Figure 5-38.  Forces exerted when pulling out of a dive.  AOA and results in increased lift. The AOA must increase  as the bank angle increases to counteract the increasing load  caused by centrifugal force. If at any time during a turn the  AOA becomes excessive, the aircraft stalls.  At this point, the action of the aircraft during a stall should be  examined. To balance the aircraft aerodynamically, the CL  is normally located aft of the CG. Although this makes the  aircraft inherently nose-heavy, downwash on the horizontal  stabilizer counteracts this condition. At the point of stall,  when the upward force of the wing’s lift diminishes below  that required for sustained flight and the downward tail  force decreases to a point of ineffectiveness, or causes it to  have an upward force, an unbalanced condition exists. This  causes the aircraft to pitch down abruptly, rotating about its  CG. During this nose-down attitude, the AOA decreases and  the airspeed again increases. The smooth flow of air over  the wing begins again, lift returns, and the aircraft begins  to fly again. Considerable altitude may be lost before this  cycle is complete.  Figure 5-40.  Inflight ice formation. Figure 5-39.  Increase in stall speed and load factor.  Airfoil shape and degradation of that shape must also be  considered in a discussion of stalls. For example, if ice, snow,  and frost are allowed to accumulate on the surface of an aircraft,  the smooth airflow over the wing is disrupted. This causes the  boundary layer to separate at an AOA lower than that of the  critical angle. Lift is greatly reduced, altering expected aircraft  performance. If ice is allowed to accumulate on the aircraft  during flight ,  the weight of the aircraft is increased while the  ability to generate lift is decreased.  [Figure 5-40]  As little as  0.8 millimeter of ice on the upper wing surface increases drag  and reduces aircraft lift by 25 percent.  Pilots can encounter icing in any season, anywhere in the  country, at altitudes of up to 18,000 feet and sometimes  higher. Small aircraft, including commuter planes, are most  vulnerable because they fly at lower altitudes where ice is more  prevalent. They also lack mechanisms common on jet aircraft  that prevent ice buildup by heating the front edges of wings.  Icing can occur in clouds any time the temperature drops  below freezing and super-cooled droplets build up on an  aircraft and freeze. (Super-cooled droplets are still liquid  even though the temperature is below 32 °Fahrenheit (F),  or 0 °Celsius (C).  The FAA along with the General Aviation Joint Steering  Committee (GAJSC) is promoting AOA indicators as one  of the many safety initiatives aimed at reducing the general  aviation accident rate. AOA indicators will specifically target  Loss of Control (LOC) accidents. Loss of control is the  number one root cause of fatalities in both general aviation  and commercial aviation. More than 25 percent of general  aviation fatal accidents occur during the maneuvering phase  of flight. Of those accidents, half involve stall/spin scenarios.  Technology such as AOA indicators can have a tremendous  impact on reversing this trend and are increasingly affordable  for general aviation airplanes.  [Figure 5-41]  The purpose of an AOA indicator is to give the pilot better  situation awareness pertaining to the aerodynamic health  Angle of Attack Indicators  W W W W  CF  L  L  CF  CF  L  L  CF  100  90  80  60  40  20  0  0°  10° 20° 30° 40° 50° 60° 70° 80° 90°  13  12  11  10  9  8  7  6  5  4  3  2  1  Percent increase in stall speed Load factor or “G”  Bank Angle  L o a d f a c t o r S $_{t}$a l $_{l}$s p e d i n c r e a s e 5-26 