A Pilot's Guide to Inflight Icing

Module I - Before You Fly

Aerodynamics of Icing

Section: Handling Effects

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By disrupting the airflow over the ailerons or elevator, small amounts of ice can alter the aerodynamic balance of the controls and potentially render the aircraft uncontrollable.

Aircraft are designed to warn the pilot of incipient handling anomalies with clean airframes. However, ice contamination can cause handling anomalies to occur without warning. Ice accretion can lead to both roll and pitch upsets caused by wing stalls and tail stalls. These handling effects are described briefly here. The recovery techniques will be fully explored in the In An Emergency Module.

Wing Stalls
Tail Stalls

Ice accumulation on wing in Icing Research Tunnel (IRT)

A small amount of ice can have a big effect

Ice ridge on wing in Icing Research Tunnel (IRT)

Ice Ridge

Related Information

Grant Besley, DC-10 Captain

MOUNT CREZZO, ITALY
OCTOBER 15, 1987
ATR-42

Icing conditions were forecast for the flight. The aircraft was climbing at an IAS of 246 kph (133 knots) when the flight crew identified ice accumulation. At an altitude of 4,880 meters (16,000 feet), the aircraft became uncontrollable, rolling from 40 degrees to more than 90 degrees, left and right. The elevator controls were unable to keep the aircraft from pitching down, suggesting that tailplane ice had formed. The aircraft flew into the ground and was destroyed.

Wing Stalls

An ice-contaminated wing will stall at a lower angle of attack or higher airspeed than a clean wing. Minute amounts of ice (equivalent to medium grit sandpaper) covering the leading edges or upper surfaces of wings can increase the stall speed up to 15 knots.

Ice on the wing also can disrupt the airflow over the ailerons and cause the aircraft to behave in unusual ways. The aileron may deflect without pilot input and cause an uncommanded roll.

Change altitude to exit icing in stratus

Comparison of clean vs. iced wing

Tail Stalls

In most aircraft designs, the center of gravity is forward of the wing center of lift. The two forces at these two points generate a nose-down pitch. The horizontal stabilizer counter-acts this nose down moment by generating downward lift.

Vertical forces and pitching moments

Vertical forces and pitching moments

Tail Stalls

To generate downward lift, the horizontal stabilizer acts like an upside-down wing. Consequently, to prevent a tail stall, the air flow must stay attached to the lower surface of the tail, just as the flow must stay attached to the upper surface of the wing to prevent a wing stall.

Tail acts like an upside-down wing

Tail acts like an upside-down wing

Tail Stalls

Ice contaminated tail stalls are almost always associated with flap extension. Lowering the flaps increases the wing downwash, and thereby greatly increases the horizontal stabilizer's angle of attack. Increasing speed and thrust can also increase the angle of attack at the horizontal stabilizer.

Increasing tailplane angle of attack with ice on the tail can disrupt the airflow under the stabilizer and make the elevator less effective. The elevator may oscillate without pilot input and cause an uncommanded pitch change.

In extreme cases, the yoke might snatch forward, immediately pitching the nose down.

Ice-contaminated tailplane stall

Ice-contaminated tailplane stall

Icing Aerodynamic Simulator

Wing stalls usually occur at slower airspeeds or following premature flap retraction.

Tail stalls almost always occur with flap extension, or at the high speed limit for flap extension.

To better understand how flap position and aircraft speed interact with ice accretion to affect the airflow over the wing and tail surfaces, experiment with the Icing Aerodynamics Simulator on the next page. To operate the simulator, select both an airspeed and flap configuration, then click the "GO" button to see how ice accretion can change the airflow over the wing and tail.

The Icing Aerodynamics Simulator has been designed to illustrate the effects of ice on airflow and is for demonstration purposes only.

Icing Aerodynamics Simulator

Instructions:
1) Select a speed setting using the buttons at right
2) Select a flap setting
3) Click the "GO!" button to view the results of the settings



Ice-contaminated tailplane stall Ice-contaminated tailplane stall

Speed indicator