Design of a Micro-Machined Flow Sensor for Aircraft Air Data Systems Application: Mechanical Considerations

The air data system (ADS) provides crucial information for aircraft navigation and control. The consolidated technology used to transduce the airflow measurements is designed based on pitot tubes and every system embedded on the aircraft shall be certified by the civil authorities to guarantee its safe operation. In the past decades, however, incidents and accidents were reported due to ice crystal formation on pitot tubes entrance, leaving the aircraft in an unsafe condition. Consequently, new technologies are currently under investigation to increase robustness against this particular risk. In this line, micro-electro-mechanical systems (MEMS) are getting attention from the aerospace industry and the micro-machined flow sensors are the best candidates to succeed in ADS application. The technology associated with thermal mass flow sensors (TMFS) can establish a reformulation on air-data system instrumentation, since it is possible to create micro heaters and thermal sensors on the same substrate, without moving parts. To explore the simplicity and robustness of such technology, here, we propose a TMFS operating in calorimetric mode and designed according to a criterion that meets aeronautic performance requirements. Based on the physical phenomena involved, we select adequate dimensions of the various structures in the sensor and materials compatible with semiconductor micro-machining processes. Additionally, the sensor is assumed to be installed in a flat plate structure constantly kept in parallel with the airflow avoiding tubular structures that could accumulate ice crystals. We expose the TMFS structures design criteria and assess the heat propagation among them throughout numeric Computation Fluid Dynamic (CFD) simulations, to validate the material selection as well as the interaction with the environment to which the sensor will be exposed when measuring airflow speeds up to 265 m/s (954 km/h). The results will show that the heating element was well defined for the application and that the thermal management among TMFS structures is a key factor to perform the speed readings