Windscreen Heat and Cracking Windscreens

[TheShadow]

Reference the Mass windscreen crackings at Denver. Most airplanes I've flown have a NESA HIGH and a NESA LOW as well as a low temp start button that feeds that low current in with around a 50% duty cycle (AFAI-recall).

The Rumors thread on Pprune seems to infer that some airplanes nowadays only have the one windscreen Heat switch (i.e. it's either ON or OFF). If so that would surely be feedback controlled for heating current via a temperature sensor embedded in the windscreen?

I've flown in some particularly nasty weather but never seen the need to use HIGH because of ice - nor have I ever cracked a windscreen. But 13 on one day? All in the same location? The weather may have been particularly icy but that would seem to me to be a design failing of some sort rather than related to a flightcrew or groundcrew screw-up. In fact the only time I used HIGH was to land at Canton Island and Christmas Island (where the birds used to rise up in very large numbers on approach).

Anybody able to elucidate about a possible cause? Maybe the ground-aircon and/or APU driven aircon just wasn't given a chance to warm things up properly on the inside first?

[SNS3Guppy]

I can't speculate as to what happened in specific case in Denver, but I can say that thermally shocking a cold soaked windscreen can cause damage, including cracking and shattering.

[Blacksheep]

The large jet transports with which I'm familiar have window heat controllers that slowly "ramp-up" the window heating current according to a carefully selected temperature/current schedule.

I can't comment on why so many windscreens cracked on one day at one airport, but it seems to me that it depends on aircraft type and the design of the window heat control system. A manually operated, double switch position system could easily be prone to delivering too much current in extremely cold conditions; but I would have thought that the Flight Manual would contain procedures for dealing with that.