if a single angle of attack sensor fails
Não.
Um A320 tem três sensores AoA. Se alguém relatar um valor diferente para os outros dois, a aviônica ignorará os valores daquele sensor AoA e usará os valores dos outros dois.
Se dois dos três falharem e relatarem valores consistentes - os aviônicos acreditarão que o terceiro é o ímpar e não confiável.
Se ambos os dois relatarem um AoA consistente e excessivamente alto, as proteções de vôo aplicarão uma entrada com o nariz para baixo na tentativa de restaurar o AoA aos valores normais e impedir uma paralisação. esta entrada é calculada com as entradas do piloto.
Pelo que entendi, todas as seguintes condições devem ser atendidas para que as proteções do AoA sejam invocadas erroneamente e causem uma entrada de nariz para baixo:
- Dois sensores AoA falham e
- Ambos informam o mesmo valor e
- O valor é aquele que causaria uma paralisação se verdadeiro.
De acordo com a revista Safety-First da Airbus :
In normal law, on a protected aircraft, exceeding the AOA value of the α PROT threshold would immediately trigger the high AOA protection, thus resulting in a nose down pitch rate ordered by the flight control laws. Further increasing the AOA by maintaining full back stick would eventually result in reaching the α MAX threshold.
are there any cases of an AOA sensor failing on an Airbus, and the plane not crashing?
Espero que haja muitos.
Há até casos de dois sensores AoA falhando simultaneamente e a aeronave continuando, depois de alguns problemas de controle.
Por exemplo, este incidente relatado por Aviação Semana
German investigators studying the cause of an uncommanded pitch-down of a Lufthansa Airbus A321-200 near Pamplona, Spain, in November 2014, hope to discover the probability of similar events linked to frozen angle-of-attack (AOA) sensors as the investigation continues. In the interim, both the European Aviation Safety Agency (EASA) and Airbus have published directives alerting pilots of the issue and the workarounds.
...
The captain continued to hold “more than 50%” rearward stick in stable flight for a period, but with help from technicians on the ground, the crew was able to reconfigure the automation into the aircraft’s alternate control law, rather than its normal “direct” law. The action removed the alpha-protection checks and canceled the nose-down input. The aircraft then continued to its destination.
...
Airbus in early December issued a Flight Operations Transmission and Operations Engineering Bulletin to all A320-family aircraft operators noting that “if two or three AOA probes are blocked at the same angle, an increase of the Mach number may activate high-angle-of-attack protection,” which results in “continuous nose-down pitch rate that may not be stopped with backward sidestick inputs, even in the full backward position.” The recommended fix is to turn off two of the three ADRs, which puts the aircraft in alternative control law, deactivating the high AOA protection.