The King Air model 200 was the first of the King Airs to have both an automatic and manual mode of propeller de-ice. All previous King Air models offered only the automatic system. The purpose of this article is to describe the system differences and to provide some insight into their use.
All King Air models have been approved for icing flight and were standardly equipped with multiple anti-ice and de-ice equipment. This included, for most models, engine auto-ignition, engine anti-ice (inertial separators), windshield heat, pitot heat, fuel vent heat, stall warning heat, engine lip boot heat (for the original cowling design) and propeller heat.
Regarding the propeller heat system, up until about 1981 this system had both an inboard and outboard electric heating element embedded in a rubber boot that extended from the propeller hub out to about one-third of the propeller blade length. Beyond that boot, there is enough centripetal force and blade flex that ice is not an issue. But near the hub, there is less velocity through the relative wind and less flex since the blade has not yet tapered from its “baseball bat” shape into an airfoil.
The heating elements are fed electric power via three brush blocks and associated slip rings (two in the hot prop system which will be explained later) that are mounted on the backside of the propeller spinner’s bulkhead. To conserve electric power, the inboard and outboard heating elements work separately, not simultaneously. Each heating element uses about 5 amps of current and they are connected in parallel not in series. That means that about 15 amps total is required to heat either the inboard or outboard elements on a three-blade propeller and 20 amps is required for a four-blade prop.
The system is activated by turning on a single Prop Heat switch located on the pilot’s right subpanel. This is a circuit breaker type of switch that will trip itself to the off or down position if excessive current is encountered. On the pilot’s left subpanel, originally, and then on 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 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, since now a normal system will be using 30 or 40 amps – 3-blade or 4-blade – this is readily observed on the generator loadmeters, each of which would increase about 6 or 8%.
The electric wires – typical, round wires – embedded in the inner and outer sections of the earlier boots, as we’ve stated, pull about 5 amps each. When BF Goodrich developed their “Hot Prop” system – with only a single heating element per blade – it would be logical to think that now 10 amps would be required. But no, it’s still 5 amps. This “magic” is accomplished by replacing the wires with metal foil that starts wider near the spinner but narrows as it moves out the blade. This causes more heat to be felt closer to the spinner and less further out. As we’ve stated, no heat at all is needed beyond the boot due to flexing and centripetal force and it follows that the farther out we go on the blade the less need for heat there is. The larger props on the 300-series take about 7 amps per blade.
I believe I am correct in stating that all four-blade King Air propellers use the Hot Prop system. With a single heating element per blade, it now sends current to one propeller for 90 seconds, then the other side for 90 seconds before it repeats. A complete heat cycle for the inner/outer style requires two minutes: 30 seconds per section times four sections. The newer Hot Prop style requires three minutes: 90 seconds per side. But here’s the rub: Ice was shed from only one-half of the boot originally. Now it’s shed from the entire boot. It is my experience and opinion that the nose of the fuselage tends to exhibit more ice “dings” with the more modern system. Oh well, it’s simply a fact of life.
With their four-blade propellers, the later 200s (and all members of the 300-series) replace the three-position inner/outer switch with a two-position switch labeled “Manual.” It is spring-loaded to remain in the down, off position and is held up by a finger of the pilot’s right hand for about a minute and a half when being used. The idea is to then wait until some prop vibration is felt and then hold it up again. Once again, current for the manual system is not displayed on the prop de-ice ammeter but the two loadmeters now show an easily observed 8% increase.
I’ll finish with an idea that is definitely not FAA-approved, yet I have it on good authority that it works like a charm. Got a big rubber band? If you use it to hold the Manual switch in the Up position – maybe hook it to a post light on the instrument panel – then the propeller de-ice system truly becomes an anti-ice system. No 90-second accumulation to fling against the nose! Of course, we are using twice the current that the auto system would use but I believe the two generators can handle it quite easily.