Reading the CRASH-AERIEN forum of french pilots I found the following post adressing the Antiicing System of the pitots. Reading 400 pages the author shows profound system knowledge and post also relevant schematics. I found it too interesting to not post it here. In the following he talks about the pitot heating logic:
au sol a une temperature relativement faible, je parle de rechauffe en temperature mais j aurais pus parler d augmentation de tension dans la resistance mais bon on fait simple, de l ordre de 25degrés. en vol ce sont les sonde de detection de givrage( ice detect) qui vont en fct de la detection envoyer un signal de rechauffage ou non au calculateur phc (probe heat computer). deux type severe ou non qui modifie ainsi le rechauffage (en gros 200 degres). ces ice detect sont surtout la pour couper le rechaufage quand il n y en a pas besoin. ensuite on a un mode de surpassement en mettant le push button "probe window heat" sur on, la c est pleine power tout le temps (encore faut il le mettre sur on). mais le hic du systeme c est que les sondes ne sont pas situé au meme endroit, deux a gauche captain et stand by (9d1 et 9d3) et une a droite celle du first officer (9d2) avec la prise du vent relatif differente, tout un merdier!!! alors cela ne givre pas au meme moment forcement, c est pourquoi la logique des adrs et qu a partir d une certaine derive d un des circuit les autres le shoot car on ne peut avoir une vitesse differente sur le pfd captain et celui du copi.
A relevant foto:
http://www.fotoshack.us/fotos/94282IceDetection.jpg
Translation:
At the ground it is kept to a temperature relatively low, about 25°C, I am talking of temperature in respect of heating, could have talked about increase of voltage in the resistor, but want to keep it simple. Inflight the Ice Detectors will send according to what they sense a signal for heating ON or OFF to the Probe Heater Computer (PHC). There are two modes 'severe' or 'not severe' that modify the heating (roundabout 200°C).
These Ice Detectors are primarily there to switch off heating when it is not required.
There is an override mode, if one pushes the 'Probe Window Heat' button to ON, which will provide constantly full heating (but you have to switch it to ON).
But the problem of this system cosists of the probes not beeing installed at the same location, two at the left side, captain and standby (9d1 and 9d3), and the First Officer's (9d2) one at the right side, with different sensing of the relative airstream, all that beeing a s..
..t!!!
Now this will not necessarily ice up at the very same moment, that's why the logic of the ADRs, as soon as a certain drift of the values of one circuit is sensed, are excluding it, because one cannot have different speeds on captains and copilots PFD.
For me that could be an answer to why the problem with pitot-icing seems to be more prominent on A32/3/40 than other types.
I remember very well that AB pilots at Lufthansa learned a big deal about Ground Sensing Logic after the Warsaw accident, they did not have all details in their Docs before ('...they don't need to know?').
I write this because sometimes there is more behind a system, than would be visibly to us pilots. Having said this, I do not want to imply any bad intention, only thinking that sometimes the 'designer/engineer-pilot interface' has to be looked at more critically.