17.5

Question 1

Effects of ice on a prop

Answer 1

Ice formation on a propeller when in operation produces a distortion to the aerofoil section, vibrations and lower propeller efficiency

Question 2

What regions are most at risk from icing?

Answer 2

The regions most at risk from icing are, the propeller blades, the spinner and the engine air intake

Question 3

What are the two types of de-icing?

Answer 3

Fluid

Electrical

Question 4

Disadvantages of fluid anti-icing?

Answer 4

The weight of fluid to be carried.
The amount 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

Question 5

What must be checked with a slinger ring?

Answer 5

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 may occur when the engine is running

Question 6

What must be checked with flow rate of a fluid anti-ice system?

Answer 6

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. When the test is carried out on a multi-engine aircraft, the difference in the
distribution of fluid to each engine should be within the limits specified

Question 7

Functional test of fluid anti-ice system?

Answer 7

To check 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 will vary with different manufacturers

Question 8

Cleaning of a fluid anti-ice system?

Answer 8

Because of the thixotropic properties of the fluid, the system and components require cleaning a 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

Question 9

Inspection of fluid anti-ice system?

Answer 9

The blade overshoes should be inspected for cuts, blisters and signs of separation at the edges. If stone impact damage is apparent then the shoes can be cut back slightly in accordance with the maintenance manual. The troughs in the overshoe must be checked for cuts and distortion. 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 should be cleaned using methylated spirit or warm soapy water as recommended by the manufacturer

Question 10

How are the electrical heating mats powered?

Answer 10

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

Question 11

How is the slip ring connected to the mats?

Answer 11

By leads

Question 12

How many heating elements are typical?

Answer 12

slip ring attach to the de-icing boot/mat element(s)
on the propeller blade. The heater element may be single, or segmented into two parts. A single element boot/mat will have two electrical leads while a twin
element will have three leads (2 Live & 1 Earth)

Question 13

Why is cyclic heating used?

Answer 13

Cyclic heating is employed to lessen the load on the aircraft electrical system and also to prevent a condition known as run-back. Continuous heating of the element would cause water to be carried rearward by the airflow, which would then freeze on the trailing edge of the blade

Question 14

How does cyclic heating work?

Answer 14

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

Question 15

How is balanced maintained on the propellers?

Answer 15

On propellers with an even number of blades, opposite blades are heated simultaneously to maintain propeller balance

Question 16

How is the heating process controlled?

Answer 16

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 may be between 15
and 30 seconds, with a complete cycle of 2 to 6 minutes

Question 17

How long is a fast cycle?

Answer 17

Approximately 2 minutes. Used for atmospheric temperatures of +10° C down to minus 6°C

Question 18

How long is a slow cycle?

Answer 18

Approximately 6 minutes. Used for atmospheric temperatures below minus 6°C

Question 19

What must be done prior to fitting electrically heated props?

Answer 19

Prior to 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 will gradually reduce an element’s insulation resistance

Question 20

What is involved in the functional test of electrically heated props?

Answer 20

It is important to comply with the maintenance manual limitations on operating the heating elements on the ground. 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 will indicate the current consumption for each cycle and a test switch can be used to check the current being drawn off each phase

Question 21

How should the brushes be inspected?

Answer 21

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 will accelerate brush wear. In this case the brush wear check must be carried out at shorter intervals. The brush blocks are
cleaned with dry lint-free cloth or a soft brush. Solvents must never be used

Question 22

How must slip rings be inspected?

Answer 22

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.
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. The brush faces can then be checked for witness marks.
When new brushes have been fitted an initial engine run is carried out to bed in the brushes prior to conducting a functional test of the system

Question 23

How are overshoes be inspected?

Answer 23

The overshoes must be inspected for cuts, blisters, wrinkling, lifting and signs of overheating. It is also important to look for signs of corrosion occurring under the boot. This can reveal itself by failure of the adhesive bonding. If a heater element is exposed, or the overshoe is sticky, swollen or affected
by contaminants, the overshoe must be replaced. If the overshoe has been rejected for overheating, the propeller blade must be examined for evidence
of damage resulting from burning. 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 back-plate