Propeller Ice Protection Systems

Propeller Ice Protection

• Aircraft and engine ice protection systems are categorized into de-icing systems (remove ice after forming) and anti-icing systems (prevent ice from forming)
• Propeller ice elimination systems prevent or eliminate ice from forming on propeller blades during flight
• Ice on propeller blades reduces aerofoil efficiency, increases propeller weight, and can cause out-of-balance conditions leading to vibrations and damage
• Icing conditions exist when the temperature is below +10°C and visible moisture (fog, rain, snow, sleet, or hail) is present
• Fog defined as visibility of less than 1,000 meters due to moisture

Fluid Anti-Icing

• Anti-icing systems prevent ice formation on the propeller
• These systems mix a fluid with moisture on propeller blades
• Effective when operating in suspected icing conditions before ice forms; ineffective after ice forms
• Known as the 'TKS system', developed by Tecalemit-Kilfrost-Sheepbridge Stokes
• Requires a fluid that readily mixes with water and has a very low freezing point
• Isopropyl alcohol is commonly used due to low cost and availability, but it is flammable

System Components

• System may include a fluid tank located in the fuselage, vented to the atmosphere with a fluid quantity indicator
• Tank size depends on the aircraft, ranging from quarts to gallons
• A fluid filter is placed between the tank and the fluid pump
• The fluid pump moves fluid from the tank to propeller feed lines, with a pressure not more than 10 psi
• An anti-siphon check valve opens at 3 to 5 psi
• The pump speed is controlled by a cockpit rheostat, varying fluid flow from 1 to 5 liters per hour
• Usually one pump supplies up to two engines
• A slinger ring feed tube is mounted on the engine case
• A slinger ring is mounted on the rear of the propeller
• Fluid flows to blades through blade feed tubes welded onto the blade propeller ring
• A check valve between the fluid pump and the slinger ring feed tube prevents siphoning and reduces evaporation

System Operation

• Rubber feed shoes (anti-icing shoes) can be attached to the leading edge for even fluid distribution, but are not required on all systems
• Rheostat controls the fluid pump, drawing fluid from the tank through the filter to the slinger feed tube
• Fluid flows from the stationary feed tube to the rotating slinger ring
• Centrifugal force moves fluid through blade feed tubes to the leading edge at the shank and outward along the blade surface

Testing

• Flow tests are applicable to systems lacking propeller overshoes and operated by electrical pumps
• Important to fill the system with fluid specified in the maintenance manual for the aircraft
• Check the pump filter's cleanliness and the tank vent system, with the electrical circuit having an ammeter
• Disconnect the delivery pipeline near the slinger ring and use a calibrated container to measure fluid delivery rate and ammeter reading for multi-engine
• Check fluid delivery rate at different rheostat settings must be within manufacturer-specified limits
• Check the slinger ring, pipelines, and tank vent system if amperage is too high or fluid delivery is too low

Functioning Test

• To ensure proper functioning of the propeller de-icing system, check during an engine ground run is recommended
• Paint the propellers with commercial whitewash, then add a dye to the de-icing fluid
• Run the system and check the distribution of the dyed fluid stained on the whitewash
• Uneven distribution may be caused by an eccentrically fitted slinger ring,incorrectly located feed pipes, or obstructions

Cleaning and Inhibiting

• Drain the supply tank and refill it with 95% methylated spirits and 5% distilled water if the de-icing system is out of service for a long period
• Operate the system to distribute the solution evenly, turning the propeller to ensure equal fluid distribution to all feed pipes
• Operators can decide to inhibit or not the fluid pump and system because de-icing fluid leaves a sticky residue when dried
• Recommended to keep about 10 liters of de-icing fluid in the tank and operate the system regularly if not inhibited

Periodic Inspection

• After each flight, propeller blades should be cleaned with methylated spirits or warm soapy water.
• The supply tank should be refilled with the fluid specified in the maintenance manual
• Check the edges and tips of overshoes for adhesion failures
• Check for and repair blisters
• Ensure overshoes are free from cuts, especially at the leading edge
• Damaged overshoes can be slightly cut back, but follow balance checks
• Ensure bottom of trough, longitudinal grooves, pipes, and valves are free from gummy deposits

Electrical De-Icing/Anti-Icing

• De-icing allows ice to form on a propeller blade before removing it using heating elements in a protective overshoe
• The overshoe is located on the blade's inboard part
• Greater centrifugal force at a greater radius causes any ice that forms there to shed more readily
• The propeller de-icing system integrates with the air intake and the propeller overshoe has two separate heater elements in each overshoe
• The element is protected by glass fiber and abrasion-resistant and oil-resistant rubber
• Heat loosens the ice adhesion surface, and ice is removed by centrifugal force and airflow
• Electrical system is switched on when Icing conditions are deemed to exist as anti-icing mode to conserve electrical power

System Operation

• Current is cycled to the heater elements at timed intervals, heating both outer and inner elements on one propeller at a time to maintain rotational balance
• In de-icing mode, the timer system sequences the heating of the outer elements first clears path for the removal of ice from the inner part in the next heating cycle
• The timer energizes each of the four phases for approximately 30 seconds
• Some aircraft have a shorter 15-second cycle
• Slip rings and spring-loaded brush packs transfer electrical power from the stationary engine to the rotating propeller

