A Pilot's Guide to Inflight Icing
Module III - In an Emergency
Aerodynamics
Section: Wing Stall
Start This SectionWing Stall In an Emergency
Although it is obvious that a large amount of ice can have a negative effect on an airfoil, it is not so obvious that a small amount of ice can also dramatically affect an aircraft's performance and controllability. These effects may not become apparent until the configuration or attitude of the aircraft is changed.
In the graph to the left, the effect of ice on the airfoil is negligible until the wing's angle of attack is increased to about 6 degrees. As the angle of attack continues to increase, the effect of the ice on the wing's ability to generate lift becomes more dramatic and the wing stalls at a much lower angle of attack.
Comparison of clean vs. iced wing
Wing Stall In an Emergency
On an uncontaminated wing, when the wing is flown at a high angle of attack, the airflow over the wing can detach from a portion of the wing and create a separated region. At the critical angle, this separated region will engulf the upper surface of wing and the wing will stall.
Example of a clean wing stall
Wing Stall In an Emergency
With ice on the wing, the flow can detach or separate at a lower angle of attack than on a clean wing.
At low angles of attack, the airflow will detach then reattach further downstream. A separation "bubble" of turbulent air is created between where the airflow detaches from the ice shape and where it reattaches to the wing. If the amount of ice on the leading edge increases or the angle of attack increases, the reattachment point will move aft and the separation bubble will grow.
If the angle of attack becomes high enough, the separation bubble will burst and the airflow will not reattach to the airfoil. This portion of the wing has stalled.
Example of an iced wing stall
Related Information
KOTZEBUE, AK
DECEMBER 21, 1988
CESSNA 207
The air taxi pilot reported that he had encountered freezing drizzle and other icing conditions en route, and that, as a result, his aircraft had accumulated about one centimeter (0.38 inch) of ice before he started his daylight approach. At the beginning of the landing flare, the aircraft stalled, and the pilot was unable to regain control. The aircraft's right wing struck the runway, causing substantial damage to the aircraft.
Wing Stall In an Emergency
A roll upset is an uncommanded roll caused by a difference in lift between the right and left wing. In the case of icing, the roll may be caused by:
Asymmetric ice shapes or
Aileron auto-deflection
During a roll upset, you may experience either high control forces and a corresponding roll, or aileron ineffectiveness.
Aircraft in unusual attitude
Related Information*
MOSINEE, WI
DECEMBER 22, 1988
ATR-42-300
The aircraft was on ILS approach at the Central Wisconsin Airport at 3,000 feet MSL in IMC conditions with moderate to heavy rain. A temperature inversion existed from 5,000 feet to 3,000 feet MSL which created conditions conducive to moderate to severe clear icing. The aircraft was in the icing conditions for approximately 12 minutes. One-half way through the procedure turn with the autopilot engaged, the captain reduced power to slow for gear extension.
At 157 knots with flaps up, the aircraft stalled and the autopilot disconnected resulting in an 80 degree roll to the left followed by a 60 degree roll to the right.
The crew applied maximum power. The aircraft was recovered with an altitude loss of 600 feet. Leading edge wing de-icing boots were not activated until the roll event occurred.
The pilot also reported that there was no stall warning prior to the onset of the aircraft buffet.
Mike Monreal, Former ATR-42 Pilot
SACRAMENTO, CA
MARCH 5, 1998
EMB-120
The aircraft departed Sacramento for San Francisco and accumulated some rime ice while climbing to 9,000 feet. After being placed in a hold, the crew climbed to 10,000 feet to avoid the clouds. At 10,000 feet the aircraft was only occasionally in IMC. During the second turn in the hold, while flying at 160 knots, the crew experienced two "shudders." The aircraft then performed an uncommanded roll. The crew added power and attempted to regain control. The aircraft rolled several times before the crew was able to regain control of the aircraft after adding full power and lowering the flaps to 15 degrees.
Michael Hudson, Former EMB-120 Pilot
*Contains pilot accounts (video)
Wing Stall In an Emergency
Asymmetric ice shapes
Ice may form asymmetrically on the wings or shed asymmetrically from the wings. In either case, the asymmetric ice accretion will cause one wing to generate more lift than the other wing. The aircraft will tend to roll towards the wing generating less lift.
Boot cycle resulting in asymmetric shed
Wing Stall In an Emergency
Aileron auto-deflection
If there is ice on the leading edge of the wing, the separation bubble may move aft of the aileron hinge point. This will alter the aileron's aerodynamic characteristics and may cause unboosted ailerons to deflect without pilot input, resulting in an uncommanded roll. In this case, the aileron is still effective in rolling the airplane, however the control forces to move it back to neutral or beyond may be quite high.
The flow over the aileron may not be stable and the control yoke may oscillate violently. The control stick or yoke may go suddenly to full deflection. This phenomenon is called 'aileron auto-deflection'.
Aileron auto-deflection*
*Only the primary wing stall mechanism has been illustrated in the animation sequence. The actual physics of a wing stall is far more complex and not fully understood. Additional wing stall effects include three-dimensionality, dynamic stability, roll, yaw, adverse yaw and power effects.
Related Information
ROSELAWN, INDIANA
OCTOBER 31, 1994
ATR-72
The aircraft was on a flight from Indianapolis to Chicago (ORD) in a holding pattern at 10,000 feet. The flaps were extended to 15 degrees. The aircraft was in the hold for approximately 32 minutes when it was cleared to descend to 8,000 feet. As the flaps were retracted and the aircraft descended through 9,130 feet, the angle of attack increased beyond 5° nose up. The ailerons rapidly deflected and the autopilot disconnected. The airplane rolled rapidly to the right. The aircraft was in a full right wing down position only 1/2 of a second after the autopilot disconnected. Following several unsuccessful attempts by the crew to correct the airplane's attitude, the aircraft impacted a soybean field.
According to the NTSB, the encounter with icing conditions in the hold resulted in a ridge of ice accreting aft of the wing deicing boots and in front of the aircraft's unpowered ailerons.
NTSB Animation