One of the most hazardous types of icing that can be encountered in the flight is a condition where the tail plane experiences icing and degrades in performance, due to changed shape of the aerofoil.
The A/c performance decreases significantly with the icing encountered, due to decreasing aerodynamic shape of the tail plane.
As we know that the tail plane is designed to produce negative lift in most phases of the flight, the recovery from this is to holding the pitch attitude level or decrease the pitch attitude in order to gain the airspeed, reduce flaps if extended since they cause high amounts of drag.
The worse part is that when the tail plane icing is encountered in the flight, it can change the shape of the aerofoil unpredictably which can cause different amount of increase in drag due to ice formation.
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Pitch Effects
When the Elevator is pulled up to bring the nose of the A/c up, it causes the upper camber of the tail plane to increase giving an increase in the downward lift, in case of a ice contaminated tail plane the airflow tends to follow the elevator up movement.
When the Elevator is pushed down to bring the nose of the A/c down, it causes the upper camber of the tail plane to decrease giving an decrease in the downward lift, in case of a ice contaminated tail plane the airflow tends to separate, since the airflow moves outward following the elevator movement.
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Flaps Effect
When the Flaps are in up position, the airflow meeting the elevator meets the airflow at a relatively horizontal angle, causing the angle of attack of the tail plane to decrease, which causes the streamline airflow to follow the tail plane curvature and produce more negative lift.
When the Flaps are in down position, the airflow meeting the elevator meets the airflow at a relatively vertical angle, due to being deflected down by the flaps causing the angle of attack of the tail plane to increase, which causes the streamline airflow to move away from the tail plane curvature and produce less negative lift.
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Centre of Gravity Effect
When the C.G is located in the most forward position, the effective weight of the A/c increases and the tail plane has to provide excess negative lift to maintain level flight. The ice contaminated tail plane provides higher Coefficient of negative lift at higher angles of attack, which causes the streamline airflow to separate from the tail plane. It also increases the Stalling speed as is the case in normal forward C.G position.
When the C.G is located in the most rearward position, the effective weight of the A/c decreases and the tail plane has to provide less negative lift to maintain level flight. The ice contaminated tail plane provides lower Coefficient of negative lift at lower angles of attack, which causes the streamline airflow to separate from the tail plane. It also decreases the Stalling speed as is the case in normal rearward C.G position.
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Propeller Engine Thrust Effect
When the engine controls are in lower throttle settings the effects of slipstream effect due to Propeller is less and due to the uneven shape of the icing accumulated on the tail plane the streamlined downwash airflow of the tail plane decreases, causing the streamlined airflow to follow the shape of the tail plane and produce more lift.
When the engine controls are in high throttle settings the effects of slipstream effect due to Propeller is more and due to the uneven shape of the icing accumulated on the tail plane the streamlined downwash airflow of the tail plane increases, causing the streamlined airflow to move away from the tail plane and produce less lift.
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Speed Effect
When the A/c is flying at a slow speed, which occurs as a result of pulling the Elevator up to bring the nose of the A/c up, it causes the upper camber of the tail plane to increase giving an increase in the downward lift, in case of a ice contaminated tail plane the airflow tends to follow the elevator up movement, producing more lift at low speeds.
When the A/c is flying at a fast speed, which occurs as a result of pushing the Elevator down to bring the nose of the A/c down, it causes the upper camber of the tail plane to decrease giving an decrease in the downward lift, in case of a ice contaminated tail plane the airflow tends to move away from the elevator shape, producing less lift at high speeds.