Sometimes you need a new approach when you understand the real problem.
It find it very significant that 19 seconds of exposure to the ice crystals/pellets was sufficient to shut down the 3 airspeed sensors. The fact that a loaded up pitot tube can begin to clear itself in 30+ seconds after the blockage is not really good news.
While blocked, a pitot essentially stops collecting new ice inside, so it would appear that what takes 19 seconds to collect requires up to 49 seconds or more inside a pitot tube to begin to melt and clear. This tells me that the pitot heating system is seriously outmatched by the environment.
I think, as some of the posters earlier have mentioned, that the key problem is coupling the melting energy to the collected ice. Conduction alone just cannot do the job and maintain an open pitot tube-at least, not at realistic pitot tube temperatures.. Settling for intermittent blockages is not really an acceptable course. What is needed is a better means of coupling energy to the ice when it is collected and a wider dynamic range of energy modulation.
During recent icing tunnel testing it was found that the electrical current drawn by air data probe heaters varied with the mach number of the airstream such that at lower mach numbers the probe current reduced due to a change in the heater element resistance.
This is very good corroboration that the pitot heaters behaved like ballast resistors.
I would like to offer the thought that using pitot tubes with different means of heating begins to create diversity in the airspeed sensing system sources. Dinking around with more of the same (higher wattage) in pitot heaters is likely to be unsuccessful and will likely retain the cyclic clearing/clogging characteristic under worst case mixed ice conditions. Sometimes KISS is an inappropriate solution if it involves undesirable adverse characteristics. Time to move on.
At least that is how I analyze this.
