Page 11 - Volume 13 Number 2

PROP HEAT
by Dean Benedict
Igot a call from a King Air pilot I know well. He used to fly a model 90 and brought it in to my shop for many years. Now he flies a 200 with four-blade Hartzell props. We got reacquainted last year during the pre-buy on this 200. The aircraft is working out well, but a recent glitch with the prop heat prompted him to give me a call.
In flight, he knew icing conditions were ahead so he turned on the prop heat. He watched his amp gauge from the start and it was reading 19 amps. He knew this reading was from the right-hand (R/H) prop because the prop heat on newer King Airs starts on the right side. He kept a close eye on the gauge and when it cycled to the left prop it dropped to 14 amps. When it cycled back to the right side, the amp gauge went back up to 19.
Then he got into icing conditions and had ice slinging off the left prop into the fuselage. This confirmed his suspicion that he had a prop heat problem. He switched to Manual (as we all know it’s a spring-loaded toggle switch that must be physically held) and after a few more thuds of ice, he was OK.
He squawked the prop heat to his shop and their first words were, “The maintenance manual says 14-18 amps is OK for four-blade props. We don’t see a problem.” Well ... yes, the manual does say that, but if the prop heat was functioning properly, then why was the left prop slinging ice?
He asked the shop to check the prop heat anyway. They did the typical check with one guy in the cockpit watching the gauge and another guy turning the prop slowly (so that the brushes do not arc on the slip ring) and feeling each boot. All boots were heating. The shop said nothing was wrong, so the pilot called me.
Prop Heat Review
When you put the prop heat switch in Auto, the prop heat timer sends power to brush blocks mounted on the engine nose case. These brushes ride on slip rings mounted on the back side of the prop bulkhead. The slip rings transfer power from the brushes, through the prop leads, to the boot. Tiny wires inside the boot resist the 28 volts coming in and generate heat. The boot gets hot, even the blade warms a little, and this prevents an accumulation of ice. Although called a “deice” system by Beech, it’s really an “anti-ice” system when used properly.
FEBRUARY 2019
The King Air prop deice system was designed to be deployed before ice starts to form. If you wait until ice has developed, the heat makes the ice slip off the prop and slam into the avionics doors on the fuselage. Paint chips and dents in the skin around the avionics area are a sign that Prop Ice Protection was deployed too late; or there is a boot that is not heating, so ice forms, builds up, and flies off the prop when it gets too heavy.
Bad Prop Leads Evade Detection
Anytime you see a drop in amperage on the amp gauge (a low cycle), you know you have a boot that is not heating properly. You’re not going to know which boot, and sometimes you’re not even sure which prop. What you do know is your prop heat system needs attention.
The boots themselves rarely go bad unless you routinely fly into unimproved strips and get rock strikes on your prop boots. This will break the small wires inside the boots and they won’t heat.
Far more common is a bad prop lead not getting power to the boot. These leads are subject to tremendous wear and tear as the props move from feather to reverse. Furthermore, the centrifugal force generated by the spinning prop pushes the leads outward against the inside of the spinner. I cannot emphasize this point strongly enough. The prop leads are under constant tension when the engine is running – even in low idle.
KING AIR MAGAZINE • 9
MAINTENANCE TIP