Unforecast Adverse Weather Inadvertent Thunderstorm Encounter A pilot should avoid flying through a thunderstorm of any  intensity. However, certain conditions may be present  that could lead to an inadvertent thunderstorm encounter.  For example, flying in areas where thunderstorms are  embedded in large cloud masses may make thunderstorm  avoidance difficult, even when the aircraft is equipped  with thunderstorm detection equipment. Therefore, pilots  must be prepared to deal with an inadvertent thunderstorm  penetration. At the very least, a thunderstorm encounter  subjects the aircraft to turbulence that could be severe. The  pilot and passengers should tighten seat belts and shoulder  harnesses, and secure any loose items in the cabin. As with any emergency, the first order of business during  an inadvertent thunderstorm encounter must be to fly the  aircraft. The pilot workload is heavy; therefore, increased  concentration is necessary to maintain an instrument scan.  If a pilot inadvertently enters a thunderstorm, it is better to  maintain a course straight through the thunderstorm rather  than turning around. A straight course minimizes the amount  of time in the thunderstorm, and turning maneuvers only  increase structural stress on the aircraft. Reduce power to a setting that maintains a speed at the  recommended turbulence penetration speed as described in the  Pilot’s Operating Handbook/Airplane Flight Manual (POH/ AFM), and try to minimize additional power adjustments.  Concentrate on maintaining a level attitude while allowing  airspeed and altitude to fluctuate. Similarly, if using the  autopilot, disengage the altitude hold and speed hold modes,  as they only increase the aircraft’s maneuvering—thereby  increasing structural stress. During a thunderstorm encounter, the potential for icing  also exists. As soon as possible, turn on anti-icing/deicing  equipment and carburetor heat, if equipped. Icing can be  rapid at any altitude and may lead to power failure and/or  loss of airspeed indication. Lightning is also present in a thunderstorm and can  temporarily blind a pilot. To reduce this risk, turn up flight  deck lights to the highest intensity, concentrate on the flight  instruments, and resist the urge to look outside. Because icing is unpredictable in nature, pilots may find  themselves in icing conditions even though they have done  everything practicable to avoid it. In order to stay alert to this  possibility while operating in visible moisture, pilots should  monitor the outside air temperature (OAT). The effects of ice on aircraft are cumulative—thrust is  reduced, drag increases, lift lessens, and weight increases.  The results are an increase in stall speed and a deterioration  of aircraft performance. In extreme cases, two to three inches  of ice can form on the leading edge of the airfoil in less than 5  minutes. It takes only  1 ⁄2 inch of ice to reduce the lifting power  of some aircraft by 50 percent and increases the frictional  drag by an equal percentage. A pilot can expect icing when flying in visible precipitation,  such as rain or cloud droplets, and the temperature is  between +02 and –10° Celsius. When icing is detected, a  pilot should do one of two things, particularly if the aircraft  is not equipped with deicing equipment: leave the area of  precipitation or go to an altitude where the temperature is  above freezing. This “warmer” altitude may not always be  a lower altitude. Proper preflight action includes obtaining  information on the freezing level and the above-freezing  levels in precipitation areas. If neither option is available, consider an immediate landing  at the nearest suitable airport. Even if the aircraft is equipped  with anti-icing/deicing equipment, it is not designed to allow  aircraft to operate indefinitely in icing conditions. Anti- icing/deicing equipment gives a pilot more time to get out of  the icing conditions. Report icing to ATC and request new  routing or altitude. Be sure to report the type of aircraft, and  use the following terms when reporting icing to ATC:  Early ice detection is critical and is particularly difficult during  night flight. Use a flashlight to check for ice accumulation on  the wings. At the first indication of ice accumulation, take  action to get out of the icing conditions. Refer to the POH/ AFM for the proper use of anti-icing/deicing equipment.  Inadvertent Icing Encounter 1.  Trace. Ice becomes perceptible. Rate of accumulation  is slightly greater than sublimation. Anti-icing/deicing  equipment is not utilized unless encountered for an  extended period of time (over 1 hour). 2.  Light. The rate of accumulation may create a problem  if flight is prolonged in this environment (over 1  hour). Occasional use of anti-icing/deicing equipment  removes/prevents accumulation. It does not present a  problem if anti-icing/deicing equipment is used. 3.  Moderate. The rate of accumulation is such that even  short encounters become potentially hazardous and  use of anti-icing/deicing equipment or flight diversion  is necessary. 4.  Severe. The rate of accumulation is such that anti- icing/deicing equipment fails to reduce or control the  hazard. Immediate flight diversion is necessary. 11-2