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flight. Therefore, the electric heat we are discussing now is definitely a ground-only system. The latching solenoid will not work in flight and even if the crew physically held the electric heat switch up, neither forward nor aft heater will operate.
During the runway line-up procedure, we are directed in these models to position the electric heat switch to the “Off” position. This reduces generator load, allowing the engine to be more likely to meet takeoff power requirements. If we ever forget this step (of course we never would!), it’s not a big deal because (1) the engines will have plenty of power capability due to the cold OAT, and (2) when the strut extends, the switch will turn itself off anyway since the holding magnetic solenoid latch releases.
Temperature sensors installed in the heater ductwork protect the system from getting too hot. The temperature at which the heater shuts off due to this over-temperature protection, about 115°F, is much higher than the temperature at which the system will permit the heater to reset and start to operate again, near 60°F. The practical result of this? Suppose the cabin reaches a comfortable temperature while taxiing out and the crew decides to switch the heater switch off. Unfortunately, there is a lengthy ATC delay, so the King Air must hold short of the runway. Dang! The cabin is getting chilly again, so it’s time to turn the electric heater back on. There is a good
chance that it won’t come on, the switch will not latch. Why? Because of that “allow to operate” temperature limit in the ducts. Conclusion? Make darn sure you have a toasty cabin before switching the heater off since there is a high probability you won’t be able to get it back on.
I will conclude this article by discussing the last of the supplemental electric heat systems, the one that was an option on the 200-series from the model’s start in 1974 through 1992. This system is comprised of radiant heat panels in the cabin’s headliner. (Just in the actual cabin; not in the cockpit nor in the toilet and baggage areas.) In my opinion, the systems that we have previously covered – the ones in the 90-, 100-, later 200- and all 300-series – are real winners that do indeed put out the heat! But the radiant heat panels? What a joke!
The single time that the panels can indeed prove useful and effective is when an external power unit can be plugged in an hour or more before the intended flight and then the heat panels can be switched on for an extended period of time. Slowly, the frigid cabin will be warmed nicely.
In the relatively short time between engine start and takeoff on a typical flight, there is not enough time for the radiant heaters to do much. Expect to remain chilly until takeoff power is applied and finally the bleed air gets nice and hot.
NOVEMBER 2023
KING AIR MAGAZINE • 25