Prop Heat

Prop Heat

Prop Heat

I got 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.

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.

Prop leads endure significant wear and tear including
constant tension when the engine is running, so they are often the culprit of lower than normal amp readings.

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.

Unfortunately, it is easy to miss a bad lead. The turn-by-hand test described earlier (which is done at every Phase Inspection) doesn’t begin to approximate the conditions in which prop leads operate. Prop leads in questionable condition often pass this test because they are not stretched or subjected to any tension. But as soon as the aircraft is cowled up and taken out for a final ground run, the amp gauge shows a low cycle.

Back when I had my shop, if a customer brought me a King Air and squawked a low cycle on the prop heat (or if I found it myself on the initial ground run) I wouldn’t even bother with the hand-turning test. Instead, I had my guys tug on each prop lead until they found the one that drops the amperage on the gauge. Or better yet, if my avionics guy was available, he’d ohm out each lead.

My first piece of advice to the pilot who called with the left-hand (L/H) prop slinging ice was to convince the shop to take a good look at the prop leads by stretching them out and watching for a low cycle on the amp gauge.

Boot Systems: Single versus Dual Segment

The backside of the propeller bulkhead with an arrow pointing to the slip rings. This example shows two, which means the propellers have single-segment boots. Props with dual-segment boots have three slip rings.

Prop boots on older King Airs may have two segments – an inner segment and an outer segment – and each heats independently. Newer King Airs have single segment boots, whereas the entire length of the boot heats up. In general, three-blade props have the dual segment system and four-blade props have single segments boots, but there are always exceptions to this rule.

The difference is the number of slip rings. Three slip rings mean a dual segment system. Two slip rings means single segment boots. Look at the back side of the prop bulkhead. There is a gap between the bulkhead and the cowling through which you can see the brass-colored slip rings.

Do you have a dual segment system? If you’re not sure, find out. The dual segment system requires that you act with greater anticipation of possible icing conditions and get your prop heat fired up well in advance. The alternating between inboard and outboard segments, delivers less consistent heating than the single segment system.

Again, I repeat, prop heat must be deployed in advance rather than after the fact. It may be called a deice system, but it functions as an anti-ice system.

Prop Amp Gauge

Whether you have a single or dual segment system, if a boot or segment is not heating as it should, you’ve got a low cycle. Ice will accumulate, fly off and slam into the fuselage. To check for a low cycle, keep your eyes glued to your prop amp gauge.

All King Airs have the same gauge regardless of single or dual segments. If the boots are operating normally, each boot (or boot segment) draws around 4-5 amps.

So, let’s do the math. A three-blade prop with three good boots should show 12-15 amps on the gauge. A four-blade prop with four good boots should show 16-20 amps. If a four-blade prop only draws 14 amps, it has one boot that is not heating. If the gauge reads 9 or 10, then two of the four boots are not heating on that side.

A lower than usual amp reading on the prop amp gauge could specify one of the prop leads is bad.

Yes, the manual says 14-18 is OK for a four-blade prop, but that’s in theory. In practice, if the gauge reads 14 and ice is slamming against the nose, something is wrong. There’s a bad lead somewhere, or there’s another issue.

Part of the conflict here is that some of the newest boots draw fewer amps, in the 3-4 range. Perhaps the manual tries to compensate for this. In my opinion, a low cycle should be investigated and not brushed off as “within limits.”

Time Will Tell

A single segment system will heat all the boots on one prop for a period (or cycle) of 60-90 seconds; then the prop heat timer switches the electrical current to the other prop for a 60-90-second cycle. As the system switches back and forth from one side to the other, the needle will flicker very slightly, but as long as all the boots on each prop are heating, the amp gauge reading will remain steady.

A dual segment system works in a similar manner but will have four cycles instead of just two: L/H prop inner segments, L/H outer segments, R/H prop inner and R/H outer (not necessarily in that order). If each cycle can be as long as 90 seconds, then it could take up to six minutes to cycle through all the boot segments one time. This is very important to know when troubleshooting prop deice because a cursory glance at the gauge now and then will not reveal the problem. You will recall the pilot mentioned he kept a steady eye on his prop amp gauge throughout several cycles.

