When covered by ice these instruments display incorrect  information thereby presenting serious hazard to instrument  flight. Detailed information on the operation of these  instruments and the specific effects of icing is presented in  Chapter 5, Flight Instruments. Stall warning systems provide essential information to pilots.  These systems range from a sophisticated stall warning vane  to a simple stall warning switch. Icing affects these systems  in several ways resulting in possible loss of stall warning to  the pilot. The loss of these systems can exacerbate an already  hazardous situation. Even when an aircraft’s stall warning  system remains operational during icing conditions, it may  be ineffective because the wing stalls at a lower AOA due  to ice on the airfoil. Accumulation of ice on flight deck windows can severely  restrict the pilot’s visibility outside of the aircraft. Aircraft  equipped for flight into known icing conditions typically have  some form of windshield anti-icing to enable the pilot to see  outside the aircraft in case icing is encountered in flight. One  system consists of an electrically heated plate installed onto  the airplane’s windshield to give the pilot a narrow band of  clear visibility. Another system uses a bar at the lower end  of the windshield to spray deicing fluid onto it and prevent  ice from forming. On high performance aircraft that require  complex windshields to protect against bird strikes and  withstand pressurization loads, the heating element often is  a layer of conductive film or thin wire strands through which  electric current is run to heat the windshield and prevent ice  from forming. Because of their small size and shape, antennas that do not lay  flush with the aircraft’s skin tend to accumulate ice rapidly.  Furthermore, they often are devoid of internal anti-icing  or deicing capability for protection. During flight in icing  conditions, ice accumulations on an antenna may cause it to  begin to vibrate or cause radio signals to become distorted  and it may cause damage to the antenna. If a frozen antenna  breaks off, it can damage other areas of the aircraft in addition  to causing a communication or navigation system failure. Ice-contaminated aircraft have been involved in many  accidents. Takeoff accidents have usually been due to failure  to deice or anti-ice critical surfaces properly on the ground.  Proper deicing and anti-icing procedures are addressed in  two other pilot guides, Advisory Circular (AC) 120-58, Pilot  Guide: Large Aircraft Ground Deicing and AC 135-17, Pilot  Guide: Small Aircraft Ground Deicing. The information in this chapter is an overview of the hazards  of aircraft icing. For more detailed information refer to  AC 91-74, Pilot Guide: Flight in Icing Conditions, AC 91- 51, Effect of Icing on Aircraft Control and Airplane Deice  and Anti-Ice Systems, AC 20-73, Aircraft Ice Protection  and AC 23.143-1, Ice Contaminated Tailplane Stall (ICTS). The pilot of an aircraft, which is certificated for flight in  icing conditions can safely operate in the conditions for  which the aircraft was evaluated during the certification  process but should never become complacent about icing.  Even short encounters with small amounts of rough icing  can be very hazardous. The pilot should be familiar with all  information in the Aircraft Flight Manual (AFM) or Pilot’s  Operating Handbook (POH) concerning flight in icing  conditions and follow it carefully. Of particular importance  are proper operation of ice protection systems and any  airspeed minimums to be observed during or after flight  in icing conditions. There are some icing conditions for  which no aircraft is evaluated in the certification process,  such as super-cooled large drops (SLD). These subfreezing  water droplets, with diameters greater than 50 microns,  occur within or below clouds and sustained flight in these  conditions can be very hazardous. The pilot should be familiar  with any information in the AFM or POH relating to these  conditions, including aircraft-specific cues for recognizing  these hazardous conditions within clouds. The pilot of an aircraft, which is not certificated or equipped  for flight in icing conditions, should avoid all icing conditions.  The aforementioned guides provide direction on how to do  this, and on how to exit icing conditions promptly and safely  should they be inadvertently encountered. Stall Warning Systems Windshields Antenna Icing Summary 4-17