Page 14 - Volume 13 Number 4

Heated Argument –
More on Prop Heat
by Dean Benedict
It’s good to know that people are reading my articles. My last one, about prop heat in the February 2019 issue of King Air, generated some discussion in the King Air section of an online forum.
Ice Slams
A King Air pilot in Alaska shared his experience. Obviously, he gets into icing conditions all the time. Not only does he get ice slams on a regular basis, he welcomes them. He warns his passengers that ice slams will occur and not to worry when they hear thumps against the fuselage. Ice slams reassure him that his prop heat is working.
In that article, I suggested that ice slams could be a sign that your prop heat is not working 100 percent. But I also stated that if you get into a lot of ice, you’ll get ice slams regardless of how soon the prop heat was deployed. I come at this from a maintenance perspective. When I see chipped paint and dents around the avionics bays, it could mean the prop heat system isn’t working 100 percent. When a prop heat segment or boot isn’t heating, it allows more ice to build up before the centrifugal force knocks it off. That makes more damage on the fuselage. So, when I see chips and dents, I want to check the prop heat.
If, in flight, you are watching the amp gauge for prop heat and you see a drop in amperage, you know for sure you have a segment or boot that is deficient; and if you’re in icing conditions, you’ll have ice slams on that side to corroborate your observation.
King Airs that get into icing on a regular basis can have ice shields installed to guard against the damage of the regular ice slams that are normal for that situation.
Cycles and Segments
With a single-segment system, all the boots on one side heat for 90-seconds, then the prop heat timer switches the current to the other prop – 90 seconds per side, back and forth. In newer King Airs the right prop heats first.
With a dual-segment system the outer segments and inner segments on each prop fire separately. The
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maintenance manual specifies the heating sequence as R/H outboard, R/H inboard, L/H outboard, L/H inboard; but then it says: “due to the fact that the timer does not return to any given point when the power is turned off, it may restart at any sequence point.” [M.M. p/n 101-590010-19, Rev D2-11-1-2015].
So, in general, the outboard segments heat before the inner segments but there is no firm guarantee which segment the prop timer will pick when prop heat is turned on.
As for timing, the manual says each cycle is 34 seconds, so a full sequence of all segments on both props is two minutes 16 seconds in duration.
Deice or Anti-Ice
In reference to King Air prop heat, the FlightSafety manual for the King Air 200/B200 says: “CAUTION – Although this system is called a prop deice system, pilot management of the system should be as an anti-ice system.” I’m sure I’ve seen that in the POH as well. That sentence was burned into my brain many years ago.
So, we have a system designed to get rid of ice after it forms, and directions to deploy it before any ice has formed. I have no argument. I just want to add that in my experience flying King Airs I’ve seen the prop heat function as anti-ice, provided it was turned on well in advance.
If I know that icing is likely in my flight plan, I’ll turn the prop heat on right after takeoff. The electrical load isn’t a big deal. As those segments heat repeatedly over an extended period, the blade absorbs and retains a measure of that heat.
When icing is finally encountered, it never forms and there are no ice slams. Again, it’s not going to work that way in chronically cold climates, so take my observation in the spirit in which it was intended.
I have a good friend that flies a 350 based in Colorado. He operates his prop heat in manual mode all the time. To avoid having to hold that switch forever, he uses a sturdy rubber band looped over a post light to hold the
APRIL 2019
MAINTENANCE TIP