17.5 Propeller Ice Protection Flashcards
what are the effects of propeller ice?
Ice formation on a propeller when in operation produces a distortion to the aerofoil section, vibrations and lower propeller efficiency.
When an aircraft is flying under icy conditions, icing protection is necessary to prevent the build up of ice on the propeller blades. The regions most at risk from icing are the propeller blades, the spinner and the engine air intake.
when visible moisture is present in the air, icing can occur in these areas at ambient air temperatures up to
+10° degrees
There are two main ice protection methods employed for protecting a propeller from icing:
The fluid anti-icing system
The electrical de-icing system
what is the Anti-icing system
Anti-icing systems are activated before the formation of ice on the propeller. Once ice builds on the propeller, this system is largely ineffective in removing ice.
what is the de-icing system
Electric de-icing systems can provide ice protection for the entire duration of the flight, if necessary
Fluid Anti-icing Equipment is usually only found on what type of aircraft
This type of system is generally found only on piston-engined aircraft. The fluid that is used can contaminate the compressor blades of gas turbine engines and is therefore not suitable for turboprop aircraft.
Fluid Anti-Icing System (2)
A very simple and problem-free method of preventing ice formation using an alcohol-based fluid (Iso-Propyl-Alcohol). The fluid is stored in a reservoir housed within the airframe. A pump, whose speed is controlled by a rheostat in the cockpit to suit the severity of the icing conditions, injects filtered fluid through a fixed nozzle on each engine into a ‘U’ shaped slinger ring on each propeller. From there the fluid is forced by centrifugal force onto grooved anti-icing rubber boots.
The disadvantages of the fluid anti-icing system are
The weight of the fluid to be carried.
The amount of fluid must be constantly monitored.
The system must be operated before icing occurs.
It is not able to remove impacted ice once it has formed.
Fluid Anti-icing Equipment Maintenance
Slinger Ring
Ensure the alignment of the fluid discharge nozzle with the slinger ring. The pipe must be positioned so that it has sufficient clearance to avoid contact with the ring, taking into account any possible vibration that can occur when the engine is running.
Fluid Anti-Icing Equipment Maintenance
Flow Test
this test is required after reassembly of the system filter to re-establish the fluid flow rate to the propeller and to compare it with the figure specified in the maintenance manual. On multi-engine aircraft, a flow test must be carried out simultaneously on each engine to check that the delivery distribution rate to each engine is balanced.
Functional Test
Fluid Anti-Icing Equipment Maintenance
Any check of the distribution of fluid over the blade surfaces must be carried out with the engine running. Generally, the test is only required on blades that are not fitted with overshoes but the requirement varies with different manufacturers.
Cleaning
Fluid Anti-Icing Equipment Maintenance
due to the thixotropic properties of the fluid, the system and components require cleaning at regular intervals
the system is cleaned by flushing it through with a mixture that consists of 95% methylated spirits and 5% distilled water. After carrying out the procedure, the blades are washed with methylated spirit or warm soapy water. The system filter, which is cleaned periodically, is cleaned using methylated spirit
Inspection
Fluid Anti-Icing Equipment Maintenance
If stone impact damage is apparent then the shoes can be cut back slightly in accordance with the maintenance manual
A reinforcing bead wire is fitted on their rim, if this is broken, the trough must be replaced. If the system has been operated then the blades must be cleaned using methylated spirit or warm soapy water as recommended by the manufacturer.
Electrical De-icing Equipment
A propeller electric de-icing system consists of an electrical energy source, a resistance heating element, system controls, and necessary wiring.
Heating boots/mats containing electrical elements are bonded to the inner third of each propeller blade
electrical power is supplied from the aircraft system and is transferred from the engine(s) by spring loaded carbon brushes rubbing against copper slip rings attached to the back of the rotating propeller hub.
.Leads connected to the slip ring attach to the de-icing boot/mat element(s) on the propeller blade. The heater element can be single, or segmented into two parts. A single element boot/mat has two electrical leads while a twin element has three leads (2 live and 1 earth)
Controls for propeller electrical de-icing systems include on-off switches, ammeters (load meters) to indicate current in the circuits, and protective devices, such as current limiters or circuit breakers
The ammeters permit the monitoring of individual circuit currents and reflect the operation of the timer. To prevent element overheating, the propeller de-icing system is used only when the propellers are rotating and for short test periods of time during the take-off check list or system inspection.
Cyclic heating is employed to lessen the load on the aircraft electrical system and also to prevent a condition known as run-back.
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The use of cyclic heating allows a thin layer of ice to form on the heater mat. This forms a thermal barrier so that when the element is energised its temperature rapidly rises. The ice melts where it contacts the mat, causing it to loosen and be dispersed by centrifugal force
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On propellers with an even number of blades, opposite blades are heated simultaneously to maintain propeller balance. A cyclic timer, installed in the cockpit, controls the whole process. It is either an electric motor-driven contactor or an electronic timer.
.The period for which the heating elements remain energised is dependent on timer selection and can be between 15 and 30 seconds, with a complete cycle of 2 to 6 minutes.
Usually a two speed cycling system is utilised to accommodate both the propeller and spinner requirements. A fast cycle used at the higher air temperatures, when water concentrations are greater and a slow cycle in the lower temperature ranges:
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Fast Cycle =
Approximately 2 minutes. Used for atmospheric temperatures of +10 °C (50 °F) down to -6 °C (21.2 °F).
Slow Cycle
Approximately 6 minutes. Used for atmospheric temperatures below -6 °C (21.2 °F).
Electrical De-icing Equipment Maintenance
Prior to the installation of a propeller, continuity and resistance checks are required on the propeller blade heating elements.
An insulation resistance check must also be carried out periodically to ensure that the heating elements are adequately insulated from the blades and the spinner. Moisture absorption from the atmosphere into the overshoes gradually reduces an element’s insulation resistance.
Electrical De-icing Equipment Maintenance
functional test
The engine must be kept running at a given speed to ensure that a sufficient airflow is maintained.
Some systems have an air/ground mode that reduces the power consumption on the ground. When checking the function of the system, the flight deck ammeter readings indicate the current consumption for each cycle and a test switch can be used to check the current being drawn off each phase.
The slip rings must be cleaned to remove any build-up of carbon and grease. Cleaning is done using white spirit and a lint-free cloth is used for drying.
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The brush gear is subjected to periodic inspection to establish the brush length and that the brushes are not jamming in their holders. Operation in damp and dusty conditions accelerates brush wear.
The brush blocks are cleaned with a dry lint-free cloth or a soft brush. Solvents must never be used.
When new brushes have to be fitted, they must be checked for satisfactory alignment and for contact on the slip ring. The contact area minimum requirement is 80%. Contact is checked by hand turning the propeller through several revolutions.
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signs of corrosion occurring under the boot. This can reveal itself by
a failure of the adhesive bonding
The blade overshoe electrical supply cables are particularly prone to damage. These must be inspected for signs of strain at the connections to the blade overshoe and the terminal block on the spinner backplate
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