October 1, 2007
Steven W. Ells
"The propellers are most important, and if I could have either wing deicers or propeller deicers, I'd take the propellers. If they are doing their job efficiently, you can pull a lot of ice-covered airplane around the sky." — Robert N. Buck, author of Weather Flying
In a perfect flying world there would be no need for propeller anti- and deicing systems. But that ocean of air that supports flight is anything but perfect. With few exceptions, in-flight icing can form on an airplane propeller anywhere within the continental United States. Whenever ice is encountered, take immediate steps to get out of icing conditions. Surviving a serious icing encounter does create some interesting hangar flying stories, but not everyone who encounters serious icing gets back to the hangar.
In 1975 I worked as an airframe and powerplant mechanic for Aero/Dyne.
I remember sitting in the Black Angus restaurant across the street from the airport in Renton, Washington, as DC-3 captain Gary Byers told the following story.
A chapter of the Sweet Adelines, a women's singing group, had hired one of the DC-3s to fly them from Renton, a suburb located southeast of Seattle, to Portland, Oregon, for a singing gig. Byers was breaking in a new co-pilot, and the weather forecast predicted icing along the route. Byers asked our mechanics to double-check the prop deicing system. We tested the pumps and topped off the fluid reservoirs. Byers launched the Douglas Racer with its precious cargo into the winter skies of western Washington. Before long the airplane started picking up ice. Lots of ice. Byers called Center and told them that he was turning around and heading back to Renton. Byers said he looked over to see his junior co-pilot sucking deeply on a cigarette in an apparent attempt to calm himself. Byers told him to put out the cigarette, turn on the alcohol pumps, and open up the flow valves that controlled the amount of alcohol to the props. Byers increased the prop rpm, and, in a minute or two, the flow of deicing fluid loosened the ice, which started slinging off the props. Some of it slammed into the fuselage skin a few feet behind the cockpit, raising an unholy ruckus. Ice soon loaded that old airplane so much that Byers ran the power up to METO (maximum except take-off), and that was enough to carry them home to Renton. That DC-3's fat wing, its legendary ice-carrying prowess — and its fluid-style prop-deice system — reinforced author Buck's belief in the value of ice-free props.
Byers chuckled as he thought back — it was the co-pilot's first real encounter with heavy ice, and between his nearly jumping out of his seat every time a chunk of ice hit the fuselage, the sickly medicinal smell of isopropyl alcohol wafting through the cockpit, and the sound of the Sweet Adelines singing "Nearer my God to Thee" in perfect harmony from the cabin — it was a flight to remember.
A propeller is an airfoil. The typical light airplane propeller converts approximately 85 to 88 percent of the engine's output into thrust. When ice forms on the individual blades of a propeller, two things happen: (1) the lift (thrust) developed by each blade decreases, and (2) in some cases, the propeller becomes unbalanced. Several sytems are now on the market to help prevent ice from forming on the propellers.
The distinctive smell of the isopropyl alcohol/distilled water mix of DC-3 days has faded from cockpits and been replaced by a fluid mix of 85 percent ethylene glycol, 5 percent isopropyl alcohol, and 10 percent filtered water. This mixture has an extremely low freezing point which melts the bond between the propeller/airframe surface and the ice, causing the ice to lose its "grip" on the propeller or airframe. This loss of grip and the centrifugal force exerted by the spinning propeller sling the ice off the propeller blades.
In addition to fluid-style propeller deicing systems, electrically powered thermal-style anti- and deicing systems have also been in service for decades and are very effective. Airplanes that aren't and can't be equipped with a propeller deicing system can get a small measure of help from Ice-Away, a spray-on compound that temporarily makes it harder for ice to stick.
All fluid-type propeller anti- and deicing systems consist of a fluid reservoir with a filler port, one or sometimes two electrically driven fluid pumps, in-line filters, rigid and flexible hoses, and a small nozzle at each propeller where fluid is squirted into a device called a slinger ring which is bolted onto the aft side of the propeller hub. Small tubes — one for each propeller blade — radiate out from the slinger ring and are positioned just above the innermost section of each blade at the leading edge. As fluid is pumped into the slinger ring, centrifugal force slings it out through the tubes onto "feed shoes," which are bonded onto the leading edge of each propeller blade. Feed shoes are very similar in appearance to thermal-electric boots but have molded-in channels to direct the flow of the fluid onto the prop blade.
Aerospace Systems and Technologies, Inc., in Salina, Kansas, also known as TKS, markets and installs all of today's fluid-based airframe and propeller anti- and deicing systems. These systems are capable enough to be certified for flight into known icing (FIKI) on a small number of light aircraft. TKS sells both FIKI and non-FIKI systems that can be retrofitted to a number of propeller-powered general aviation aircraft under supplemental type certificates (STC). TKS systems have also been developed for new airplane manufacturers Cirrus Design and Columbia Aircraft — neither system is FIKI-approved — to provide short-term protection in case of an inadvertent icing encounter. The propeller anti- and deicing system components of TKS systems use — you guessed it — a slinger ring fluid delivery system that's almost identical to the one that Douglas installed on the DC-3 more than 70 years ago. TKS does not sell its prop anti- and deicing system as a standalone system. Unless it's done via the field approval process, currently there's no way to legally install a fluid-based propeller deicing system to existing airplanes unless the propeller system is part of an STCed TKS system.
