Window Heat question: OVHT vs PWR TEST

[mizra108]

Hi there, wondering if someone can explain to me the logic behind the switch that tests an overheat condition vs the one that overrides the temp control and powers the heat to the window. Based on the FCOM, when switch is selected to OVHT with window heat switches ON, it simulates an overheat condition and therefore the OVERHEAT lights come ON and power to the window heat is removed after about a minute. On the other hand, when switch is selected to PWR TEST, it will apply power to the windows if any of the windows is not being provided heat so as to be able verify it is indeed working. Well, as it turns out, in the ixeg 737, the behavior of the switch when placed in the PWR TEST simulates the overheat condition instead of simulating it when the switch is actually placed in the OVERHEAT position. Then, when switch is placed in the OVHT position, it does not seem to do anything at all. I wonder if this was modeled in reverse or is it something particular to this 733 model that was simulated.

[Litjan]

Hi mizra,

this is indeed not portrayed in the correct direction on our 737 yet. I have had a ticket open for this for quite some time, it needs some artwork and some logic work and in 6 years no one ever noticed - except for you, good job!

You are right in your description of how it should work - we currently have the OVHT test assigned to the DOWN position (it works correctly). We have no label for the PWR TEST and it is not working - but the only way to see this working in the real aircraft would be to hold it to that position and feel if the windows will heat up - after a while they should also trigger the overheat protection, especially in the summer, as the window heat is forced to full power with this switch.

Cheers, Jan