The modern air data computers and their associated
instrumentation are used to measure a number of critical air mass properties
during the flight of an aircraft. The computer must track pressure changes as
the aircraft climbs, descends, accelerates, decelerates and then accurately
predict for example capture points for the autopilot. The atmospheric pressure
will decrease monotonically with the distance above the surface. Air Data
Computers have been with us for many years now and have become increasingly more
important, never more so than now as the Reduced Vertical Separation Minima
(RVSM) mandate deadline approaches. The air data computer has become most
important in maintaining optimum performance of the aircraft in this
ever-demanding environment we call the airspace system. As an example, RVSM
provides additional flight levels, increases airspace capacity and most likely
saves hundreds of millions in fuel burn each year.
There are three critical measurements for the computer which
- Airspeed (indicated, true and Mach number),
- Altitude, and
The computer must be capable of providing a stable and extremely accurate measurement of these parameters over long sustained periods of flight.
The two fundamental pressure measurements required is impact pressure as measured at the pitot tube and the ambient static air pressure sensed at the static ports.
Airspeed is probably the most important single piece of information the pilot needs. Virtually every phase of flight is conducted at a prescribed airspeed or range of airspeeds.
The second most important piece of information is altitude which is used:
- by the air traffic control system to provide vertical separation between aircraft,
- to avoid terrain (assuming one knows where the aircraft is in relation to the terrain),
- to convert indicated airspeed to true airspeed,
- to control the pressurization system in the aircraft (to avoid rapid changes in pressure for instance).
The final measurement that must be considered and one that influences many calculations is the air temperature. The air temperature is used to calibrate the impact pressure as well as in determining air density. Temperature information is therefore used to compute Mach number and true airspeed and to indicate when external conditions are such that icing is likely.
The heart of any air data computer is the pressure sensor/transducer itself. The accuracy of the entire system is based on the sensor. The two types of pressure sensors used are absolute sensor for the static port and a differential sensor for the pitot system. MEMSCAP proven sensor design employed based on ion implanted piezoresistive elements acting as strain gauge. Our pressure sensors/transducers are electro-mechanical devices for translating fluid pressure values into voltages across a high-impedance bridge. Our piezo-resistive sensor design employs a fully active Wheatstone Bridge coupled with temperature sensors located a few tens of microns away from the measurement location. Applied pressure presents a distributed load to the diaphragm, which provides bending stresses. This stress creates a strain proportional to the applied pressure, which results in a bridge imbalance. With an applied voltage across the bridge, the unbalance produces a millivolt output. The more common technique used now in our products is an analog to digital interface directly interfacing with the microprocessor. The microprocessor then can monitor and control the transducer temperature to insure a predictable output. Our sensors are specifically designed, packaged and calibrated to provide the high accuracy and stability required by ADCs and RVSM regulations.