Ice Induced Roll Upset
Ice induced roll upset is an uncommanded, and potentially uncontrollable, rolling moment caused by airflow separation in front of the ailerons which results in self-deflection of unpowered control surfaces.
Roll upset is associated with flight in icing conditions that are severe enough to result in water droplets flowing back behind those wing surfaces that are protected by Ice Protection Systems before freezing. The ice from this "flow-back" forms ridges that cannot be removed by aircraft de-icing equipment. These ridges can cause flow separation which can result in a roll upset.
Roll upset is most often associated with icing conditions involving Supercooled Large Droplets (SLD). In theory, it can occur in conventional icing conditions when temperatures are just slightly below 0°C to -40°C. A roll upset can occur well before the normal symptoms of ice accretion are evident to the pilot and at an angle of attack (AOA) less than what would be the critical AOA for the ice contaminated wing. The control forces necessary to overcome the self-deflection of the ailerons may be beyond the pilot's physical capability.
Conventional ailerons are balanced; that is, in normal flight with the lateral control centred, the hinge moment in one direction on one aileron is compensated by the hinge moment on the opposite aileron. The net force on the pilot’s lateral control wheel is very low. However, should the compensating hinge moment on one side change significantly, the ailerons will automatically self-deflect resulting in an uncommanded roll of the aircraft.
In one accident attributed to an ice induced roll upset, the aircraft was in a holding pattern with the flaps partially extended and the autopilot engaged. Just prior to the Loss of Control, the flaps were retracted. The increase in the wing AOA due to the flap retraction resulted in a flow separation at the wing tip due to the ice accretion. This flow separation induced a hinge moment imbalance at the aileron, which in turn caused the ailerons to self-deflect to full deflection. The autopilot was unable to correct the overbalance, and the aeroplane departed controlled flight from which recovery was not accomplished.
Note that it is the (unpowered) ailerons producing the rolling motion, rather than an aerodynamic stall as might be encountered with leading edge icing.
The symptoms of an incipient roll upset may be masked by the autopilot. If the autopilot is disconnected when operating in icing conditions, warning signs of incipient roll upset, in the form of sloppy aileron response or abnormal roll control forces, may be detected.
Note that in ALL cases, manufacturer's procedures and Aircraft Flight Manual (AFM) guidance take precedence over any recommendations of this article
If icing conditions (especially severe icing conditions) are inadvertently encountered, pilots should consider the following actions to help avoid roll upset:
Exit the icing conditions as soon as possible. Activate ice protection systems and verify that the wing ice protection system is operating symmetrically by visual observation. If it is not, follow the AFM procedures.
Disengage the autopilot and manually fly the aeroplane. As indicated previously, the autopilot may mask important clues of an incipient upset. It may also self-disconnect, should control forces exceed limits, thu
s presenting the pilot with an abrupt unusual attitude and abnormal control forces.
Reduce the angle of attack by increasing speed. If turning, roll the wings level and maintain.
If flaps are extended, do not retract them unless it can be determined that the upper surface of the wing is clear of ice. Retracting the flaps will increase the AOA at any given airspeed, possibly leading to the onset of roll upset.
Set appropriate power and monitor airspeed and AOA. If it is not possible to climb out of the icing conditions, a controlled descent is far preferable to an uncontrolled descent.
Note that the procedures for recovery from an incipent roll upset are, for all intents, the same as those for an incipient stall recovery and are essentially the opposite of those required to recover from a tailplane stall. Should the situation be mis-diagnosed and the wrong procedures applied, a critical situation can be made significantly worse.
Skybrary Nov 3, 2014