Prop Heat: Anti- or De-Ice?

HighFlyingA380

HighFlyingA380

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Jim F.
So what exactly is prop heat classified as?

According to the FAA's AMT Airframe Handbook, chapter 15 page 23, prop heat is considered a de-icing system. According to my training, it should not be activated unless there is ice adhering, otherwise it may melt and run-back to the unprotected outer portion of the blade and re-freeze. McCauley's Propeller Operator Manual also refers to the heated boots as a de-ice system, explicitly stating that ice should be allowed to build up before activating the heat in order for it to shed properly. These all indicate that prop heat is a de-ice system.

However, the PIM procedures indicate that it should be used as an anti-ice system... Section 3, "Inadvertent Icing Encounter" procedures state that prop heat must immediately be put into the "auto" position. ("Auto" on the Caravan is a 90 seconds on 90 seconds off sequence.) Company SOPs also state it must be turned on prior to entering icing conditions, or immediately upon inadvertent encounters. In addition, the FAA's Instrument Flying Handbook, chapter 4 page 16 states: "Propeller areas on which ice may accumulate and be ingested into the engine normally are anti-iced rather than deiced to reduce the probability of ice being shed into the engine." These are all clearly treating prop heat as an anti-ice system.

In practice, I've tried treating it as both to see what worked better. Any time I've been in icing and turned it to "auto," I've always had it run-back and refreeze. It may feel fine at first, but then even when the heat is on, the prop starts vibrating indicating it's running back and refreezing on the unprotected portion, requiring me to cycle the prop to shed that ice. On the other hand, when I keep it off and wait until I start feeling the ice imbalance, I hit it and it all sheds clean within 10 seconds...

Obviously I'll be contacting my company regarding our procedures, but I'm curious to hear what ya'll have to say about it? Thanks for your input!
 
What does your AFM/POH call it? But from the description of electrothermal propeller heating systems in AC 91-74A, I'd say it's generally considered "de-icing", as in removing icing which has already formed, rather than "anti-icing" to prevent it from forming in the first place.
d. Electrothermal System.
(1) [FONT=Times New Roman,Times New Roman][FONT=Times New Roman,Times New Roman]The electrothermal system deices a surface by heating the surface to a temperature above freezing to break the bond of accumulated ice. The shattered ice is then carried away by the airflow. The surface is allowed to cool to allow ice to form, and the heat is activated again to shed the ice, the cycle being repeated. Such systems are common on propellers and helicopter main rotors, and have been recently introduced on wing and tail leading edges.
[/FONT][/FONT](2) [FONT=Times New Roman,Times New Roman][FONT=Times New Roman,Times New Roman]Propellers are deiced using rubber boots with embedded heater wires to break the adhesion of ice to the propeller blades. Sometimes the blades are heated alternately in sections due to limits of available electrical power. The alternate sections are heated symmetrically to avoid an imbalance of the propeller while sections of ice are being removed and dislodged from the propeller by centrifugal force. Often, on aircraft that have such systems, the skin surrounding the airframe is reinforced with doublers to strengthen the skin where ice is most likely to be flung from the propellers. However, the initial imbalance caused by ice accumulation and the loud noise created by ice shedding and hitting the airframe can be unsettling to passengers and distracting to flightcrews.
[/FONT][/FONT]
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Ron Levy said:
What does your AFM/POH call it? But from the description of electrothermal propeller heating systems in AC 91-74A, I'd say it's generally considered "de-icing", as in removing icing which has already formed, rather than "anti-icing" to prevent it from forming in the first place.
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I'm not seeing anything explicit in my generic .pdf PIM. I'll look in the official PIM tomorrow while at work. I'm guessing it may be an additional page in section 7 for the specific aircraft configuration...? But again, all the checklist procedures treat it as anti-ice as it always says to turn it on immediately.
 
Everskyward said:
From the King Air 200 training manual...



http://www.aviaco-va.es/WP/BE20_Technical_Manual.pdf
Page 10-11 in case you are curious.

That is how I remember using it.

In any case it's probably airplane-specific.
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I forgot about my FSI materials, which also refer to it as an anti-ice system. It just seems weird to me. As I said, it always seems to work so much better just waiting a bit and hitting the switch to "manual" for a few seconds...
 
HighFlyingA380 said:
I forgot about my FSI materials, which also refer to it as an anti-ice system. It just seems weird to me. As I said, it always seems to work so much better just waiting a bit and hitting the switch to "manual" for a few seconds...
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We ran it as anti-ice in the 1900. I never had any issues with it not shedding ice. You sure knew when it was shedding.
 
