Aircraft manufactures use different methods of powering and controlling window heat systems. Commercial and larger corporate aircraft use alternating current (A/C) as primary window heat power. Control functions can make use of alternating or direct current depending on the specific application.
A/C power is usually supplied in one of two fashions. Either current is “pulsated” on and off or constantly applied with a varying voltage.
Hawker aircraft use the pulsating method. Three phase power from the alternators is supplied directly to the window heating elements. Longer bursts are needed for initial heat and then the duration tapers off as the window temperature comes up.
Three thermisters (sensors) are used for control and overheat shutdown. Hawker sensors are Negative Temperature Coefficient thermisters. NTC sensors resistance decreases as temperature increases.
Hawker windows have three heating elements per window, each connected to one phase of A/C power. Each section has its own sensor. The three sections should receive the same amount of power and the resistance readings of the sensors should be fairly close at all times.
In normal operation, the overheat relay contacts are closed and the control relay cycles to pulsate power to the window. If either overheat sensor’s resistance value falls below a set norm, the overheat relay will open. A warning annunciator will also alert the crew for any overheat problem.
(I recently had a problem with a window overheating approximately 5 minutes after power application. The problem turned out to be an open heating element for the section that had the control sensor. The sensor would never show a resistance change and power remained applied until the overheat sensors shut it down.)