Ice and Rain Minimum Performance Standard for Angle of Attack (AOA) and Angle of Sideslip (AOS) Sensors
The new standard will establish minimum performance requirements for angle of attack (AOA) and angle of sideslip (AOS) sensors in ice and rain conditions. The new standard will cover the various sensor technologies used to measure these flow angles and is limited to the sensor itself as defined from the portion of the sensor that is directly exposed to the ice and rain environment to the means by which its output is relayed to the relevant aircraft systems.
The user of this standard must evaluate the aircraft level installation requirements for the probe to ensure adequate coverage for the application. It may be necessary to modify the test conditions in order to meet the applicable installation requirements.
Rationale: Applicants seeking TSO authorization for their flow angle sensors are able to apply for authorization as a part of TSO-C54, the minimum performance standard for stall warning instruments because it contains some requirements for flow angle sensors. However, this TSO was written in 1961 and is based on SAE AS403 [aerospace standard for a stall warning instrument] as revised in 1958. AS403 does not take into account the current body of knowledge on atmospheric icing and does not provide the required performance level of safety in many applications. Regulators, airframers, and probe manufacturers resorted to using test conditions from SAE AS5562 [Ice and Rain Minimum Qualification Standards for Pitot and Pitot-static Probes] to qualify flow angle sensing devices for use in ice and rain conditions. The test points in AS5562 are tailored to target the failure modes of common pitot and pitot-static probe designs, and are not necessarily applicable to all flow angle sensor designs. In addition, the pass/fail criteria for pitot and pitot-static probes does not apply to flow angle probes. This approach causes these TSO applicants to request many deviations to TSO-C54 and AS403 thereby essentially creating an entirely new specification each time.
The new standard seeks to address the shortcoming of the current approach so that fewer deviations are taken to the minimum performance standard in the TSO and seeks to provide a consistent means to evaluate the performance of flow angle sensors in ice and rain conditions by taking into account the current body of knowledge on atmospheric icing and the current state of the art of flow angle sensors. This specification, when used in conjunction with AS7984 [Minimum Performance Standards, for Angle of Attack (AOA) and Angle of Sideslip (AOS) Sensors], provides a comprehensive design and test standard for the devices that fall within the scope of this document.