I must not understand the question. All of my experience is with multi engine, and they are de ice. All of the hot props I have experienced cycle between inner and outer boots ( the boots are actually two separate windings in each boot, that is the reason there are three wires in each boot). For example left engine outer boots heat, then cycle off. Then the inner boots heat and cycle off. Switch to right side and repeat. By the time you finish the forth cycle you will have ice build up on the left prop and you start the cycle again. If it is shedding ice then by definition it is de ice not anti ice.
I am not familiar with a hot prop that can be made to stay hot all the time, might be a single engine thing:dunno:
 
ronnieh said:
I am not familiar with a hot prop that can be made to stay hot all the time, might be a single engine thing:dunno:
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HighFlyingA380 said:
"Auto" on the Caravan is a 90 seconds on 90 seconds off sequence.
...
On the other hand, when I keep it off and wait until I start feeling the ice imbalance, I hit it and it all sheds clean within 10 seconds...
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As I said in the original post, it's a standard on/off cycle. There is a spring-loaded manual switch to override if the timer fails. What I'm finding is when in auto, ice can build quite a bit when it's off for those 90 seconds off, and the 90 seconds on is enough to get run-back and ice outboard of the boot...
 
Yes, I understood what you said. My point was that I have never seen a "hot prop" system that was not designed to be de ice. However if you have a manual over ride that permits continuous operations of the boots then your system appears to be able to operate as anti ice. I have never seen a system in my limited experience that has a way to over ride the timer. Timer fails, you have a problem.
Also I have a hard time understanding water running out past the boots and freezing. Appears to me if your prop can ice up outside the boots it would do that with or without boot operation. Again:dunno:
 
HighFlyingA380 said:
I'm not seeing anything explicit in my generic .pdf PIM. I'll look in the official PIM tomorrow while at work. I'm guessing it may be an additional page in section 7 for the specific aircraft configuration...? But again, all the checklist procedures treat it as anti-ice as it always says to turn it on immediately.
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Immediately upon what? If it's upon seeing ice accumulating, that would suggest it's de-ice. If it's upon entering conditions where icing is likely (like turning on pitot heat in the clouds below 5C/40F), then it's anti-icing. Also, you haven't mentioned what aircraft you're in, so the type specific comments above wouldn't apply unless you're flying the same type. And the answer may be in a system-specific AFM supplement (Section 9) rather than Section 7 of the AFM/POH if it's optional equipment.
 
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Exactly what Captain said. Pitot heat is an excellent example of anti ice. You want no ice to form, not remove ice that accumulates. With prop de ice you normally remove ice that forms. The ice melting part and running out board of the boots is what confuses me. If ice has accumulated on the props, when the heat is applied via boots the props should quickly start shedding ice. Us flying twins are familiar with the noise made as ice bounces off the fuselage cringing at what is happening to the paint. Thus as the timer(s) steps to each section of the prop it sheds the accumulated ice. I have never seen prop boots designed to prevent ice.
I would suggest that perhaps you are feeling the vibration of the ice next to the spinner and the area of the prop close to the hub that does not have heat. It is common practice to change RPM on occasion to help remove that ice. Just my experience.
 
Ron Levy said:
Immediately upon what? If it's upon seeing ice accumulating, that would suggest it's de-ice. If it's upon entering conditions where icing is likely (like turning on pitot heat in the clouds below 5C/40F), then it's anti-icing. Also, you haven't mentioned what aircraft you're in, so the type specific comments above wouldn't apply unless you're flying the same type. And the answer may be in a system-specific AFM supplement (Section 9) rather than Section 7 of the AFM/POH if it's optional equipment.
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Immediately upon inadvertantly entering icing conditions, or on before purposeful entry. Post 1 I did state this is in reference to the Caravan.

ronnieh said:
Exactly what Captain said. Pitot heat is an excellent example of anti ice. You want no ice to form, not remove ice that accumulates. With prop de ice you normally remove ice that forms. The ice melting part and running out board of the boots is what confuses me. If ice has accumulated on the props, when the heat is applied via boots the props should quickly start shedding ice. Us flying twins are familiar with the noise made as ice bounces off the fuselage cringing at what is happening to the paint. Thus as the timer(s) steps to each section of the prop it sheds the accumulated ice. I have never seen prop boots designed to prevent ice.
I would suggest that perhaps you are feeling the vibration of the ice next to the spinner and the area of the prop close to the hub that does not have heat. It is common practice to change RPM on occasion to help remove that ice. Just my experience.
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the area next to the spinner is the only heated part of the prop. The heating element extends from the spinner approx. 8-10 inches outboard. I have landed and after shut-down, noticed clear ice on the outboard portion of the blades, indicating re-frozen runoff after using it continuously in "auto" per the book.

I read the icing supplement of the PIM today while in cruise, and it does explicitly categorize it as an anti-ice system. However, in the description, it then explicitly states that the 90 second cycle is to allow ice to build and then be shed, indicating it's being used to de-ice... It just seems that in my experience (albeit fairly limited so far) the "best practice" as outlined in the PIM is a less efficient and less comfortable means to an end than simply manually de-icing when needed. Shoot, if they're so convinced that the auto timer has a better feel for the build-up than the pilot at the wheel, why not also put a timer on the boots...? :rolleyes:
 
Actually on most (many) prop boots the actual heating starts an inch or so just outside the spinner. The boot goes almost to the hub but, not the heating elements. Just seems hard to understand how water could migrate out the out board sections of blade. The ice should simply turn loose upon activation. Shouldn't be any significant melting.The Caravan may just differ from what I have seen.:dunno:
 
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