Page 18 - Volume 12, Number 3

Ask the Expert Heater Operation
(and ADF Tuning)
Darn it, the heater is out!
Now that we have been in winter’s icy grip (well, here in Phoenix, it’s more like a warming caress!), your King Air’s heater is probably getting some use. This discussion applies to the electric heater in C90s (and all the C90 variants), E90s, F90s, 100s, A100s, and B100s. In these models, the electric heater is a supplement to bleed air heat. Although most useful on the ground, it may also be used in flight whenever bleed air alone is insufficient to comfortably heat the cabin.
On the ground, bleed air is usually quite cool since the compressors are turning slowly at Idle. In fact, there may be no bleed air at all: With the engines not yet started, it surely is nice to be able to heat the cabin using the electric heater in conjunction with a Ground Power Unit (GPU).
Remember that the heater is composed of eight identical elements or grids, four wired together in parallel to make up the Normal heater and an identical four wired in parallel to create the Ground Maximum heater. When all eight grids operate on the ground, it is satisfying how quickly the cabin warms even on the most frigid of winter mornings.
Yet there are some operators who have never fully utilized this wonderful system due to a lack of understanding and/or a lack of recent practice with it. Let’s review how to make the heater operate properly and effectively and remind ourselves of some reasons why the heater may not be cooperating today.
First, the vent blower. The heat grids – each using about 36 amps of current – get so hot that they would damage themselves and the heater casing if that heat energy were not carried away by sufficient airflow. Yes, the heater has an overtemperature protection switch, but it is located in the heater’s discharge duct, not in the heater core itself. Thus, it will not feel the excessive internal temperature quickly enough to prevent damage. Therefore, the vent blower has an airflow pressure sensor (think pitot tube) that will not allow the Normal or Ground Max heater to operate unless the device senses a good amount of air flow exiting the blower. If ever your vent blower dies, the heater will die with it but then come back to life when the vent blower is replaced.
16 • KING AIR MAGAZINE
I suggest that you always position the vent blower switch to the High position when using the electric heater. This not only provides an increased volume of airflow across the grids to move the heat energy into the cabin more quickly, but also ensures that the airflow pressure sensor is more likely to function correctly. In fact, in most serial numbers – LJ-620 and after, LW-120 and after, B-208 and after, as well as in all F90s and B100s – the blower automatically kicks into High speed mode whenever the heater is operating. To prolong the life of the blower, remember to move its switch back to Low or Auto once airborne when the abundance of bleed air heat causes the heater to no longer be operating.
Do a little math with me: 36 amps per grid times four Normal grid elements equals 144 amps. When the Ground Max elements are also running, we double that to 288 amps. Yet, the heater cannot operate without the Vent Blower, so that adds another 20 amps or so. Over 300 amps required to get full heater operation!
With the exception of the momentary demand of the starter motor, this is by far the largest electrical load that the airplane ever experiences in normal operations. A few consequences of this huge demand: First, the GPU must have enough capacity to handle this demand. Using a simple battery cart to run the heater is not a good idea. A powerful GPU is needed.
Second, a single generator has insufficient output to do the job, since its maximum continuous rating is 250 amps. Make sure both generators are on line before running the heater. Also, for you people with three-blade props and hence have your Low Idle speeds set near 50 percent N1, you will be exceeding generator cooling limits if you apply this much generator load while still at Low Idle. Set both condition levers for about 60 percent N1 before turning on the heater! And for you people with four-blade props, you may also need to tweak your condition levers a bit forward if the generator load bogs down the engine enough such that the propeller speed drops below the minimum Np (propeller speed) limit.
Third, the system designers ensured that the high electrical demand of this comfort item – the electric heater – would never rob power from more important safety of flight items. When switching on any of these anti-ice systems, a relay is activated that prevents the
by Tom Clements
MARCH 2018