS
The improvements in aircraft instrumentation have resulted in the determination of the angle of attack readings through a special pitot tube with 3 or 4 different pressure measurement ports for data acquisition.
These are Ps, Pt, AoA and also 2nd AoA. To test these new types of pneumatic readings the ADTS needs 3 or 4 different pressures generated simultaneously, or it is necessary to use 2 different ADTS with the resultant extra cost and more complication.
DMA produces 3 types of ADTS, two different instruments with 3 independent pressure generators and one instrument with 4 pressures generators. All these instruments have all the capabilities previously described, such as battery, multiple isolator option, complete leak test, various different HHRCU, Bluetooth useful to control the ADTS from a PC or notebook, etc.
Another recently introduced product is the smallest ADTS available today: housed in a very small box (11.8 x 9.8 x 4.7 in) and with a total weight of 8.8 lbs there is a fully automatic ADTS , RVSM capable with color touch screen display, and a great deal more features such as user programmed test profiles, UUT readings, automatic LEAK, different measure units, ultra-low speed readings etc.