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engine P1
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Originally Posted by ettore
Would air speed measurments at the engine intake, compared against the tremendous engine's thrust, be reliable ?
Not a bad idea at all. Very likely each engine has its own probes for measuring intake total pressure P1 and intake total temperature T1, which are used by the engine electronic control system (FADEC). The difference between Pitot pressure and P1 is engine intake total pressure loss, which is quite small in cruise. I don't know if the engine's P1 probes have ever experienced similar icing problems as the airplane's pitots.
HN39
Given that the need for true backup airspeed is rare here is another possible option:
Deploy the RAT (ram air turbine) emergency generator and use the RPM and generated power to compute air speed.
This would also require air temperature and static pressure for decent accuracy, although even without those a usable estimate might be possible.
I don't know if this is at all practical (max speed etc) but it would certainly be an alternate method.
(I created a similar message yesterday but appear to have not actually submitted it, so pardon in advance if this shows up as a dupe.)