Installation and Maintenance

• Refer to aircraft and propeller maintenance manuals and approved schedules for complete details
• Routinely inspect overshoes and anti-erosion strips for condition and attachment security
• Look for splits, tears, wrinkling, and overheating discoloration
• Replace anti-erosion strips with any signs of splitting or advanced erosion
• Replace overshoes if damaged exposings and heaters of are are tacky or swollen
• Inspect cable assemblies for cracking or fretting near propeller blades, slip rings, and brush block housings
• Cables should be checked for strain after turning blades through pitch range
• Remove a burned-out overshoe, and inspect blade metal before installing a serviceable one

Brush and Slip Rings Care

• Brushes must be checked to ensure they are not worn, damaged, dirty
• Brushes must move freely in their holder
• The aircraft and propeller maintenance manual to describe the limits, use gauges
• Replace worn brushes and springs beyond their limits
• Clean brush holders with a dry cloth or small spiral hairbrush instead of using solvents
• Ensure brushes can slide, and position with respect to installation
• With Brush installation, avoid side loads. Check at least 80% of face of New brush face of Slip ring contact
• Align brushes with clearance between brush block and rings whenever fitting a block or pack
• After installing a new brush, postpone functional testing until completing engine ground running

Slip Ring Maintenance

• Slip rings should be checked for security and signs of scoring
• Discoloration caused by burning; deposits of oil, grease, or dirt
• The insulation filling between slip rings should be inspected for separation or damage
• After insulation repairs, test insulation resistance
• Clean dirty slip rings with a lint-free cloth moistened with white spirit or specified cleaning fluid

Electrical Checks and Tests

• Perform electrical checks to ensure proper function of propeller de-icing system
• Perform heater element continuity and resistance checks before installing
• The resistance values must meet the limits for the propeller
• Check for damage in element insulation, blades, and spinner
• Check insulation resistance between brush gear and the earth
• Test the insulation resistance after service, maintain min prescribed values (2 to 4 megohm)
• Account for the type of cement in certain de-icing systems, referring to relevant manuals for cement specifications

Voltage Proof Check

• After repairing element overshoes, check a high voltage apply (1,360 V DC or 960 V AC) to leads and blade
• Maintain the voltage for one minute, ensures the insulation resistance there is no breakdown.

Function Tests

• After maintenance operations, to ensure correct performance of an entire de-icing system, with correct periods from the approved maintenance schedules
• Functional tests that involve applying heating current to blade and spinner elements
• It can be determined by operation and read off the ammeter
• Monitor supply voltages and duration during equipment ground runs
• If any circuit devices are isolated for testing, the circuit must be returned to original status after testing

Repairs

• Guidelines are provided for repairing damage to an overshoe
• If the metal is not damaged, cuts, nicks, and tears in the overshoe can be mended with minor repairs if there are no electrical issues
• Cropping or cutting the overshoe tip are restricted when damaged
• Damaged anti-erosion strips on the side edges or blade should be replaced as a similar repair, blade leading
• Blades are to be prepared from worn edges prior applying any new strips in place
• Repair steps, as well as materials needed, and tools and equipment is to found in the manual

Surface Preparation and Bonding

• Chemical cleaning guarantees proper adhesion
• Use clean, lin free cloth, with the cleaning agent in place during cleaning
• Avoiding use excessive quantities of the agent by swabbing
• Shield adjacent surface or equipment with agent in use
• Check bonding medium after a time laps as needed for repairs, also avoid moisture
• Polyvinyl Chloride(PVC) gloves are needed as needed contamination
• Dry all damaged parts before fixing for failures during installation
• Initially fitting blades paint from relevant spot, likewise the sealer paint off after overshoe

Overshoe Fitting

• Worn overshoe edges peeled back carefully where damaged when using sealer paint
• Exposed space inspected if needed to treat and remove
• Blade space blended with blade repair per maintenance manual
• Primer should have clean space to have under coat sealer paint prior to blade being applied
• Cement mixing is of ready to use or required to mix, mix defined portions

Adhesion

• Time to be mixed bonding related to temp and also moisture
• Bond Strength should check prior use is very helpful
• Duralumin should ready as sealer applied with 1’ section of sealer, dried as needed, with weight is key. Rubber weight is 10lbs and also 1’ spacing
• Seal nicks and edges with agent applied. With tacky parts together is also beneficial; you’ll need a thin rubber strip in conjunction with soft pliable pad
• Rubble from existing surfaces where damaged, repair with epoxy
• Blade mask should be remove with all prime agent and cement, clean sealer, rub
• If an overshoe has no rebate, you’ll need template with the sealer on, this marks outline with Crayon

Overshoe Cement Process

• Wire brush a coating from old bonded surfaces and use cleaning agent and bonding related part, no electrical parts will cause failure, dry completely.
• Brush agent on surfaces with clean tool and dry time
• Correct spacing to match centre, install sheath of PVC between blade and side, PVC stops adhesive from spilling
• Heat agent, make blade has heated too
• Allow bonded time by the adhesive label is recommended. Check and remove bond using instructions, re bond if needed
• Check instructions manuals to see if needed. Moment balanced aircraft blades. Need hub or blade is needed