A340 Probes Questions
 

Anyone know why the A340 TAT probes are'nt heated on the ground and the Pitot tubes at a lower level? What reason?

Could well be proved wrong, but if the TAT is heated on the ground the ECAM will overread the temperature due to the heating and the FADECs and ADCs will then get the wrong temp also. The pitots are not heated to save life. Very high temps would shorten time to failure and cause corrosion which would affect accuracy.

IDG wins.
The Rosemont (TAT) probe isn't heated on the ground because it is designed to maintain 750 degrees F and would quickly burn up without cooling. (Not to mention what it has done to hands, clothing, and other items when inadvertantly heated on the ground.)
The way it works is: the current used to maintain that temp is measured and translated into a temperature or mach number reading by the ADC.

Let's see someone of you simplify that explanation.

Pitots on the 340 are heated on the ground after engine 2 or 3 have been started, albeit at a reduced level until the a/c is airborne.