Fluid-based propeller anti- and deicing systems do have some drawbacks. The fluid reservoir must be large enough to hold from three to eight gallons of deicing fluid, and it must be installed where in-flight changes in the fluid level won't adversely affect the aircraft weight and balance. Fluid-type systems weigh more than thermal-electric systems, and allowances must be made for the loss of useful load when the reservoir is filled.
Thermal-electric (or heated propeller) anti- and deicing systems consist of either a series of heating wires or a layer of metal foil encapsulated in synthetic rubber "boots." These boots are glued onto the inner part of each propeller blade's leading edge. A typical system has a pilot-controlled switch that controls aircraft bus voltage to a timer module, a slip ring assembly bolted onto the back of the prop hub, a set of carbon brushes to convey electrical power to the rotating propeller by bearing against two or three electrically conductive circular rings on the slip ring assembly, a system ammeter, a boot for each propeller blade, and wiring harnesses to connect the components.
Larger "boots" have two separate heated sections that are heated in sequence by the timer; smaller boots have a single heated section. Typically the timer module energizes the circuit that heats the outer section of the boot for a short period of time — cycle lengths vary depending on the installation and typically range from 20 to 90 seconds — before switching the power to the inboard section of the same boot for a cycle period. A typical system on a single-engine airplane then begins the outer-inner cycling again. Multi-engine airplane systems typically flip-flop the boot heating cycles back and forth between the two propellers, going through a complete outer-inner heating cycle before each switch. Typical current draws range from 14 to 18 amps, although some single-engine systems can draw as high as 35 amps. Goodrich of Akron, Ohio, has been the pioneer in developing and improving electro-thermal anti- and deicing systems and components for GA aircraft propellers for decades. Its product list covers a very wide range of aircraft and systems. Hartzell Propeller Inc. of Piqua, Ohio, recently began manufacturing and stocking a complete line of electro-thermal system components ranging from boots to brushes and including everything in between. Michael R. Disbrow, Hartzell's senior vice president of marketing and customer services, told Pilot that this decision was made to establish Hartzell as a "one-stop shopping approach for propeller systems."
Rapco, Inc., of Hartland, Wisconsin, also markets a wide range of electro-thermal propeller anti- and deicing components that are FAA approved via the parts manufacturer approval (PMA) process. The Rapco Web site provides some helpful reference and service information on these systems.
A number of STCs exist for the aftermarket installations of electro-thermal prop deicing systems for light single- and twin-engine airplanes, and Goodrich does list kits on its Web site, but the demand has been very low over the past decade.
Since so few GA airplanes are being retrofitted with FAA-approved prop deicing systems these days, pilots that fly airplanes without any propeller de- or anti-ice systems may probably wonder whether they are totally defenseless against the hazards of propeller icing during an inadvertent encounter. The answer is "not quite." Oregon Aircraft Design LLC has developed a spray-on product called Ice-Away. According to Bill Larson, president of Oregon Aircraft Design, an application of Ice-Away will improve the ice-shedding ability of propeller blades for about three flight hours. Ice-Away is inexpensive and is available at most aviation supply houses. Increasing the propeller rpm, which increases the centrifugal force acting on accumulated ice, is another prop ice-shedding tip that several experienced pilots mentioned.
Goodrich markets products called AgeMaster and ShineMaster that slow boot aging and approve the appearance of boots. ICEX II, another Goodrich product, lessens the adhesion of ice to wing and prop boots. According to the company, an application of ICEX II is effective for up to 50 hours on wing boots and 15 hours on prop boots.
Jet Stream markets boot maintenance and protection products. Pbs Prep is used first to clean and is followed by an application of PBS Boot Sealant, which protects against weather, wear, and tear. All of these products are also available at most aviation supply houses.
Every fall the airframe and powerplant mechanics at Aero/Dyne were tasked with cleaning the filters, hoses, and nozzles of the isopropyl alcohol/distilled water system installed on the DC-3s. For some reason — probably evaporation — after each summer the systems would be clogged with crud. Preflight actions consisted of checking the fluid level; turning on the pumps and listening for the characteristic click-click noise that meant they were moving fluid; and checking to see that the fluid was being delivered to the slinger ring, where it would overflow and drip to the ground.
Rich Fischler, the repair station manager at Rapco, sent the following hints for maintaining electro-thermal systems: Proper maintenance of the slip rings is critical. Keep the rings clean by wiping with lacquer thinner or isopropyl alcohol at least once every 100 hours — more often in dusty conditions. Dirty slip rings accelerate the wear of the carbon brushes. Fischler said that Rapco brushes have a life of approximately 1,100 hours when the slip rings are kept clean and properly maintained. The length of each brush must also be checked regularly. If the brushes are permitted to get too short they can cock and in some cases jam in the brush holders. Maintenance manuals outline brush length checking procedures.
All thermal deicing system service manuals go into great detail about the proper way to secure the deice electrical lead straps, boots, and required clamps to each propeller blade. This isn't just busywork — these procedures must be followed to maintain system security in the presence of the centrifugal forces generated by the spinning propeller.
The typical GA airplane is no match for in-flight icing. A proper weather briefing, an escape plan, and immediate action are the tools that will help pilots savor a more familiar application of ice — in a cool drink after a safe flight.
E-mail the author at [email protected].
Tricks for dealing with ice
Under the "right" conditions ice can accrete on an airplane at an extremely rapid rate. As little as a one-eighth inch of rough ice can cause a 50 percent loss of climb rate and a 10 percent loss in cruise speed. Any icing encounter must be taken seriously.
A fluid system preflight consists of checking the reservoir for adequate fluid level and visually seeing fluid drip out of each slinger ring during system activation.
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