Zero Cycles and Brush Blocks

A zero cycle means no boots on the prop are heating. It is highly unlikely that all the prop leads would fail simultaneously. But if the brush block assembly is worn down to the point where it fails to make contact with the slip ring, that will give you a zero cycle every time.

Brush blocks have springs that push the brushes outward, keeping them in constant contact with the slip rings. When the spring becomes fully extended there is not enough pressure to keep that contact.

Brush Blocks and Prop Overhauls

Saving my customers money was always the uppermost in my mind, so when their props came back from overhaul, if the brush blocks looked reasonably good, I would not change them. Over time, however, I noticed prop heat failures occurring within months or maybe a year of the prop overhaul.

During a propeller overhaul, the slip rings are cleaned up and surfaced so that the brush blocks make optimum contact. This, in turn, makes the brushes wear down faster as they sit with the newly surfaced slip rings. So, even though the prop brush blocks looked like they had some time left in them, they wore down much faster after the prop was overhauled.

Ultimately, I reversed my earlier practice and installed new prop brush blocks whenever props came back from overhaul. I then noticed that new brush blocks together with fresh slip rings often lasted the full six years between prop overhauls (depending on hours flown, of course).

Regarding the problem of the pilot who called me, I remembered that his props had been overhauled recently. So, in addition to checking the prop leads carefully, I suggested he have the shop check the brush blocks and verify good contact with the slip rings. Brush blocks that are on their way out can create intermittent prop heat performance. When they wear out completely, they make zero contact and you get a zero cycle.

Manual Prop Deice

Let’s say you are mid-flight, you get some prop ice slinging against the nose so you check your amp gauge and find a zero cycle. It could be a brush block issue, or it could be a problem with the prop heat timer. That is the time to use the Manual mode.

When the spring-loaded Manual switch is toggled and held in place, the prop heat timer is bypassed and the prop heat runs on both props simultaneously.

When this spring-loaded switch is toggled and held in place, the prop heat timer is bypassed. Now, the prop heat on both props runs simultaneously.

With the dual-segment systems, the Manual switch stays in the center position until you push it up and hold it to heat the inboard segments of both props; then toggle it down and hold it to heat the outboard segments of both props.

Unfortunately, whether you have single or dual-segment prop heat, the Manual Prop Deice bypasses the amp gauge, so you can’t rely on the gauge to verify that the manual mode is working. But if the ice slams go away, you’ll know the Manual switch did the trick (or you are no longer in icing conditions). Either way, you’ll be squawking prop heat when you get on the ground.


Ice slams are never a good thing. Either the prop heat was turned on too late, there’s a malfunction in the prop heat system, or you’ve gotten yourself into icing conditions so heavy that the system just can’t handle it.

Better to be safe than sorry. Test your prop heat on the next ground run. Keep your eye on that gauge and allow sufficient time for several cycles. Address low or zero cycles before flying into potential icing conditions.

If you report a low cycle to a shop and they come back with “unable to duplicate discrepancy” or tell you that your low cycle is “within limits,” you should tactfully suggest a closer look at those deice leads. Some mechanics don’t understand the extreme conditions endured by these leads. All they know is to turn the prop and feel the boots for heat.

A prop lead that went bad and had passed the turn-by-hand test.

Prop leads are the prime suspects if there’s a low cycle. Brush blocks can also be the culprit. Occasionally the prop timer goes bad; that will give you ice slams. Beyond that, you need a good mechanic to troubleshoot the problem to full conclusion. The idiosyncrasies of brush block installation and alignment and the vagaries of mechanical or electronic prop heat timers is what he or she is all about.


As for my pilot friend, following my advice he returned to the shop and had them do a pull test on each prop lead. Guess what? They found a bad one – a lead that passed the hand-turning test. The pilot decided to have all the prop leads replaced as a precautionary measure.

I propose a toast: Here’s to hot props in cold weather!

Fly safely.

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