Page 27 - Dec 2022

“Overall, Prop Heat has proven to be a reliable and trustworthy system.”
labeled “Auto” and works exactly as we have been describing. The right switch is labeled “Inner/Outer” or, in later airplanes – 1981 and after, “Manual.” Let’s talk about that new switch.
This switch – the Inner/Outer earlier one – activates electric relays that send current to both propellers simultaneously. The “Inner” position – the up position on the switch that is spring-loaded into its center position – as you would expect, activates the inner heating elements of both propellers simultaneously. Likewise, the “Outer” position – down – does the same for the outer elements. Remember to use Outer before Inner so that the shedding of the inner ice is not impeded by outer ice.
If ever the automatic system fails – for example, perhaps the circuit breaker switch trips itself off – now the pilot has another option to use. Here’s what’s important to realize: The manual system switch has its own circuit breaker as well as breakers for both left and right prop heaters. These are totally independent of the circuit breaker switch that is used in Auto mode. These three switches are located on the left cockpit side wall, in the bottom of the two rows beneath the fuel panel.
Furthermore, the ammeter used for measuring the operation of the automatic system is also removed from the manual system. If it were not, it would always be pegged out on the high end of the scale since twice as much current is flowing. However,
the overhead panel in later year models, is an analogue ammeter with a needle that moves on a fixed background. There is a green arc painted on the background that designates the proper current usage. The green runs from 14 to 18 amps for the three-blade propellers and from 19 to 23 amps for the four- blade ones.
Again, to decrease the electric power required by this system, a timer unit cycles the current in the following fashion: Current is supplied to the right prop’s outer heating elements for 30 seconds, then for 30 seconds it goes to the right prop’s inner elements, then to the left prop’s outer elements for 30 seconds, and finally to the left prop’s inner elements for the same time. By removing the outboard ice first, it makes it easier for the inboard ice to be slung off by the spinning propeller.
In most of these timers – but not the latest version – when the switch is turned off and then back on, the timer starts on the next stage. For example, if the right prop’s inner elements were in use when the system was shutdown, it would restart on the left prop’s outer elements. To verify that all heat boots are working properly for the whole system, two minutes must elapse as the prop ammeter is monitored so that all four heating combinations are checked. If your timer is the earlier type, a complete check can also be made by cycling the Prop Heat switch on and off four times. If you’re not sure which timer
you have, this action may have just checked one section four times!
Usually the ammeter will show a small but noticeable “jump” as the timer switches to a new section. If a very noticeable drop on the ammeter is exhibited – perhaps going from 15 amps to about 10 amps – and then the ammeter jumps back up to 15 after 30 seconds, it is telling you that one blade’s heating element has come disconnected: Current is only flowing on two, not three, blades. (Of course, for a four-blade propeller the ammeter would have gone from 20 amps down to about 15.)
Overall, Prop Heat has proven to be a reliable and trustworthy system. If there is a weaker link in the system, it is probably the timer unit. With the advent of the model 200 in 1974, Beech addressed the issue of a defective timer. Read on...
In the 200-series and 300-series, a second switch is installed in the pilot’s subpanel next to the other Prop Heat switch. The left switch is
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KING AIR MAGAZINE • 25