A Pilot's Guide to Inflight Icing
Aerodynamics of Icing
Section: Handling EffectsStart This Section
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.
A small amount of ice can have a big effect
Grant Besley, DC-10 Captain
MOUNT CREZZO, ITALY
OCTOBER 15, 1987
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.
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.
Comparison of clean vs. iced wing
Vertical forces and pitching moments
Tail acts like an upside-down wing
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.
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.
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