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An air data computer calculates critical air data parameters that provide information such as altitude and speed; accurate data helps a flight crew to fly safely and precisely.
The simplest air data computer provides, at a minimum, barometric altitude (ALT) and indicated airspeed (IAS). Modern air data computers generate the complete International Civil Aviation Organization (ICAO) air data parameter set, providing critical parameters for aircraft flight and navigation.
Air data computer design has been influenced by significant advancements in modern aircraft technology, the main influences being
- Having to operate at higher altitudes, greater airspeeds and more extreme temperatures.
- Increased use of air data parameters by other aircraft systems such as autopilot, engine control, altitude reporting and long range navigation.
- The close proximity of increasingly complex panel instrumentation and associated avionic equipment can create higher operating temperatures.
- The use of data buses for communicating between aircraft systems.
Importance of Pressure Accuracy
The two fundamental pressure measurements required for accurate air data measurement are static (barometric) air pressure and the air pressure induced by the movement of the aircraft through the air, called pitot pressure. When coupled with temperature measurements and error correction, the parameters of interest are calculated by the air data computer (ADC).
Accurate, reliable pressure measurement is therefore critical in the development of an air data computer. The biggest challenge in air data system design is the selection or development of a suitable pressure transducer. The transducer must provide highly accurate measurements for an extended period, often receiving data from sensors that are exposed to the outside and extreme conditions (high heat or ice etc.) and therefore needs to be extremely reliable yet affordable enough to be cost effective.
Curtiss-Wright’s next generation of air data computers contain industry leading pressure transducers which employ vibrating cylinder technology to produce highly accurate, reliable pressure measurements. The transducers are integrated with a processing core and make up the Curtiss-Wright Air Data Engine (ADE).
Vibrating Cylinder Sensor Accuracy
At the core of the Curtiss-Wright ADC is a vibrating cylinder pressure sensor. This sensor is regarded as the premium choice for air data pressure measurement systems when compared to silicon as they are less prone to drift over time and temperature variations. This reduces the number of aircraft system calibration cycles, thus reducing aircraft down time typically by 4 or 5 fold, as well as operator costs.
A vibrating cylinder pressure sensor is designed to measure the absolute air pressure using the vibrating element principle. The sensor provides a frequency output from which the pressure can be computed. The vibrating element, contained in a thin walled metal cylinder, is set into motion at its natural frequency through applied pressure. Electromagnetic pickup coils detect the resonant frequency of the cylinder as the inside air pressure changes, and feedback the frequency to the drive circuitry. This enables the system to maintain the resonant state where the ‘new’ resonant frequency can be equated to the absolute pressure. Thus, the natural frequency of the cylinder depends on the applied pressure.
An air data computer can be extremely accurate after temperature correction and linearization at manufacture. However, the pedigree of an air data computer comes from the stability of its high accuracy pressure transducers over time. The vibrating cylinders used in the Curtiss-Wright’s next generation air data computer products have a stability specification of ‹ 0.145 mbar (4 ft) change per year and typically much less. It is this stability which reduces the cost of maintenance activities and hence cost of ownership of the air data computer.
Curtiss-Wright has tested a number of transducer types as well as having field experience gathered from some 50 years in the Air Data Market. The figure above shows the stability and accuracy of a vibrating pressure sensor over time.