[Certainly is for both FR and US probes]
I hate to confess ignorance, but FR probes? US probes? Either side of the Atlantic or something else?
[Both probes showed limitations.]
Agreed, but one design (from a maker with a previously good record) starts to show problems after being in service for what, seven years? I am not aware of any changes to route structure and if you exclude global warming
the atmosphere hasn't changed much, so what did? I don't think one can rule out some sort of time dependent change in the physical state of the probes.
[Look, i am trying to simplify (not always possible) because the AS probe principle of operation is VERY SIMPLE.]
Again agreed, but as always the devil is in the detail
[The short duration of the "failures" suggest that:
The heating may be near the required amount or not being applied fast enough due thermal inertia due geometry, geometry of the "heater" or a combination of aspects related to thermal characteristics.]
Again agreed, and Machinbird's listing of the possible apparently minor variations in probe design is exactly what I had in mind when I suggested the answer might be too subtle for outsider analysis (but we still try!)
Like RR NDB, I feel that the heat being applied is probably near what is required, since the UAS events have been transitory, but it may not have been applied in the right area. I have heard of problems on other aircraft where pitot icing was linked to the absence of an insulating washer between probe and fuselage. Extrapolating that thought, the fuselage (and the unheated parts of the probes) constitute a thermal mass that would be at much lower temperature than the heated part of the probe. There will be a temperature gradient along the probe and maybe the details of that matter.
I must say that this is one of the more sensible discussions on this thread
Machinbird
[It is if someone in conceptual design had decided that the AOA systems were not to be trusted. ]
Which is not consistent with the exhortations to "Believe the stall warning". If AoA is good enough for triggering stall warning it is good enough to freeze the THS.
PS - could somebody please remind me how to put quotes in those little blue boxes?