occur at a lower AOA and higher speed. Even a thin layer of  ice at the leading edge of a wing, especially if it is rough, can  have a significant effect in increasing stall speed. For large  ice shapes, especially those with horns, the lift may also be  reduced at a lower AOA. The accumulation of ice affects the  coefficient of drag of the airfoil.  [Figure 4-20]  Note that the  effect is significant even at very small AOAs. A significant reduction in C$_{L-MAX}$ and a reduction in the  AOA where stall occurs can result from a relatively small  ice accretion. A reduction of C$_{L-MAX}$ by 30 percent is not  unusual, and a large horn ice accretion can result in reductions  of 40 percent to 50 percent. Drag tends to increase steadily  as ice accretes. An airfoil drag increase of 100 percent is not  unusual, and for large horn ice accretions, the increase can  be 200 percent or even higher. Ice on an airfoil can have other effects not depicted in these  curves. Even before airfoil stall, there can be changes in the  pressure over the airfoil that may affect a control surface at  the trailing edge. Furthermore, on takeoff, approach, and  landing, the wings of many aircraft are multi-element airfoils  with three or more elements. Ice may affect the different  elements in different ways. Ice may also affect the way in  which the air streams interact over the elements. Ice can partially block or limit control surfaces, which  limits or makes control movements ineffective. Also, if the  extra weight caused by ice accumulation is too great, the  aircraft may not be able to become airborne and, if in flight,  the aircraft may not be able to maintain altitude. Therefore  any accumulation of ice or frost should be removed before  attempting flight. Another hazard of structural icing is the possible uncommanded  and uncontrolled roll phenomenon, referred to as roll upset,  associated with severe inflight icing. Pilots flying aircraft  certificated for flight in known icing conditions should be  aware that severe icing is a condition outside of the aircraft’s  certification icing envelope. Roll upset may be caused by  airflow separation (aerodynamic stall), which induces self- deflection of the ailerons and loss of or degraded roll handling  characteristics  [Figure 4-21] . These phenomena can result  from severe icing conditions without the usual symptoms of  ice accumulation or a perceived aerodynamic stall. Most aircraft have a nose-down pitching moment from the  wings because the CG is ahead of the CP. It is the role of the  tailplane to counteract this moment by providing a downward  force.  [Figure 4-22]  The result of this configuration is that  actions which move the wing away from stall, such as  Since the tailplane is ordinarily thinner than the wing, it is a  more efficient collector of ice. On most aircraft the tailplane  is not visible to the pilot, who therefore cannot observe how  well it has been cleared of ice by any deicing system. Thus, it  is important that the pilot be alert to the possibility of tailplane  stall, particularly on approach and landing. deployment of flaps or increasing speed, may increase the  negative AOA of the tail. With ice on the tailplane, it may  stall after full or partial deployment of flaps.  [Figure 4-23] Angle of Attack  Upper Surface Frost  Leading Edge Ice Formations  C$_{L-MAX }$ w f i C of L  CG  Weight  Tail download Figure 4-22.  Downward force on the tailplane. Figure 4-21.  Effect of ice and frost on lift. 4-15